Canadian Open Textbook 1st Edition (2023)

too hot to touch

The somatic nervous system has traditionally been viewed as a division within the peripheral nervous system. However, this overlooks an important point: somatic refers to a functional division, while peripheral refers to an anatomical division. The somatic nervous system is responsible for our conscious perception of the environment and our voluntary responses to that perception through skeletal muscle. Peripheral sensory neurons receive information from environmental stimuli, but the neurons that produce motor responses come from the central nervous system. The distinction between the structures (i.e. anatomy) of the central and peripheral nervous systems and the functions (i.e. physiology) of the somatic and autonomic systems can be most easily demonstrated through a simple reflex action. If you touch a hot stove, withdraw your hand. Sensory receptors in the skin detect extreme temperatures and early signs of tissue damage. This initiates an action potential that travels along the sensory fiber from the skin through the dorsal spinal root to the spinal cord and directly activates a ventral horn motor neuron. This neuron sends a signal down its axon to excite the biceps brachii, causing the muscle to contract and flexing the forearm at the elbow to take your hand off the hot stove. The withdrawal reflex has other components, such as inhibition of the opposing muscle and balancing posture while the arm is being pulled back forcefully, which will be explored further at the end of this chapter.

The basic withdrawal reflex discussed above involves sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes the biceps brachii to contract). Expanding the explanation of the withdrawal reflex can include inhibition of the opposite muscle or transverse stretching, both of which increase the complexity of the example by including more central neurons. A collateral branch of the sensory axon would inhibit another ventral horn motor neuron so that the triceps brachii would not contract and slow the retraction. The crossed extensor reflex provides a compensating movement on the other side of the body, which requires another sensory axon collateral to trigger extensor contraction in the contralateral limb.

A more complex example of somatic function is conscious muscle movement. For example, reading this text begins with visual sensory input to the retina, which is then projected to the thalamus and then to the cerebral cortex. A number of regions of the cerebral cortex process visual information, beginning in the primary visual cortex of the occipital lobe, and leading to the conscious perception of these letters. Subsequent cognitive processing leads to understanding of the content. As you read, regions of the cerebral cortex in the frontal lobe plan how to move your eyes to follow the lines of text. Output from the cortex causes activity in brainstem motor neurons, which cause movement of extraocular muscles via the third, fourth, and sixth cranial nerves. This example also includes sensory input (the projection of the retina to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brainstem resulting in coordinated nerve contraction). extraocular muscles) .

An important function of sensory receptors is to help us learn about the environment around us or the state of our internal environment. Stimuli from different sources and of different types are received by the nervous system and converted into electrochemical signals. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to generate an action potential that is transmitted to the central nervous system (CNS), where it is integrated with other sensory information or sometimes with higher cognitive functions to become a conscious perception of that stimulus. Central integration can then lead to a motor response.

Describing the sensory function with the term sensation or perception is a conscious distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli in a meaningful pattern. Perception depends on sensation, but not all sensations are perceived. Receptors are the cells or structures that perceive sensations. A receptor cell is directly modified by a stimulus. A transmembrane protein receptor is a protein on the cell membrane that mediates a physiological change in a neuron, most commonly through the opening of ion channels or changes in cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, not specifically called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flux across the membrane and can generate an action potential or graded potential in sensory neurons.

sensory receptors

Environmental stimuli activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are recognized by different types of receptor cells. Recipient cells can be classified into types based on three different criteria: cell type, location, and function. Receptors can be structurally classified based on the cell type and their position in relation to the stimuli they recognize. They can also be classified functionally based on thetransmissionof stimuli, or how mechanical, light, or chemical stimulation changes the potential of the cell membrane.

Types of structural receptors

The cells that interpret information about the environment can be (1) a neuron that afree nerve endings, with dendrites embedded in tissue that would receive sensation; (2) a neuron with aencapsulated endin which the sensory nerve endings are encapsulated in connective tissue, increasing their sensitivity; or (3) a specialistrecipient cell, which has different structural components that interpret a particular type of stimulus (Figure 1. Classification of receptors by cell type). Pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also found in the dermis of the skin are laminated bodies, neurons with encapsulated nerve endings that respond to pressure and touch. Cells in the retina that respond to light stimuli are an example of a specialized receptor, aphotoreceptor.

Classification of receptors by cell type

Another way receptors can be classified is based on their position in relation to stimuli. AExteroceptorIt is a receptor that is close to a stimulus in the external environment, such as B. somatosensory receptors located in the skin. AinterceptorIt is the one that interprets the stimuli of the internal organs and tissues, such as the receptors that detect the increase in blood pressure in the aorta or carotid artery. Finally onePropiozeptorIt is a receptor that is close to a moving part of the body, e.g. B. a muscle that interprets the positions of tissues as they move.

Types of functional receptors

A third classification of receptors is based on how the receptor translates stimuli into changes in membrane potential. Stimuli are of three general types. Some stimuli are ions and macromolecules that affect transmembrane receptor proteins as these chemicals diffuse across the cell membrane. Some stimuli are physical changes in the environment that affect the membrane potentials of receptor cells. Other stimuli include visible light electromagnetic radiation. For humans, the only electromagnetic energy our eyes perceive is visible light. Some other organisms have receptors that humans lack, such as snakes' heat sensors, bees' ultraviolet light sensors, or migratory birds' magnetic receptors.

Receptor cells can be further categorized based on the type of stimuli they transmit. Chemical stimuli can be interpreted by aChemorezeptorwhich interprets chemical stimuli such as the taste or smell of an object.of osmoreceptorsrespond to dissolved concentrations of body fluids. In addition, pain is primarily a chemical sense, interpreting the presence of tissue-damaging chemicals or similar intense stimuli through aNozizeptor. Physical stimuli such as pressure and vibration as well as the perception of sound and posture (balance) are interpreted by aMechanorezeptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed by aThermorezeptorsensitive to temperatures above (heat) or below (cold) normal body temperature.

sensory modalities

Ask anyone what the senses are and they will likely list the five main senses: taste, smell, touch, hearing and sight. However, these are not all senses. The most obvious omission from this list is balance. Additionally, what is known simply as touch can be broken down into pressure, vibration, stretch, and hair follicle position, depending on what type of mechanoreceptors perceive those tactile sensations. Other overlooked senses are temperature perception through thermoreceptors and pain perception through nociceptors.

Within physiology, the senses can be classified as general or specific. Acommon senseIt is one that is distributed throughout the body and has receptor cells in the structures of other organs. Examples of this are mechanoreceptors in the skin, muscles or blood vessel walls. The general senses often contribute to or contribute to the sense of touch, as described abovePropriozeption(body movement) andKinästhesie(body movement) or to agut feeling, which is more important for autonomous functions. Aspecial senseIt is one to which a specific organ is dedicated, namely the eye, inner ear, tongue or nose.

Each of the senses is called uponsensory modality. Modality refers to the way information is encoded, which is similar to the idea of ​​transduction. The main sensory modalities can be described on the basis of their transduction. The chemical senses are taste and smell. The general sense, commonly referred to as touch, involves chemical sensations in the form of nociception or pain. Mechanoreceptors detect pressure, vibration, muscle stretch, and hair movement from an external stimulus. Mechanoreceptors also detect hearing and balance. Finally, vision involves the activation of photoreceptors.

Listing all the different sensory modalities, which can be as many as 17, involves breaking down the five major senses into more specific categories, orsubmodalitiesIn the broadest sense. A single sensory modality represents the sensation of a particular type of stimulus. For example, the general sense of touch known asNo somatosensory, can be divided into light pressure, deep pressure, vibration, itching, pain, temperature or hair movement.

taste (taste)

There are few recognized submodalities within the sense of taste, ortaste. Until recently, only four tastes were known: sweet, salty, sour, and bitter. Research in the early 20th century led to the discovery of the fifth taste, umami, in the mid-1980s.umamiis a Japanese word meaning "delicious taste" and is often translated as savory. Recent research suggests there may also be a sixth taste for fats or lipids.

Taste is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Called raised bumpspapillae(singular = papilla) contain the structures for taste transmission. There are four types of papillae, depending on their appearance (Figure 2. The tongue): equatorial, laminar, filiform, and fungiform. Within the structure are the papillaetaste budscontains specializedtaste receptor cellsfor the transmission of taste stimuli. These receptor cells are sensitive to chemicals in the food that is eaten and release neurotransmitters based on the amount of the chemical in the food. Taste cell neurotransmitters can activate sensory neurons in the cranial facial, glossopharyngeal, and vagus nerves.

language

The salty taste is simply the perception of sodium ions (Na⁺) in saliva. When you eat something salty, the salt crystals dissociate into the component Na ions.⁺y Kl^–that dissolve in the saliva of the mouth. then one⁺The concentration increases outside the taste cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. Nas entrance⁺in these cells it leads to a depolarization of the cell membrane and the generation of a receptor potential.

The bitter taste is the perception of H⁺Concentration. Like the sodium ions in salt flavors, these hydrogen ions enter the cell and trigger depolarization. Acid flavors are essentially the perception of acids in our food. Increasing concentrations of hydrogen ions in saliva (decreasing salivary pH) trigger increasingly more graded potentials in taste cells. For example, orange juice, which contains citric acid, tastes acidic because it has a pH of around 3. Of course, it's often sweetened to mask the sour taste.

The first two tastes (salty and sour) are caused by Na cations.⁺y H⁺. The other tastes result from the binding of food molecules to a G protein-coupled receptor, a G protein signaling system eventually leading to depolarization of the taste cell. The sweet taste is the sensitivity of the taste cells to the presence of dissolved glucose in the saliva. Other monosaccharides such as fructose or artificial sweeteners such as aspartame (NutraSweet™), saccharin or sucralose (Splenda™) also activate sweet receptors. The affinity for each of these molecules varies, and some taste sweeter than glucose because they bind differently to the G protein-coupled receptor.

