Module 9: The Somatic Nervous System

Lesson 1: Somatic Nervous System: Sensory Perception

Hệ Thần Kinh Bản Thể: Nhận Thức Bằng Giác Quan

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Dưới đây là danh sách những thuật ngữ Y khoa của module The Somatic Nervous System.
Khái quát được số lượng thuật ngữ sẽ xuất hiện trong bài đọc và nghe sẽ giúp bạn thoải mái tiêu thụ nội dung hơn. Sau khi hoàn thành nội dung đọc và nghe, bạn hãy quay lại đây và luyện tập (practice) để quen dần các thuật ngữ này. Đừng ép bản thân phải nhớ các thuật ngữ này vội vì bạn sẽ gặp và ôn lại danh sách này trong những bài học (lesson) khác của cùng một module.

Medical Terminology: The Somatic Nervous System

alkaloid
substance, usually from a plant source, that is chemically basic with respect to pH and will stimulate bitter receptors
amacrine cell
type of cell in the retina that connects to the bipolar cells near the outer synaptic layer and provides the basis for early image processing within the retina
ampulla
in the ear, the structure at the base of a semicircular canal that contains the hair cells and cupula for transduction of rotational movement of the head
anosmia
loss of the sense of smell; usually the result of physical disruption of the first cranial nerve
anterior corticospinal tract
division of the corticospinal pathway that travels through the ventral (anterior) column of the spinal cord and controls axial musculature through the medial motor neurons in the ventral (anterior) horn
aqueous humor
watery fluid that fills the anterior chamber containing the cornea, iris, ciliary body, and lens of the eye
ascending pathway
fiber structure that relays sensory information from the periphery through the spinal cord and brain stem to other structures of the brain
association area
region of cortex connected to a primary sensory cortical area that further processes the information to generate more complex sensory perceptions
audition
sense of hearing
auricle
fleshy external structure of the ear
basilar membrane
in the ear, the floor of the cochlear duct on which the organ of Corti sits
Betz cells
output cells of the primary motor cortex that cause musculature to move through synapses on cranial and spinal motor neurons
binocular depth cues
indications of the distance of visual stimuli on the basis of slight differences in the images projected onto either retina
bipolar cell
cell type in the retina that connects the photoreceptors to the RGCs
Broca’s area
region of the frontal lobe associated with the motor commands necessary for speech production
capsaicin
molecule that activates nociceptors by interacting with a temperature-sensitive ion channel and is the basis for “hot” sensations in spicy food
cerebral peduncles
segments of the descending motor pathway that make up the white matter of the ventral midbrain
cervical enlargement
region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of and finer control of muscles of the upper limb
chemoreceptor
sensory receptor cell that is sensitive to chemical stimuli, such as in taste, smell, or pain
chief sensory nucleus
component of the trigeminal nuclei that is found in the pons
choroid
highly vascular tissue in the wall of the eye that supplies the outer retina with blood
ciliary body
smooth muscle structure on the interior surface of the iris that controls the shape of the lens through the zonule fibers
circadian rhythm
internal perception of the daily cycle of light and dark based on retinal activity related to sunlight
cochlea
auditory portion of the inner ear containing structures to transduce sound stimuli
cochlear duct
space within the auditory portion of the inner ear that contains the organ of Corti and is adjacent to the scala tympani and scala vestibuli on either side
cone photoreceptor
one of the two types of retinal receptor cell that is specialized for color vision through the use of three photopigments distributed through three separate populations of cells
contralateral
word meaning “on the opposite side,” as in axons that cross the midline in a fiber tract
cornea
fibrous covering of the anterior region of the eye that is transparent so that light can pass through it
corneal reflex
protective response to stimulation of the cornea causing contraction of the orbicularis oculi muscle resulting in blinking of the eye
corticobulbar tract
connection between the cortex and the brain stem responsible for generating movement
corticospinal tract
connection between the cortex and the spinal cord responsible for generating movement
cupula