Bitter taste is similar to sweet in that the food molecules bind to G-protein coupled receptors. However, this can happen in a number of ways as there is a wide variety of bitter taste molecules. Some bitter molecules depolarize taste cells while other taste cells hyperpolarize. Likewise, some bitter molecules increase G-protein activation in the taste cells, while other bitter molecules decrease G-protein activation. The specific response depends on which molecule binds to the receptor.

An important group of bitter-tasting molecules are the alkaloids.alkaloidsare nitrogenous molecules commonly found in bitter-tasting plant products such as coffee, hops (in beer), tannins (in wine), tea and aspirin. The toxic alkaloid content makes the plant less susceptible to microbial infection and less attractive to herbivores.

Therefore, the function of bitter taste may be mainly related to stimulating the gag reflex to avoid poison ingestion. For this reason, many normally eaten bitter foods are often combined with a sweet component to make them tastier (e.g. cream and sugar in coffee). The highest concentration of bitter receptors appears to be on the back of the tongue, where a gag reflex might yet spit out toxic food.

The taste known as umami is often referred to as a salty taste. Like sweet and sour, it is based on the activation of G protein-coupled receptors by a specific molecule. The molecule that activates this receptor is the amino acid L-glutamate. Therefore, the umami taste is often perceived when eating high-protein foods. Not surprisingly, meaty dishes are often described as tasty.

Once taste molecules activate taste cells, they release neurotransmitters in the dendrites of sensory neurons. These neurons are part of the cranial facial and glossopharyngeal nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to the taste buds in the front third of the tongue. The glossopharyngeal nerve connects to the taste buds in the back two-thirds of the tongue. The vagus nerve connects to the taste buds at the back of the tongue, at the rim of the pharynx, which are more sensitive to noxious stimuli such as bitterness.

look at thisVideoto learn more about dr. Danielle Reed of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, who was interested in science from an early age because of her sensory experiences. He realized that his sense of taste was unique compared to other people he knew. Now he is studying the genetic differences between people and their sensitivity to taste stimuli. A brief image of a person sticking out their tongue, which is covered in colored dye, can be seen in the video. This is how dr. Reed Visualize and count the papillae on the surface of the tongue. Humans are divided into two groups known as “tasters” and “non-tasters” based on the density of the taste buds on the tongue, which also indicates the number of taste buds. Non-tasters can taste food but are not as sensitive to certain flavors, such as B. Bitter. dr Reed found that it didn't taste good, which explains why she perceived bitterness differently than other people she knew. Are you very sensitive to taste? Can you see similarities between your family members?

smell (smell)

Such as taste, smell orOdorIt also responds to chemical stimuli. Olfactory receptor neurons are located in a small region within the upper nasal cavity (Figure 3. The olfactory system). This region is known asolfactory epitheliumand contains bipolar sensory neurons. Everyoneolfactory sensory neuronIt has dendrites extending from the apical surface of the epithelium to the mucus lining the cavity. When airborne molecules are inhaled through the nose, they pass through the olfactory epithelial region and are dissolved in mucus. Areodor moleculesThey bind to proteins that keep them suspended in mucus and help transport them to the olfactory dendrites. The odorant-protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G-protein coupled and produce a graded membrane potential in olfactory neurons.

The axon of an olfactory neuron extends from the basal surface of the epithelium through an olfactory foramen in the ethmoid plate and into the brain. The group of axons called the olfactory tract connects to theolfactory bulbon the ventral surface of the frontal lobe. From there, the axons divide to travel to different regions of the brain. Some migrate to the brain, particularly the primary olfactory cortex, located in the inferior and medial portions of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells are associated with long-term memory and emotional responses. Certain smells trigger emotional memories, such as the smell of food associated with the place of birth. Smell is the only sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This close connection between the olfactory system and the cerebral cortex is one of the reasons why smells can be a powerful trigger for memories and emotions.

The nasal epithelium, including the olfactory cells, can be affected by airborne toxic chemicals. Olfactory neurons are therefore regularly replaced within the nasal epithelium, whereupon the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons already in the cranial nerve.

the olfactory system

audition (listen)

audience, thatAudition, is the conversion of sound waves into a neural signal, which is possible thanks to the structures of the ear (Figure 4. Structures of the ear). The large, fleshy structure on the side of the head is known as theAtrium. Some sources also refer to this structure as the pinna, although that term is more appropriate for a structure that can move, like a cat's outer ear. The C-shaped curves of the pinna direct sound waves into the ear canal. The canal enters the skull through the external auditory canal of the temporal bone. At the end of the ear canal is theeardrum, or eardrum vibrating after being hit by sound waves. The pinna, ear canal, and eardrum are often referred to asouter ear. Thatmiddle earIt consists of a space traversed by three small bones, the theoscula. The three auditory ossicles are theMaleo,Anvil, jstirrup, these are Latin names that can roughly be translated as hammer, anvil and stirrup. The hammer is attached to the eardrum and articulates with the anvil. The anvil, in turn, articulates with the stirrup. Then the stirrup is attached to itinner ear, where the sound waves are converted into a neural signal. The middle ear is connected to the pharynx by the Eustachian tube, which helps equalize air pressure across the eardrum. The tube is normally closed, but it opens when the muscles of the pharynx contract during swallowing or yawning.

Ear structures

The inner ear is often described as a bony labyrinth as it consists of a series of canals embedded in the temporal bone. It has two separate regions thatSnailand thelobbywho are responsible for hearing and balance. Neuronal signals from these two regions are transmitted to the brainstem via separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brainstem as the vestibulocochlear nerve. Sound is translated into neural signals within the cochlear region of the inner ear, which contains the sensory neuronsspiral ganglia. These nodes are located inside the spiral-shaped cochlea of ​​the inner ear. The cochlea is attached by the stapesoval window.

The oval window is at the beginning of a fluid-filled tube inside the cochlea called the cochleavestibular scale. The vestibular ramp extends from the oval window and moves across theCochlea-Kanal, which is the central cavity of the cochlea that contains the sound-transmitting neurons. At the top of the cochlea, the vestibular scale curves over the top of the cochlear duct. The fluid-filled tube, now calledtympanic scale, returns to the base of the cochlea, this time traveling under the cochlear canal. The scala tympanic ends onround windoww, covered by a membrane containing the fluid within the scale. As vibrations travel from the ossicles through the oval window, the fluid in the vestibular scales and tympanic scales move in an undulating motion. The frequency of the liquid waves matches the frequencies of the sound waves (Figure 5. Transmission of sound waves to the cochlea). The membrane covering the round window bulges or puckers with the movement of fluid within the scala tympani.

Transmission of sound waves to the cochlea

A cross-sectional view of the cochlea shows that the scala vestibularis and scala tympani run on either side of the cochlear canal (Figure 6. Cross-section of the cochlea). The cochlear duct contains severalorgans of the court, which translate the wave motion of the two scales into neuronal signals. The organs of Corti are on top of thatBasilarmembran, which is the side of the cochlear duct that sits between the organs of Corti and the scala tympani. As the waves of fluid move through the scala vestibulare and scala tympanica, the basilar membrane moves at a certain point depending on the frequency of the waves. The higher frequency waves move the area of ​​the basilar membrane that is near the base of the cochlea. The low-frequency waves move the area of ​​the basilar membrane near the top of the cochlea.

Cross section of the cochlea

Containing the organs of CortiHairzels, which take their name from the shape of their hairStereozilienExtension from the apical surfaces of the cell (Figure 7. Hair cell). Stereocilia are a series of microvilli-like structures arranged from largest to smallest. Protein fibers bind adjacent hairs in each bundle, causing the bundle to bend in response to movements of the basilar membrane. Stereocilia extend from the hair cells to those above.tectorial membrane, which is attached medially to the organ of Corti. When the scale pressure waves move the basilar membrane, the tectorial membrane slides over the stereocilia. This bends the stereocilia towards or away from the highest member of each set. When the stereocilia bend toward the highest member of their bundle, tension in the protein bands opens ion channels in the hair cell membrane. This depolarizes the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend towards the shorter limb of their bundle, the tension on the bonds is released and the ion channels close. When there is no sound and the stereocilia are erect, there is still some tension in the ligaments, leaving the hair cell membrane potential slightly depolarized.

Haarzelle

Cochlea und Corti-Organ

Check out the University of Michigan WebScopehttp://virtualslides.med.umich.edu/Histology/Central%20Nervous%20System/080a_HISTO_40X.svs/view.apmlto examine the tissue sample more closely. The basilar membrane is the thin membrane that stretches from the central nucleus of the cochlea to the rim. What is anchored to this membrane so that it can be activated by the movement of fluids in the cochlea?

As mentioned above, a specific area of ​​the basilar membrane only moves when the incoming sound is of a specific frequency. Because the tectorial membrane moves only where the basilar membrane moves, the hair cells in that region also only respond to sounds of that particular frequency. So when the frequency of a sound changes, different hair cells along the basilar membrane are activated. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which corresponds to the sound range that human ears can perceive. The Hertz unit measures the frequency of sound waves in terms of cycles generated per second. Hair cells at the top or tip of the cochlea detect frequencies down to 20 Hz. Frequencies in the higher 20 kHz ranges are encoded by hair cells at the base of the cochlea near the round and oval windows (Figure 9. Frequency encoding in the cochlea). Most auditory stimuli contain a mixture of tones at a variety of frequencies and intensities (represented by the amplitude of the sound wave). Hair cells along the cochlear canal, sensitive to a specific frequency, allow the cochlea to separate auditory stimuli by frequency, much like a prism separates visible light into its color components.

Frequency coding in the cochlea

look at thisVideoto learn more about how structures in the ear convert sound waves into a neural signal by moving the "hairs" or stereocilia in the cochlear canal. Specific locations along the line encode specific frequencies or tones. The brain interprets the meaning of the sounds we hear as music, speech, noise, etc. Which ear structures are responsible for amplifying and transmitting sound from the outer ear to the inner ear?

look at thisAnimationto learn more about the inner ear and see how the cochlea rolls up, with the base at the back of the image and the tip at the front. Certain wavelengths of sound cause certain areas of the basilar membrane to vibrate, much like piano keys produce sounds of different frequencies. From the animation, where do the frequencies, from high to low tones, cause activity in the hair cells within the cochlear duct?

Balance

In addition to hearing, the inner ear is responsible for encoding informationbalancethe sense of balance. A similar mechanoreceptor, a hair cell with stereocilia, detects head position, head movement, and whether our body is moving. These cells are located in the vestibule of the inner ear. The position of the head is recorded by theGlasjbags, while the movement of the head is captured by thesemicircular canals. The neural signals generated in theVestibular ganglionthey are transmitted to the brainstem and cerebellum via the vestibulocochlear nerve.

The utricle and saccule consist largely ofmanchaTissue (plural = stains). The macula consists of hair cells surrounded by supporting cells. The hair cell stereocilia are spread out in a viscous gel calledotolithische Membran(Figure 10. Linear acceleration encoded by maculae). Above the otolithic membrane is a layer of calcium carbonate crystals called otoliths. Otoliths essentially add weight to the top of the otolith membrane. The otolithic membrane moves separately from the macula in response to head movements. Head tilt causes the otolith membrane to slide in the direction of gravity over the macula. The moving otolithic membrane, in turn, bends the stereocilia, causing some hair cells to depolarize while others hyperpolarize. The brain interprets the exact position of the head based on the depolarization pattern of the hair cells.

Macule linear acceleration encoding

The semicircular canals are three annular extensions of the vestibule. One is oriented in the horizontal plane while the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees to the sagittal plane (Figure 11. Rotational encoding by semi-circular canals). The base of each semicircular canal, where it joins the vestibule, connects with an enlarged region known as theBottle. The ampoule contains the hair cells that respond to rotational motion, such as B. Turning your head while saying "no." The stereocilia of these hair cells extend towards themake me, a membrane attached to the top of the bladder. As the head rotates in a plane parallel to the semicircular canal, the fluid is decelerated and deflects the cupula in the opposite direction of head movement. The semicircular canals contain multiple ampullae, some oriented horizontally and others vertically. By comparing the relative movements of the horizontal and vertical ampoules, the vestibular system can discern the direction of most head movements in three-dimensional (3-D) space.

Rotary coding by semi-circular channels

Somatosensory (touch)

Somatosensory is considered a general sense as opposed to the specific senses discussed in this section. Somatosensory is the group of sensory modalities associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickling, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not assigned to a specialized organ, but are distributed throughout the body in a variety of organs. Many of the somatosensory receptors are found in the skin, but also in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.

Two types of somatosensory signals transmitted by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperature deviates from body temperature. Some thermoreceptors only react to cold, others only to heat. Nociception is the perception of potentially noxious stimuli. Mechanical, chemical, or thermal stimuli beyond a specified threshold cause painful sensations. Stressed or damaged tissue releases chemicals that activate receptor proteins in nociceptors. For example, the feeling of heat associated with spicy foods is linked to capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37 °C. The dynamics of capsaicin binding to this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, the ability of other stimuli to elicit pain sensations through the activated nociceptor will decrease. Because of this, capsaicin can be used as a topical pain reliever, for example in products like Icy Hot™.

When you drag your finger across a textured surface, the skin on your finger vibrates. Such low-frequency vibrations are picked up by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are found in the basal layer of the epidermis. Deep pressure and vibration are transmitted through laminated (pacini) bodies, which are receptors with encapsulated ends located deep in the dermis or subcutaneous tissue. Light touches are transformed by encapsulated ends known as tactile bodies (Meissner). The follicles are also wrapped in a network of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hairs on the skin's surface, such as when an insect walks on the skin. Skin stretch is transmitted through stretch receptors known as tubercle bodies. Tubers are also known as Ruffini bodies or type II cutaneous mechanoreceptors.

Other somatosensory receptors are located in joints and muscles. Stretch receptors control the stretching of tendons, muscles, and joint components. For example, have you ever stretched your muscles before or after a workout and found that you can only stretch so far before your muscles return to a less stretched state? This spasm is a reflex triggered by stretch receptors to prevent muscle tears. These stretch receptors can also prevent a muscle from over-contracting. In skeletal muscle tissue, these stretch receptors are called muscle spindles. The Golgi tendon organs similarly convert the degrees of stretching of the tendons. Tubers are also present in joint capsules, where they measure strain in components of the skeletal system within the joint. The types of nerve endings, their location and the stimuli they transmit are presented in Table (Somatosensory mechanoreceptors).

*No corresponding eponymous name.

view

viewIt is the special sense of sight that is based on the transmission of light stimuli received through the eyes. The eyes are in one of the orbits of the skull. The bony orbits surround and protect the eyeballs and anchor the soft tissues of the eye (Figure 12. The eye in the orbit). The eyelids, with lashes on the leading edges, protect the eye from abrasions by blocking particles that may fall on the surface of the eye. The inner surface of each cap is a thin membrane known aspalpebral conjunctiva. The conjunctiva stretches across the white areas of the eye (the sclera) and connects the eyelids to the eyeball. Tears are produced bytear gland, located under the lateral edges of the nose. Tears produced by this gland flow through thetear ductto the medial corner of the eye, where tears flow across the conjunctiva, removing foreign particles.

The eye in orbit

The movement of the eye within the orbit is accomplished by contraction of sixextraocular muscleswhich originate from the bones of the orbit and are inserted into the surface of the eyeball (Figure 13. Extraocular muscles). Four of the muscles are located at cardinal points around the eye and are named for those locations. You are theTop right,recto medial,bottom right, jstraight forward. As each of these muscles contracts, the eye moves to the contracting muscle. For example, when the upper rectus contracts, the eye rotates to look up. Thatoblique at the topIt originates in the posterior socket of the eye, near the origin of the four rectus muscles. However, the tendon of the oblique muscles runs through a pulley-like piece of cartilage called the tendon cartilageTrochlea. The tendon attaches obliquely to the upper surface of the eye. The angle of the tendon over the trochlea means that contraction of the superior oblique muscle rotates the eye medially. Thatgo diagonally downThe muscle originates at the floor of the orbit and inserts on the inferolateral surface of the eye. When it contracts, it rotates the eye sideways, as opposed to the upper oblique. The rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned in the sagittal plane. Also, when the eye is looking up or down, the eye must rotate slightly to compensate for the pull of the upper rectus at an angle of about 20 degrees, rather than up. The same applies to the lower rectus, which is compensated by the contraction of the lower oblique muscle. A seventh muscle in the orbit is theOberlidstraffer, which is responsible for raising and retracting the upper eyelid, a movement that generally occurs in conjunction with raising the eye through the upper rectus (see Figure 12. The eye in the orbit).

The eye muscles are innervated by three cranial nerves. The lateral rectus, which causes the eye to be abducted, is innervated by the oculomotor nerve. The superior oblique muscle is innervated by the trochlear nerve. All other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves are connected to the brainstem, which coordinates eye movements.

extraocular muscles

The eye itself is a hollow sphere made up of three layers of tissue. The outermost layer is thephased Tunika, which contains whitescleroticAnd I cleancornea. The sclera makes up five-sixths of the eye's surface, most of which is not visible, although humans are unique compared to many other species in having so much of the "white of the eye" visible (Figure 14. Structure of the eye) . The transparent cornea covers the front of the eye and allows light to enter the eye. The middle layer of the eye is theTunic vascular, which consists mainly of the choroid, ciliary body, and iris. ThatchoroidIt is a layer of highly vascularized connective tissue that supplies blood to the eyeball. The choroid lies behind the ciliary body, a muscular structure attached to the choroidFederby suspension straps thatzonular fibers. These two structures bend the lens and allow it to focus light on the back of the eye. Superimposed on the ciliary body and visible in the anterior eye is the colored portion of the eye. The iris is a smooth muscle that opens or closes the eyepupils, which is the hole in the center of the eye that allows light to enter. The iris contracts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is theneural Tunica, ÖRetina, which contains the nerve tissue responsible for light reception.

The eye is also divided into two cavities: the anterior chamber and the posterior chamber. The anterior cavity is the space between the cornea and the lens, including the iris and ciliary body. It is filled with a watery liquid calledaqueous humor. The posterior cavity is the space behind the lens that extends to the back of the inner eyeball where the retina is located. The posterior cavity is filled with a more viscous liquid calledvitreous.