specialized structure within the base of a semicircular canal that bends the stereocilia of hair cells when the head rotates by way of the relative movement of the enclosed fluid
decussate
to cross the midline, as in fibers that project from one side of the body to the other
dorsal column system
ascending tract of the spinal cord associated with fine touch and proprioceptive sensations
dorsal stream
connections between cortical areas from the occipital to parietal lobes that are responsible for the perception of visual motion and guiding movement of the body in relation to that motion
encapsulated ending
configuration of a sensory receptor neuron with dendrites surrounded by specialized structures to aid in transduction of a particular type of sensation, such as the lamellated corpuscles in the deep dermis and subcutaneous tissue
equilibrium
sense of balance that includes sensations of position and movement of the head
executive functions
cognitive processes of the prefrontal cortex that lead to directing goal-directed behavior, which is a precursor to executing motor commands
external ear
structures on the lateral surface of the head, including the auricle and the ear canal back to the tympanic membrane
exteroceptor
sensory receptor that is positioned to interpret stimuli from the external environment, such as photoreceptors in the eye or somatosensory receptors in the skin
extraocular muscle
one of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye
extrapyramidal system
pathways between the brain and spinal cord that are separate from the corticospinal tract and are responsible for modulating the movements generated through that primary pathway
fasciculus cuneatus
lateral division of the dorsal column system composed of fibers from sensory neurons in the upper body
fasciculus gracilis
medial division of the dorsal column system composed of fibers from sensory neurons in the lower body
fibrous tunic
outer layer of the eye primarily composed of connective tissue known as the sclera and cornea
fovea
exact center of the retina at which visual stimuli are focused for maximal acuity, where the retina is thinnest, at which there is nothing but photoreceptors
free nerve ending
configuration of a sensory receptor neuron with dendrites in the connective tissue of the organ, such as in the dermis of the skin, that are most often sensitive to chemical, thermal, and mechanical stimuli
frontal eye fields
area of the prefrontal cortex responsible for moving the eyes to attend to visual stimuli
general sense
any sensory system that is distributed throughout the body and incorporated into organs of multiple other systems, such as the walls of the digestive organs or the skin
gustation
sense of taste
gustatory receptor cells
sensory cells in the taste bud that transduce the chemical stimuli of gustation
hair cells
mechanoreceptor cells found in the inner ear that transduce stimuli for the senses of hearing and balance
incus
(also, anvil) ossicle of the middle ear that connects the malleus to the stapes
inferior colliculus
last structure in the auditory brainstem pathway that projects to the thalamus and superior colliculus
inferior oblique
extraocular muscle responsible for lateral rotation of the eye
inferior rectus
extraocular muscle responsible for looking down
inner ear
structure within the temporal bone that contains the sensory apparati of hearing and balance
inner segment
in the eye, the section of a photoreceptor that contains the nucleus and other major organelles for normal cellular functions
inner synaptic layer
layer in the retina where bipolar cells connect to RGCs
interaural intensity difference
cue used to aid sound localization in the horizontal plane that compares the relative loudness of sounds at the two ears, because the ear closer to the sound source will hear a slightly more intense sound
interaural time difference
cue used to help with sound localization in the horizontal plane that compares the relative time of arrival of sounds at the two ears, because the ear closer to the sound source will receive the stimulus microseconds before the other ear
internal capsule
segment of the descending motor pathway that passes between the caudate nucleus and the putamen
interoceptor
sensory receptor that is positioned to interpret stimuli from internal organs, such as stretch receptors in the wall of blood vessels
ipsilateral
word meaning on the same side, as in axons that do not cross the midline in a fiber tract
iris
colored portion of the anterior eye that surrounds the pupil
kinesthesia
sense of body movement based on sensations in skeletal muscles, tendons, joints, and the skin
lacrimal duct