The retina is made up of several layers and contains specialized cells for the initial processing of visual stimuli. Photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential changes the amount of neurotransmitters that photoreceptor cells releaseBipolar Cellsinsideouter synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to aretinal ganglion cells (RGC)insideinner synaptic layer. Over there,amacrine cellsIn addition, they contribute to retinal processing before the RGC generates an action potential. RGC axons, found in the innermost layer of the retina, gather in theoptical discand let the eye than thatoptic nerve(See Figure 14. Structure of the eye). Because these axons pass through the retina, there are no photoreceptors at the back of the eye, where the optic nerve begins. This creates a “blind spot” on the retina and a corresponding blind spot in our field of vision.

structure of the eye

Note that the photoreceptors in the retina (rods and cones) are located behind the retinal axons, RGCs, bipolar cells, and blood vessels. These structures absorb a significant amount of light before the light reaches the photoreceptor cells. However, right in the center of the retina is a small area known as theFovea. In the fovea, the retina lacks supporting cells and blood vessels and contains only photoreceptors. That's why,visual acuity, or acuity of vision, is greatest in the fovea. This is because other structures in the retina in the fovea absorb the least amount of incoming light (see Figure 14. Structure of the eye). Moving in any direction from this central point on the retina decreases visual acuity significantly. In addition, each photoreceptor cell in the fovea is connected to a single RGC. Therefore, this RGC does not need to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Towards the edges of the retina, different photoreceptors on the RGCs (via bipolar cells) converge in a 50 to 1 ratio. The difference in visual acuity between the fovea and the peripheral retina is easily seen by looking directly at a word in the middle looks . this paragraph. The visual stimulus in the center of the visual field falls on the fovea and is most sharply focused. Notice, without taking your eyes off that word, that the words at the beginning or end of the paragraph are not in focus. Images in your peripheral vision are focused by the peripheral retina and have fuzzy, blurry edges and words that aren't as clearly identifiable. As a result, much of the neural function of the eyes involves eye and head movement, so that important visual stimuli are focused on the fovea.

Light falling on the retina causes chemical changes in the pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, theinner segmentand theouter segment(Figure 15. Photoreceptor). The inner segment contains a cell's nucleus and other common organelles, while the outer segment is a specialized region where photoreception occurs. There are two types of photoreceptors, rods and cones, which differ in the shape of their outer segment. The rod-shaped outer segments of therod photoreceptorcontain a stack of membrane-bound discs that contain the light-sensitive pigmentRhodopsin. The cone-shaped outer segments of the photoreceptor cone contain its light-sensitive pigments in folds of the cell membrane. There are three cone-shaped photopigments called opsins, each sensitive to a specific wavelength of light. The wavelength of visible light determines its color. The pigments in the human eye are specialized to perceive three different primary colors: red, green and blue.

photoreceptor

At the molecular level, visual stimuli cause changes in the photopigment molecule that result in changes in the membrane potential of the photoreceptor cell. A single light unit is calledPhoton, which is described in physics as an energy package with particle and wave properties. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a specific color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall in the infrared range, while wavelengths shorter than 380 nm fall in the ultraviolet range. Light with a wavelength of 380 nm is blue, while light with a wavelength of 720 nm is dark red. All other colors are at different points along the wavelength scale between red and blue.

Opsin pigments are actually transmembrane proteins that contain a cofactor known asretina. Retinal is a carbohydrate molecule related to vitamin A. When a photon strikes retinal, the molecule's long hydrocarbon chain is biochemically altered. In particular, the photons cause some of the double-bonded carbons in the chain to switch from one direction to the other.cisstilltransconformation. This process is calledphotoisomerization. Before interacting with a photon, the flexible double bond carbons of the retinal are in thecisconformation. This molecule is known as 11-cis-retina. A photon interacting with the molecule causes the flexible double-bonded carbons to change totrans– conformation, all forming –trans-Retinal, which has a straight hydrocarbon chain (Figure 16. Retinal isomers).

The change in retinal shape in the photoreceptors initiates visual transduction in the retina. Activation of the retinal and opsin proteins leads to activation of a G protein that changes the membrane potential of the photoreceptor cell, which then releases fewer neurotransmitters into the outer synaptic layer of the retina. Until the retinal molecule returns to the 11-cis-retinal form, the opsin cannot respond to light energy, which is called opacification. When a large group of photopigments is bleached, the retina sends information as if opposing visual information is being perceived. Afterimages usually appear negative after a bright flash of light. Photoisomerization is reversed through a series of enzymatic changes, causing the retina to respond to more light energy.

retinal isomers

Opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment of rods, is most sensitive to light at a wavelength of 498 nm. Tricolor opsins have maximum sensitivities of 564 nm, 534 nm, and 420 nm, which roughly correspond to the primary colors red, green, and blue (Figure 17. Comparison of color sensitivities of photopigments). Rhodopsin absorption in rods is much more sensitive than in cone opsins; In particular, the rods are sensitive to vision in low light and the cones are sensitive to brighter conditions. In normal sunlight, the rhodopsin will continue to bleach as long as the cones are active. In a dark room, there isn't enough light to activate the opsins in the cones, and vision is entirely dependent on the rods. Rods are so sensitive to light that a single photon can generate an action potential of the corresponding RGC of a rod.

Sensitive to different wavelengths of light, all three types of cone opsins allow us to see color. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light with a wavelength of about 450 nm would activate the "red" cones minimally, the "green" cones slightly, and the "blue" cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, the cones cannot respond to dim light, and the rods do not perceive the color of light. Therefore, in low light, our vision is essentially greyscale. In other words, in a dark room, everything appears a shade of gray. If you think you can see colors in the dark, your brain probably knows what color something is and builds on that memory.

Comparison of the color sensitivity of photopigments

look at thisVideofor more information on a cross-section of the brain showing the visual pathway from the eye to the occipital cortex. The first half of the way is the projection from the RGCs via the optic nerve to the lateral geniculate nuclei in the thalamus on both sides. This first fiber in the signaling pathway is synapsed to a thalamic cell, which is then projected to the visual cortex in the occipital lobe where "seeing" or visual perception takes place. This video provides an abridged overview of the visual system by focusing on the pathway from the eyes to the occipital lobe. The video states (at 0:45) that "specialized cells in the retina called ganglion cells convert light rays into electrical signals". What aspect of retinal processing does this statement simplify? explain your answer

sensory nerves

As soon as a sensory cell converts a stimulus into a nerve impulse, this impulse has to travel along the axons to reach the CNS. In many of the specialized senses, the axons exiting sensory receptors have atopographicalArrangement, meaning that the location of the sensory receptor is related to the location of the axon in the nerve. For example, in the retina, the axons of the foveal RGCs are located in the center of the optic nerve, where they are surrounded by axons of the more peripheral RGCs.

Spinal nerves

Spinal nerves generally contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons that travel to muscles or other effector organs. As the spinal nerve approaches the spinal cord, it divides into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, while the ventral roots contain only the axons of motor neurons. Some of the branches synapse with local neurons in the spinal ganglion, dorsal horn (dorsal) or even anterior horn (ventral) at the level of the spinal cord where they enter. Other branches travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch can also become the posterior (dorsal) column of white matter to connect to the brain. For convenience, we use the terms ventral and dorsal to refer to the structures within the spinal cord that are part of these pathways. This will help emphasize the relationships between the various components. The spinal nervous systems connected to the brain are normalcontralateralwhere the right side of the body is connected to the left side of the brain and the left side of the body is connected to the right side of the brain.

Brain nerves

The cranial nerves carry certain sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body is connected to the right side of the brain. While spinal information is contralateral, the cranial nervous systems mostly areipsilateral, meaning a cranial nerve on the right side of the head connects to the right hemisphere of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not connected to the somatic nervous system). The general senses of facial somatosensation travel through the trigeminal system.

chapter overview

The senses are smell (smell), taste (taste), somatosensation (sensations related to the skin and body), hearing (hearing), equilibrium (balance), and sight. With the exception of the somatosensory system, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or the eye. Somatosensory belongs to the general senses, these are those sensory structures that are distributed throughout the body and in the walls of various organs. The specialized senses are all primarily part of the somatic nervous system as they are consciously perceived through brain processes, although some specialized senses contribute to autonomic function. The general senses can be divided into somatosensory, which is generally considered to be touch but includes perception of touch, pressure, vibration, temperature and pain. The general senses also include the visceral senses, which are separated from the functioning of the somatic nervous system in that they do not normally reach the level of conscious perception.

Cells that convert sensory stimuli into electrochemical signals in the nervous system are classified based on structural or functional aspects of the cells. Structural classifications are based on the anatomy of the cell interacting with the stimulus (free nerve endings, encapsulated endings, or specialized receptor cells) or on the cell's position in relation to the stimulus (interoceptor, exteroceptor, proprioceptor). Third, functional classification is based on how the cell translates the stimulus into a neural signal. Chemoreceptors respond to chemical stimuli and are the basis for smell and taste. The chemoreceptors are related to the osmoreceptors and nociceptors for fluid balance and pain reception. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensory function, as well as the basis for hearing and balance in the inner ear. Thermoreceptors are sensitive to temperature changes and photoreceptors are sensitive to light energy.

The nerves that transmit sensory information from the periphery to the CNS are the spinal nerves, which connect to the spinal cord, or the cranial nerves, which connect to the brain. Spinal nerves have mixed fiber populations; some are motor fibers and others are sensory. Sensory fibers connect to the spinal cord via the dorsal root, which is connected to the spinal ganglion. Sensory information from the body, transmitted through the spinal nerves, is projected to the opposite side of the brain for processing by the cerebral cortex. Cranial nerves can be purely sensory fibers, such as the olfactory, optic, and vestibulocochlear nerves, or mixed sensory and motor nerves, such as the trigeminal, facial, glossopharyngeal, and vagus nerves. The cranial nerves connect to the same side of the brain that sensory information originates from.

sensory pathways

Specific regions of the CNS coordinate various somatic processes using sensory input and motor output from peripheral nerves. A simple case is a reflex caused by a synapse between the axon of a dorsal sensory neuron and a motor neuron in the anterior horn. More complex arrangements are possible to integrate peripheral sensory information with higher level processes. The important regions of the CNS involved in somatic processes can be divided into the spinal cord, brainstem, diencephalon, cerebral cortex and subcortical structures.

spinal cord and brainstem

A sensory pathway that carries peripheral sensations to the brain is referred to asway up, or ascending orbits. Each of the different sensory modalities follows specific pathways through the CNS. Tactile and somatosensory stimuli activate receptors in skin, muscles, tendons and joints throughout the body. However, the somatosensory pathways are divided into two separate systems based on the location of the receiving neurons. Somatosensory stimuli from below the neck travel through the sensory pathways of the spinal cord, while somatosensory stimuli from the head and neck travel through the cranial nerves, particularly the trigeminal system.

Thatsystem of the dorsal column(sometimes referred to as the dorsal-column-medial lemniscus) and theSpinothalamic tractThere are two main pathways that carry sensory information to the brain (Figure 1. Ascending sensory pathways from the spinal cord). The sensory pathways in each of these systems are made up of three consecutive neurons.

The dorsal column system begins with the axon of a spinal ganglion neuron that enters the dorsal root and connects to the white matter of the dorsal column in the spinal cord. When the axons enter the dorsal column from this pathway, they become positionally located such that axons from lower body levels are located medially, while axons from upper body levels are located laterally. The dorsal cleft separates into two partial tracts, thefasciculus graciliscontains axons from the legs and lower body, and thewedged with a bundlecontains axons from the torso and arms.

Dorsal column axons terminate in spinal nuclei, where each synapse connects to the second neuron in its respective pathway. ThatKern gracilisis the target of the fibers in the gracilis fasciculus, while thetrapped at the coreit is the target of the fibers in the cuneatus fasciculus. The second neuron in the system projects from one of the two nuclei and thensorry, or crosses the center line of the cord. These axons then continue up the brainstem in a bundle called theLemnisko media. These axons terminate in the thalamus, where each synapse connects to the third neuron in its respective pathway. The third neuron in the system projects its axons to the postcentral gyrus of the cerebral cortex where initially somatosensory stimuli are processed and conscious perception of the stimulus occurs.

The spinothalamic tract also begins with neurons in a spinal ganglion. These neurons extend their axons to the dorsal horn, where they synapse with the second neuron in their respective pathway. The name "spinothalamic" comes from this second neuron, which has its cell body in the gray matter of the spinal cord and is connected to the thalamus. The axons of these second neurons then cross in the spinal cord and ascend to the brain and enter the thalamus where each synapses with the third neuron in its respective pathway. Neurons from the thalamus then project their axons to the spinothalamic tract, which synapses in the postcentral gyrus of the cerebral cortex.

These two systems are similar in that they both start with spinal ganglion cells, as does most of the general sensory information. The dorsal column system is primarily responsible for tactile sensation and proprioception, while the spinothalamic tract is primarily responsible for pain and temperature sensations. Another similarity is that the second neurons in both pathways are contralateral in that they project across the midline to the other side of the brain or spinal cord. In the dorsal column system, this discussion takes place in the brainstem; in the spinothalamic pathway, it takes place in the spinal cord at the same level of the spinal cord where the information was entered. The third neurons in the two pathways are essentially the same. In both, the second neuron synapses in the thalamus, and the thalamic neuron projects to the somatosensory cortex.

Ascending sensory pathways of the spinal cord

The trigeminal pathway carries somatosensory information from the face, head, mouth, and nasal cavity. As with the neural pathways discussed above, each of the sensory pathways of the trigeminal tract involves three consecutive neurons. First, axons from the trigeminal ganglion enter the brainstem at the level of the pons. These axons project to one of three locations. Thatspinal trigeminal nucleusFrom the medulla it receives similar information as the spinothalamic tract, such as pain and temperature sensations. Other axons go to themost important sensory coreon the bulge or themidbrain nucleiin the midbrain. These nuclei receive information like that carried by the spinal cord system, such as touch, pressure, vibration, and proprioception. The axons of the second neuron cross and ascend to the thalamus along the trigeminothalamic tract. In the thalamus, each axon synapses with the third neuron in its respective pathway. The axons of the third neuron then project from the thalamus to the brain's primary somatosensory cortex.

The sensory pathway for taste runs along the cranial facial and glossopharyngeal nerves, which synapse with neurons in the brain.lonely corein the brainstem. Axons from the nucleus solitaire then project to theventral posterior nucleusthalamus Finally, axons project from the ventral posterior nucleus to the gustatory cortex of the cerebral cortex, where taste is consciously processed and perceived.

The sensory pathway to hearing runs along the vestibulocochlear nerve, which synapses with neurons in the cochlear nuclei of the superior medulla oblongata. Within the brainstem, inputs from both ears are combined to extract location information from auditory stimuli. While the initial auditory stimuli received at the cochlea accurately represent the frequency (or pitch) of the stimuli, the locations of the tones can be determined by comparing the information arriving at both ears.

Sound localization is a feature of central processing in the auditory nuclei of the brainstem. Sound localization is achieved when the brain calculates itinteraural time differenceand theinteraural intensity difference. A sound emanating from a specific location will reach each ear at different times unless the sound is directly in front of the listener. If the sound source is slightly to the left of the listener, the sound will reach the left ear microseconds before the right ear (Figure 2. Auditory brainstem mechanisms of sound localization). This time difference is an example of an interaural time difference. Also, the sound in the left ear is slightly louder than the right ear because the head is blocking some of the sound waves from reaching the opposite ear. This is an example of an interaural intensity difference.

Auditory brainstem mechanisms of sound localization

Auditory processing continues to a nucleus in the midbrain called thelower colliculus. The axons of the inferior colliculus project at two sites, the thalamus and the thalamusOberer Colliculus. ThatNucleus geniculado medialThe thalamus receives auditory information and then projects it to the auditory cortex in the temporal lobe of the cerebral cortex. The superior colliculus receives input from the visual and somatosensory systems, as well as the ears, to initiate stimulation of the muscles that rotate the head and neck in the direction of the auditory stimulus.

Balance is coordinated by the vestibular system, whose nerves consist of vestibular ganglion axons that carry information from the utricle, sacculus, and semicircular canals. The system helps control head and neck movements in response to vestibular signals. An important function of the vestibular system is to coordinate eye and head movements to maintain visual attention. Most axons end in thevestibular nucleifrom the medulla Some axons project from the vestibular ganglion directly to the cerebellum without intervening synapses in the vestibular nuclei. The cerebellum is primarily responsible for initiating movements based on balance information.

Neurons in the vestibular nuclei project their axons to targets in the brainstem. One of the targets is the reticular formation, which affects respiratory and cardiovascular functions related to body movements. A second target for the axons of neurons in the vestibular nuclei is the spinal cord, which triggers spinal reflexes related to posture and balance. To support the visual system, fibers project from the vestibular nuclei to the oculomotor, trochlear, and oculomotor nuclei to affect signals sent along the cranial nerves. These connections form the path ofvestibulo-ocular reflex (VOR), which compensates for head and body movements by stabilizing images on the retina (Figure 3. Vestibulo-ocular reflex). Finally, the vestibular nuclei project to the thalamus to join the proprioceptive pathway of the thoracic column system, allowing for a conscious awareness of balance.

Vestibulo-ocularer Reflex

Optic nerve connections are more complicated than those of other cranial nerves. Instead of having the connections between each eye and the brain, visual information is separated between the left and right sides of the visual field. Also, some information is projected from one side of the visual field to the opposite side of the brain. In each eye, axons projecting from the medial side of the retina cross at theoptic chiasm. For example, axons from the medial retina of the left eye cross to the right side of the brain at the optic disc. However, in each eye, the axons that protrude from the lateral side of the retina do not cross. For example, axons project from the lateral retina of the right eye to the right side of the brain. Thus, the left visual field of each eye is processed in the right brain hemisphere, while the right visual field of each eye is processed in the left brain hemisphere (Figure 4. Separation of visual field information).

Separation of information from the visual field at the optic chiasm

A unique clinical presentation associated with this anatomical arrangement is the loss of lateral peripheral vision known as bilateral hemianopsia. This differs from "tunnel vision" in that the upper and lower peripheral fields are not lost. Visual field deficits can be bothersome to a patient, but in this case the cause is not in the visual system itself. Pituitary growth presses against the optic chiasm and interferes with signal transmission. However, axons that project to the same side of the brain are unaffected. As a result, the patient loses the outermost areas of his field of vision and cannot see objects on the right and left.

Starting from the optic chiasma, the axons of the visual system are calledoptical tractinstead of the optic nerve. The visual tract has three main goals, two in the diencephalon and one in the midbrain. The connection between the eyes and the diencephalon is demonstrated during development, when the neural tissue of the retina differs from that of the diencephalon by the growth of secondary sacs. The retinal connections to the CNS are a remnant of this developmental association. Most of the connections of the visual tract are with the thalamus, especially with theNucleus geniculatum lateralis. Axons from this nucleus then project into the brain's visual cortex, located in the occipital lobe. Another target of the optic tract is the superior colliculus.

Also, a very small number of RGC axons project from the optic nerve chiasm to the optic nerve chiasm.suprachiasmatischen Nucleusof the hypothalamus. These RGCs are light sensitive as they respond to the presence or absence of light. However, unlike photoreceptors, these light-sensitive RGCs cannot be used to perceive images. By simply responding to the absence or presence of light, these RGCs can send information about the length of the day. The perceived relationship between sunlight and darkness determines thecircadian rhythmof our body, causing certain physiological events to occur at approximately the same time each day.

midbrain

The diencephalon lies below the brain and includes the thalamus and hypothalamus. In the somatic nervous system, the thalamus is an important relay for communication between the brain and the rest of the nervous system. The hypothalamus has somatic and autonomic functions. In addition, the hypothalamus communicates with the limbic system, which controls emotions and memory functions.

Sensory input to the thalamus comes from most specialized senses and ascending somatosensory pathways. Each sensory system is transmitted through a specific nucleus in the thalamus. The thalamus is a necessary transmission point for most sensory pathways reaching the cerebral cortex where conscious sensory perception begins. The only exception to this rule is the olfactory system. The axons of the olfactory tract from the olfactory bulb, along with the limbic system and hypothalamus, project directly into the cerebral cortex.

The thalamus is a collection of different nuclei that can be divided into three anatomical groups. The white matter that runs through the thalamus defines the three main regions of the thalamus, namely an anterior nucleus, a medial nucleus, and a lateral group of nuclei. The anterior core serves as a link between the hypothalamus and the limbic system, which produces emotions and memories. The medial nuclei serve as relays of information from the limbic system and the basal ganglia to the cerebral cortex. This enables memory formation during learning, but also determines attention. The special and somatic senses connect to the lateral nuclei, where their information is relayed to the appropriate sensory cortex of the brain.

cortical processing

As described above, many of the sensory axons are located in the same way as their corresponding receptor cells in the body. This allows the location of a stimulus to be identified based on which receptor cells are sending information. The cerebral cortex also maintains this sensory topography in certain areas of the cerebral cortex that correspond to the location of receptor cells. The somatosensory cortex provides an example where essentially the locations of somatosensory receptors in the body are mapped onto the somatosensory cortex. This assignment is often represented with asensory homunculus(Figure 5. The sensory homunculus).

The term homunculus comes from the Latin word for "little man" and refers to a map of the human body placed on a portion of the cerebral cortex. In the somatosensory cortex, the external genitalia, feet, and lower legs are shown on the medial side of the gyrus within the longitudinal fissure. As the gyrus exits the fissure and along the surface of the parietal lobe, the body map continues through the thighs, hips, trunk, shoulders, arms, and hands. The head and face are lateral to the fingers as the gyrus approaches the lateral sulcus. The representation of the body in this topographical map is medial to lateral from the bottom to the top of the body. It is a continuation of the topographical arrangement seen in the dorsal column system, where axons from the lower body are carried in the gracile fasciculus, while axons from the upper body are carried in the cuneate fasciculus. As the dorsal column system continues toward the medial lemniscus, these relationships are maintained. In addition, head and neck axons run from the trigeminal nuclei to the thalamus alongside the trunk fibers. Connections through the thalamus preserve topography, so anatomical information is preserved. Note that this correspondence does not result in a perfect miniature version of the body, but instead exaggerates the more sensitive areas of the body, like the fingers and lower face. Less sensitive areas of the body, such as the shoulders and back, are mapped to smaller areas in the cortex.

The sensory homunculus

Likewise, the topographical relationship between the retina and the visual cortex is maintained along the visual pathway. As described above, the visual field projects onto both retinas with division at the optic chiasm. The right peripheral visual field falls on the medial part of the right retina and the lateral part of the left retina. The right medial retina then projects across the midline through the optic chiasm. As a result, the right visual field is processed in the left visual cortex. Likewise, the left visual field is processed in the right visual cortex (see Figure 4. Separation of visual field information in the optic chiasm). Although the chiasm helps sort left and right visual information, upper and lower visual information stays topographically in the visual path. Light from the upper visual field falls on the lower retina and light from the lower visual field falls on the upper retina. This topography is maintained such that the upper visual cortex processes the lower visual field and vice versa. Therefore, information from the visual field is reversed and inverted as it enters the visual cortex: up is down and left is right. However, the cortex processes visual information in such a way that the final conscious perception of the visual field is correct. The topographic relationship is demonstrated by processing information from the foveal region of the retina at the center of the primary visual cortex. Information from the peripheral regions of the retina is processed correspondingly to the edges of the visual cortex. Similar to the exaggerations in the sensory homunculus of the somatosensory cortex, the foveal processing area of ​​the visual cortex is disproportionately larger than the processing areas of peripheral vision.

In an experiment conducted in the 1960s, subjects wore prism glasses so that the field of view was reversed before it reached the eye. On the first day of the experiment, the subjects crouched as they approached a table, thinking it was hanging from the ceiling. However, after a few days of acclimatization, the subjects behaved as if everything had been displayed correctly. Therefore, the visual cortex is somewhat flexible in adapting to the information it receives from our eyes (Figure 6. Topographic map of the retina in the visual cortex).

Topographical mapping of the retina in the visual cortex

The cortex has been described as having specific regions responsible for processing specific information; there is the visual cortex, the somatosensory cortex, the taste cortex, etc. However, our experience of these senses is not divided. Instead, we experience what can be called a perfect perception. Our perceptions of the various sensory modalities, which differ in terms of content, are integrated by the brain so that we experience the world as a continuous whole.

In the cerebral cortex, sensory processing begins in the brain.primary sensory cortex, then continues with adressing area, and finally in amultimodaler Integrationsbereich. For example, the visual pathway projects from the retina through the thalamus to the primary visual cortex in the occipital lobe. This area is mainly located on the medial wall within the longitudinal fissure. Here the visual stimuli begin to be recognized as basic forms. Object edges are recognized and built into more complex shapes. Additionally, the inputs from both eyes are compared to extract depth information. Because of the overlapping field of view between the two eyes, the brain can begin to estimate the distance of stimuli based on thatbinocular depth cues.

look at thisVideoto learn more about how the brain perceives three-dimensional motion. Much like retinal disparity offers 3D moviegoers a way to extract three-dimensional information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. When a visual stimulus moves to the left in one eye and to the right in the other eye, the brain interprets it as moving toward (or away from) the face along the median line. If both eyes see an object moving in the same direction but at different speeds, what would that mean for spatial motion?

DAILY CONNECTIONS
Depth perception, 3D movies and optical illusions

The field of view is projected onto the surface of the retina, where photoreceptors convert light energy into neural signals that the brain can interpret. The retina is a two-dimensional surface, so it does not encode any three-dimensional information. However, we can perceive depth. How is this done?

Two ways we can extract depth information from the two-dimensional retinal signal are based on monocular cues and binocular cues, respectively. Monocular depth cues are those that are the result of information within the two-dimensional field of view. An object that overlaps another object must be in front. Relative size differences are also a sign. For example, if a basketball appears larger than the hoop, then the hoop must be farther away. Based on experience, we can estimate how far away the basket is. Binocular depth cues compare the information presented on the two retinas because they don't see the visual field exactly the same.

The centers of the two eyes are separated by a small distance, around 6-6.5 cm in most people. Because of this shift, visual stimuli do not fall on exactly the same point on both retinas unless we are directly fixated on them, and they fall on the fovea of ​​each retina. All other objects in the field of view, whether closer or farther than the fixed object, fall on different points on the retina. When vision is directed at an object in space, the nearest objects fall on the lateral retina of each eye and the farthest objects fall on the medial retina of each eye (Figure). This is easily observed by raising a finger in front of your face while looking at an object further away. You will see two images of your finger representing the two different images that land on each retina.

These depth cues, both monocular and binocular, can be exploited to trick the brain into believing that two-dimensional information has three dimensions. This is the basis of 3D movies. The image projected onto the screen is two-dimensional but embedded with disparate information. The 3D glasses available in cinemas filter the information so that only one eye sees one version of the screen content and the other eye sees the other version. When you take off your glasses, the image on the screen is blurred to varying degrees because both eyes see both layers of information and the third dimension is not visible. Some optical illusions can also take advantage of depth cues, although more commonly they use monocular cues to trick the brain into seeing different parts of the scene as if they are at different depths.

retinal disparity

There are two main regions surrounding the primary cortex, commonly referred to as the V2 and V3 areas (the primary visual cortex is the V1 area). These surrounding areas are the visual association cortex. Visual association regions develop more complex visual perceptions by adding color and motion information. Information processed in these areas is then sent to the temporal and parietal lobe regions. Visual processing has two separate processing streams: one in the temporal lobe and one in the parietal lobe. These are the ventral and dorsal streams, respectively (Figure 8. Ventral and dorsal visual streams). Thatabdominal streamrecognizes visual stimuli and their meaning. As the ventral stream uses temporal lobe structures, it begins to interact with the non-visual cortex and may be important in making visual stimuli part of memories. Thatback currentlocates objects in space and helps control body movements in response to visual input. The dorsal current enters the parietal lobe where it interacts with somatosensory cortical areas important to our perception of the body and its movements. The dorsal current can then affect activity in the frontal lobe, where motor functions originate.

ventral and dorsal visual streams

The inability to recognize people by their faces is an annoying problem. It can be caused by trauma or it can be congenital. look at thisVideoMore information about a person who lost the ability to recognize faces as a result of an injury. She cannot recognize the faces of her close relatives or herself. What other information can a person suffering from prosopagnosia use to know who they are seeing?

chapter overview

Sensory information travels to the brain via pathways that travel through the spinal cord (for the body's somatosensory information) or the brainstem (for everything else except the visual and olfactory systems) to reach the diencephalon. In the diencephalon, sensory pathways reach the thalamus. This is necessary for all sensory systems to reach the cerebral cortex except for the olfactory system, which is directly connected to the frontal and temporal lobes.

The two main pathways of the spinal cord that arise from sensory neurons in the spinal ganglia are the dorsal column system and the spinothalamic tract. The main differences between the two lie in the type of information that is sent to the brain and where the pathways meet. The dorsal column system carries mainly touch and proprioceptive information and crosses the midline in the medulla oblongata. The spinothalamic tract is primarily responsible for the sensation of pain and temperature and crosses the midline in the spinal cord at the level at which it enters. The trigeminal nerve adds similar sensory information from the head to these pathways.

The auditory pathway runs through several nuclei in the brain stem, where additional information is extracted from the fundamental frequency stimuli processed by the cochlea. The localization of sound is made possible by the activity of these structures in the brainstem. The vestibular system enters the brainstem and affects the activity of the cerebellum, spinal cord, and cerebral cortex.

The visual pathway separates information from the two eyes so that half of the visual field is projected onto the other side of the brain. Within the visual cortical areas, the perception of stimuli and their position is transmitted along two streams, one ventral and one dorsal. The ventral visual current connects to structures in the temporal lobe that are important in the formation of long-term memory. The dorsal visual streaming interacts with the somatosensory cortex in the parietal lobe and together they can affect the activity of the frontal lobe to produce body movements related to visual information.

The defining characteristic of the somatic nervous system is that it controls skeletal muscle. The somatic senses inform the nervous system of the external environment, but the response is through voluntary muscle movement. The term “honorary work” suggests a conscious decision to take a step. However, some aspects of the somatic system use voluntary muscles without conscious control. An example is our breathing's ability to switch to unconscious control while we are focused on another task. However, the muscles responsible for the basic process of breathing are also used for speech, which is entirely voluntary.

cortical responses

Let's start with the sensory stimuli registered by receptor cells and the information transmitted up the ascending pathways to the CNS. In the cerebral cortex, initial sensory processing proceeds via associative processing and then integration into multimodal areas of the cerebral cortex. These levels of processing can lead to the incorporation of sensory perceptions into memory, but more importantly, they lead to a response. The completion of cortical processing through primary, associative, and integrative sensory areas initiates a similar progression of motor processing, usually in different cortical areas.

While sensory cortical areas are located in the occipital, temporal, and parietal lobes, motor functions are largely controlled by the frontal lobe. The foremost regions of the frontal lobe, the prefrontal areas, are important forexecutive functions, these are the cognitive functions that lead to goal-oriented behavior. These higher cognitive processes includeworking memory, which is called a "mental scratch pad" and can help organize and present information not found in the immediate environment. The prefrontal lobe is responsible for aspects of attention such as B. Inhibiting distracting thoughts and actions to allow a person to focus on a goal and direct behavior towards achieving that goal.

The functions of the prefrontal cortex are an integral part of an individual's personality, as it is largely responsible for what a person intends to do and how they achieve those plans. A famous case of damage to the prefrontal cortex is that of Phineas Gage in 1848. He was a railroad worker whose prefrontal cortex was pierced by a metal spike (Figure 1. Phineas Gage). He survived the accident, but according to secondhand reports, his personality changed drastically. Friends described him as nothing like himself. If he was a kind and hardworking man before the accident, after the accident he became an irritable, temperamental and easygoing man. Many of the accounts of his change may have been inflated in the retelling, and some of the behavior is likely attributed to alcohol being used as a pain reliever. However, the reports suggest that some aspects of his personality have changed. Additionally, there is new evidence that, despite his life changing drastically, he was able to become a functioning stagecoach driver, suggesting that the brain has the ability to heal itself from severe trauma like this recover.

phineas gage

Secondary motor cortices

In generating motor responses, the executive functions of the prefrontal cortex must initiate actual movement. One way to define the prefrontal area is any region of the frontal lobe that does not cause movement when electrically stimulated. These are mainly located in the front part of the frontal lobe. The remaining frontal lobe regions are the regions of the cortex that produce motion. The prefrontal areas project onto the secondary motor cortices, which include thepremotor cortexand thesupplementary motor area.

Two important regions that help plan and coordinate movement lie adjacent to the primary motor cortex. The premotor cortex is more lateral while the supplemental motor area is more medial and superior. The premotor area helps control core muscle movements to maintain posture during movement, while the supplemental motor area is said to be responsible for planning and coordinating movement. The supplementary motor area also addresses sequential movements based on previous experience (i.e. learned movements). Neurons in these areas are most active, leading to the initiation of movement. These areas can, for example, prepare the body for the movements required to drive a car before the light changes.

Adjacent to these two regions are two centers specializing in engine design. Thatfrontal eye fieldsThey are responsible for moving the eyes in response to visual stimuli. There are direct connections between the frontal eye fields and the superior colliculus. Also anterior is the premotor cortex and primary motor cortexBroca's place. This area is responsible for controlling the movements of the speech production structures. The area is named after a French surgeon and anatomist who examined patients who could not speak. They had no impairment in speech comprehension, only in the production of speech sounds, suggesting damaged or underdeveloped Broca's area.

primary motor cortex

The primary motor cortex is located in the precentral gyrus of the frontal lobe. One neurosurgeon, Walter Penfield, described much of the basic understanding of the primary motor cortex by electrically stimulating the brain's surface. Penfield examined the surface of the cortex while the patient was under local anesthesia only so responses to the stimulation could be observed. This led to the assumption that the precentral gyrus directly stimulated muscle movement. We now know that the primary motor cortex receives inputs from various areas that help with movement planning, and its main output stimulates neurons in the spinal cord to stimulate skeletal muscle contraction.

The primary motor cortex is arranged similarly to the primary somatosensory cortex, mapping the body topographically and forming a motor homunculus (cf[Shortcut]). Neurons responsible for the musculature of the feet and lower legs lie in the medial wall of the precentral gyrus, with the thighs, trunk, and shoulder lying at the apex of the longitudinal fissure. The hand and face are on the lateral side of the gyrus. In addition, the relative space allocated to different regions is exaggerated in muscles with higher innervation. Most of the cortical space is given to the muscles that perform fine, agile movements, such as the B. the muscles of the fingers and the lower part of the face. The "power muscles" that perform grosser movements, like the glutes and back muscles, take up much less space in the motor cortex.

descending paths

Motor output from the cortex travels to the brainstem and spinal cord to control muscles via motor neurons. Neurons in the primary motor cortex, calledBetz cells, are large cortical neurons that synapse with lower motor neurons in the brainstem or spinal cord. The two descending pathways traversed by the axons of the Betz cell are thecorticobulbar tractand theCorticospinal tract, respectively. Both pathways are named for their origin in the cortex and their destinations, either the brainstem (the term "bulbar" refers to the brainstem as the bulb or extension at the top of the spinal cord) or the spinal cord.

These two descending pathways are responsible for the conscious or voluntary movements of skeletal muscle. Each motor command from the primary motor cortex is sent down Betz cell axons to activate upper motor neurons in the cranial motor nuclei or ventral horn of the spinal cord. The corticobulbar tract axons are ipsilateral, meaning they project from the cortex to the motor nucleus on the same side of the nervous system. In contrast, axons from the corticospinal tract are largely contralateral, meaning they cross the midline of the brainstem or spinal cord and synapse on the opposite side of the body. Thus, the brain's right motor cortex controls the muscles on the left side of the body and vice versa.

The corticospinal tract descends from the cortex through the deep white matter of the brain. It then passes between the caudate nucleus and the putamen basal ganglia as a bundle called theinner capsule. The tract then passes through the midbrain as thecerebral stalks, after which it digs through the bulge. The tracts entering the medulla oblongata form the great white matter tract, termed thepyramids(Figure 2. Corticospinal tract). The defining landmark of the medullary-spinal junction is thePyramid Diskussion, where most fibers from the corticospinal tract cross to the opposite side of the brain. At this point, the tract separates into two parts that control different areas of the musculature.

Corticospinal tract

attachment control

Thatlateral corticospinal tractit consists of the fibers that cross the midline at the pyramidal junction (see Figure 2. Corticospinal tract). Axons cross from the anterior position of the pyramids in the medulla oblongata to the lateral column of the spinal cord. These axons are responsible for controlling the appendix muscles.

This influence on the appendicular muscles means that the lateral corticospinal tract is responsible for the movement of the arm and leg muscles. The ventral horn in both the lower cervical spinal cord and the lumbar spinal cord has wider ventral horns that are responsible for the greater number of muscles controlled by these motor neurons. Thatenlarged cervixit is particularly large because the fine muscles of the upper extremities, especially the fingers, can be better controlled. Thatlumbar enlargementAppearance isn't as important, as fine motor skills are lower in the lower extremities.

steer axially

Thatanterior corticospinal tractIt is responsible for controlling the muscles of the trunk (see Figure 2. Corticospinal tract). These axons do not cross in the medulla. Instead, they remain in an anterior position as they travel down the brainstem and enter the spinal cord. These axons then travel to the level of the spinal cord, where they synapse with a lower motor neuron. Upon reaching the appropriate level, the axons decuss and enter the ventral horn on the opposite side of the spinal cord from where they entered. In the anterior horn, these axons synapse with their corresponding lower motor neurons. Lower motor neurons are located in the medial regions of the anterior horn because they control the axial muscles of the trunk.

Since trunk body movements involve both sides of the body, the anterior corticospinal tract is not fully contralateral. Some collateral branches of the tract project into the ipsilateral ventral horn to control synergistic muscles on that side of the body or to inhibit antagonistic muscles via interneurons within the ventral horn. Due to the influence of both sides of the body, the anterior corticospinal tract can coordinate the postural muscles during large body movements. These coordinating axons in the anterior corticospinal tract are often considered bilateral because they are both ipsilateral and contralateral.

look at thisVideofor more information on the descending motor pathway of the somatic nervous system. Autonomous connections are mentioned, which are discussed in another chapter. In this short video only part of the descending motor pathway of the somatic nervous system is described. Which division of the path is described and which division is omitted?

Extrapyramidal controls

Other descending connections between the brain and spinal cord are mentionedsistema extrapiramidal. The name comes from the fact that this system lies outside of the corticospinal pathway, which includes the pyramids in the spinal cord. Some pathways that originate in the brainstem contribute to this system.

ThatTectospinal tractIt projects from the midbrain to the spinal cord and is important for postural movements driven by the superior colliculus. The tract's name comes from an alternative name for the superior colliculus, which is the tectum. ThatReticulospinal tractconnects the reticular system, a diffuse region of gray matter in the brainstem, to the spinal cord. This tract influences the muscles of the trunk and proximal limbs related to posture and locomotion. The reticulospinal tract also contributes to muscle tone and affects autonomic functions. ThatVestibulospinaltraktconnects the brainstem nuclei of the vestibular system to the spinal cord. This allows posture, movement and balance to be modulated based on balance information provided by the vestibular system.

The pathways of the extrapyramidal system are influenced by subcortical structures. For example, the connections between the secondary motor cortices and the extrapyramidal system modulate movements of the spine and skull. The basal ganglia, which are important in regulating CNS-initiated movement, influence the extrapyramidal system as well as its thalamic feedback to the motor cortex.

Consciously moving our muscles is more complicated than simply sending a single command from the precentral gyrus to the appropriate motor neurons. During the movement of any part of the body, our muscles transmit information to the brain, and the brain is constantly sending “revised” instructions to the muscles. The cerebellum contributes significantly to the motor system as it compares cerebral motor commands to proprioceptive feedback. Corticospinal fibers, which project to the ventral horn of the spinal cord, have branches that also synapse in the pons, which project to the cerebellum. In addition, proprioceptive sensations from the thoracic column system have a collateral projection to the medulla oblongata, which projects to the cerebellum. These two streams of information are compared in the cerebellar cortex. Conflicts between motor commands sent by the brain and information about body position provided by proprioceptors cause the cerebellum to stimulate the midbrain red nucleus. Thatred corethen sends correction commands to the spinal cord along theRubrospinaltrakt. The name of this contract comes from the word red seen in the English word "ruby".

A good example of how the cerebellum corrects cerebral motor commands can be illustrated by walking in water. An initial brain motor command to walk results in a highly coordinated set of learned movements. In water, however, the body cannot perform a typical walking motion as instructed. The cerebellum can alter motor control and stimulate the leg muscles to take larger strides to overcome the resistance of the water. The cerebellum can make the necessary changes via the rubrospinal tract. The modulation of the basic walking command is also based on spinal reflexes, but the cerebellum is responsible for calculating the appropriate response. When the cerebellum is not functioning properly, coordination and balance are severely affected. The most dramatic example of this is binge drinking. Alcohol inhibits the cerebellum's ability to interpret proprioceptive feedback, making it difficult to coordinate body movements, such as walking. B. walking in a straight line or guiding the hand movement to touch the tip of the nose.

visit this oneOrtRead about an elderly woman who gradually loses the ability to control fine movements such as speech and movement of her limbs. Many of the usual causes have been ruled out. It wasn't a stroke, Parkinson's, diabetes, or thyroid dysfunction. The next obvious cause was medication, so she had to see her pharmacist. A side effect of a drug that was supposed to help her sleep had caused changes in motor control. Which regions of the nervous system are likely to be the focus of haloperidol side effects?

ventrals Hornausgang

The somatic nervous system delivers power exclusively to the skeletal muscles. Lower motor neurons responsible for contracting these muscles are located in the anterior horn of the spinal cord. These large multipolar neurons have a crown of dendrites surrounding the cell body and an axon that protrudes from the anterior horn. This axon travels through the ventral nerve root to join the exiting spinal nerve. The axon is relatively long because it needs to reach the muscles on the periphery of the body. Cell body diameters can be on the order of hundreds of microns to support the long axon; Some axons are a meter long, such as the lumbar motor neurons that innervate the muscles of the first toes.

The axons also branch to innervate multiple muscle fibers. Together, the motoneuron and all the muscle fibers it controls form a motor unit. Motor units vary in size. Some may contain as many as 1,000 muscle fibers, like in the quadriceps, or as few as 10 fibers, like in an eye muscle. The number of muscle fibers that are part of a motor unit corresponds to the precision of control of that muscle. Also, muscles with finer motor control have more motor units attached, and this requires a larger topographical field in the primary motor cortex.

Motor neuron axons connect to muscle fibers at a neuromuscular junction. This is a specialized synaptic structure in which multiple axon terminals synapse with the muscle fiber sarcolemma. The synaptic terminal bulbs of motor neurons secrete acetylcholine, which binds to receptors on the sarcolemma. Binding of acetylcholine opens ligand-gated ion channels, which increases the movement of cations across the sarcolemma. This depolarizes the sarcolemma and initiates muscle contraction. While other synapses result in graded potentials that must reach a threshold at the postsynaptic target, activity at the neuromuscular junction reliably results in muscle fiber contraction with each nerve impulse received from a motor neuron. However, the strength of the contraction and the number of contracting fibers can be affected by the frequency of the motor neuron impulses.

reflexes

This chapter began by introducing reflexes as an example of the basic elements of the somatic nervous system. Simple somatic reflexes do not involve the higher centers discussed for the conscious or voluntary aspects of movement. Reflexes can be spinal or cranial, depending on which nerves and central components are involved. The example described at the beginning of the chapter involved sensations of heat and pain from a hot stove causing the arm to retract through a connection in the spinal cord, resulting in contraction of the biceps brachii. The description of this withdrawal reflex has been simplified for the introduction to emphasize parts of the somatic nervous system. But to fully account for reflections, more attention should be paid to this example.

When you take your hand off the stove, you don't want to slow down this reflex. When the bicep contracts, the opposite tricep must relax. Since the neuromuscular junction is strictly excitatory, the biceps contract when the motor nerve is active. Skeletal muscles do not actively relax. Instead, the motor neuron needs to "quiet down" or be inhibited. In the hot spot withdrawal reflex, this occurs via an interneuron in the spinal cord. The cell body of the interneuron is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron, which detects that the hand is on fire. In response to this sensory neuron stimulation, the interneuron inhibits the motor neuron that controls the triceps brachii muscle. It does this by releasing a neurotransmitter or other signal that hyperpolarizes the motor neuron connected to the triceps brachii muscle, making it less likely to fire an action potential. When this motor neuron is inhibited, the triceps brachii relax. Without the counter-contraction, removal from the hot oven is faster and prevents further tissue damage.

Another example of a withdrawal reflex occurs when you step on a painful stimulus, such as a foot. B. a thumbtack or a sharp stone. Nociceptors, which are activated by the pain stimulus, activate motor neurons responsible for contracting the anterior tibialis muscle. This leads to dorsiflexion of the foot. An inhibitory interneuron, activated by a collateral branch of the nociceptor fiber, inhibits motor neurons in the gastrocnemius and soleus muscles to stop plantar flexion. An important difference in this reflex is that when the foot presses on the saddle, plantar flexion is most likely in progress. The contraction of the tibialis anterior is not the most important aspect of the reflex, as continued plantar flexion will cause more damage as you step on the turn.

Another type of reflection is astretch reflex. In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated. The axon of this receptor structure causes the muscle to contract directly. A muscle spindle fiber collateral also inhibits the motor neuron of the antagonistic muscles. The reflex helps keep the muscles at a constant length. A common example of this reflex is the knee jerk induced by a rubber mallet being struck against the patellar ligament during the physical exam.

A specialized reflex to protect the surface of the eye is theHornhautreflex, or the blink reflex. When the cornea is stimulated by a tactile stimulus or even bright light in a related reflex, blinking is elicited. The sensory component travels through the trigeminal nerve, which carries somatosensory information from the face, or through the optic nerve when the stimulus is a bright light. The motor response travels through the facial nerve and innervates the orbicularis oculi muscle on the same side. This reflex is usually tested during a physical exam with a puff of air or a gentle touch of a cotton swab.

look at thisVideoLearn more about the corneal reflex arc. What happens to the left eye when the right cornea perceives a tactile stimulus? explain your answer

look at thisVideoto learn more about newborn reflexes. Newborns have a set of reflexes that are thought to have been essential for survival before modern times. These reflexes disappear as the baby grows as some of them become unnecessary as the baby gets older. The video shows a reflex called the Babinski reflex, in which the foot dorsiflexes and the toes spread when the sole of the foot is lightly scratched. This is normal in newborns, but is a sign of reduced spinal myelination in adults. Why would this reflex be a problem for an adult?

chapter overview

The motor components of the somatic nervous system begin in the brain's frontal lobe, where the prefrontal cortex is responsible for higher functions such as working memory. The integrative and associated functions of the prefrontal lobe feed the secondary motor areas that aid in movement planning. The premotor cortex and the supplemental motor area feed the primary motor cortex, which initiates movement. Large Betz cells project through the corticobulbar and corticospinal pathways to synapse with lower motor neurons in the brainstem and anterior horn of the spinal cord, respectively. These compounds are responsible for generating skeletal muscle movements.

The extrapyramidal system includes projections from the brainstem and higher centers that affect movement, primarily to maintain balance and posture and muscle tone. The superior colliculus and red nucleus in the midbrain, the vestibular nuclei in the medulla oblongata, and the reticular formation throughout the brainstem all have pathways that project to the spinal cord in this system. Descending inputs from the secondary motor cortices, basal ganglia, and cerebellum connect to the brainstem origins of these pathways.

All of these motor pathways project to the spinal cord to synapse with motor neurons in the ventral horn of the spinal cord. These lower motor neurons are the cells that connect to skeletal muscle and cause contractions. These neurons project through the spinal nerves to connect to the muscles at the neuromuscular junctions. A motor neuron connects to multiple muscle fibers within a target muscle. The number of fibers innervated by a single motor neuron depends on the precision and strength required of that muscle and motor unit. The quadriceps, for example, has many fibers controlled by individual motor neurons for powerful contractions that don't have to be precise. The eye muscles only have a small number of fibers controlled by each motor neuron because moving the eyes doesn't require much force but needs to be very precise.

Reflexes are the simplest circuits within the somatic nervous system. A withdrawal reflex from a painful stimulus requires only that the sensory fiber enter the spinal cord and project the motor neuron to a muscle. Antagonist and postural muscles can be coordinated with withdrawal, adding complexity to the connections. The simple single neural connection is the basis of somatic reflexes. The corneal reflex is the contraction of the orbicularis oculi muscle to blink the eyelid when something touches the surface of the eye. Stretch reflexes maintain constant muscle length by causing a muscle to contract to compensate for a stretch, which can be detected by a specialized receptor called the muscle spindle.

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