duct in the medial corner of the orbit that drains tears into the nasal cavity
lacrimal gland
gland lateral to the orbit that produces tears to wash across the surface of the eye
lateral corticospinal tract
division of the corticospinal pathway that travels through the lateral column of the spinal cord and controls appendicular musculature through the lateral motor neurons in the ventral (anterior) horn
lateral geniculate nucleus
thalamic target of the RGCs that projects to the visual cortex
lateral rectus
extraocular muscle responsible for abduction of the eye
lens
component of the eye that focuses light on the retina
levator palpebrae superioris
muscle that causes elevation of the upper eyelid, controlled by fibers in the oculomotor nerve
lumbar enlargement
region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of muscles of the lower limb
macula
enlargement at the base of a semicircular canal at which transduction of equilibrium stimuli takes place within the ampulla
malleus
(also, hammer) ossicle that is directly attached to the tympanic membrane
mechanoreceptor
receptor cell that transduces mechanical stimuli into an electrochemical signal
medial geniculate nucleus
thalamic target of the auditory brain stem that projects to the auditory cortex
medial lemniscus
fiber tract of the dorsal column system that extends from the nuclei gracilis and cuneatus to the thalamus, and decussates
medial rectus
extraocular muscle responsible for adduction of the eye
mesencephalic nucleus
component of the trigeminal nuclei that is found in the midbrain
middle ear
space within the temporal bone between the ear canal and bony labyrinth where the ossicles amplify sound waves from the tympanic membrane to the oval window
multimodal integration area
region of the cerebral cortex in which information from more than one sensory modality is processed to arrive at higher level cortical functions such as memory, learning, or cognition
neural tunic
layer of the eye that contains nervous tissue, namely the retina
nociceptor
receptor cell that senses pain stimuli
nucleus cuneatus
medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the upper body and arms
nucleus gracilis
medullary nucleus at which first-order neurons of the dorsal column system synapse specifically from the lower body and legs
odorant molecules
volatile chemicals that bind to receptor proteins in olfactory neurons to stimulate the sense of smell
olfaction
sense of smell
olfactory bulb
central target of the first cranial nerve; located on the ventral surface of the frontal lobe in the cerebrum
olfactory epithelium
region of the nasal epithelium where olfactory neurons are located
olfactory sensory neuron
receptor cell of the olfactory system, sensitive to the chemical stimuli of smell, the axons of which compose the first cranial nerve
opsin
protein that contains the photosensitive cofactor retinal for phototransduction
optic chiasm
decussation point in the visual system at which medial retina fibers cross to the other side of the brain
optic disc
spot on the retina at which RGC axons leave the eye and blood vessels of the inner retina pass
optic nerve
second cranial nerve, which is responsible visual sensation
optic tract
name for the fiber structure containing axons from the retina posterior to the optic chiasm representing their CNS location
organ of Corti
structure in the cochlea in which hair cells transduce movements from sound waves into electrochemical signals
osmoreceptor
receptor cell that senses differences in the concentrations of bodily fluids on the basis of osmotic pressure
ossicles
three small bones in the middle ear
otolith
layer of calcium carbonate crystals located on top of the otolithic membrane
otolithic membrane
gelatinous substance in the utricle and saccule of the inner ear that contains calcium carbonate crystals and into which the stereocilia of hair cells are embedded
outer segment
in the eye, the section of a photoreceptor that contains opsin molecules that transduce light stimuli
outer synaptic layer
layer in the retina at which photoreceptors connect to bipolar cells
oval window
membrane at the base of the cochlea where the stapes attaches, marking the beginning of the scala vestibuli
palpebral conjunctiva
membrane attached to the inner surface of the eyelids that covers the anterior surface of the cornea
papilla
for gustation, a bump-like projection on the surface of the tongue that contains taste buds
photoisomerization
chemical change in the retinal molecule that alters the bonding so that it switches from the 11-cis-retinal isomer to the all-trans-retinal isomer
photon
individual “packet” of light
photoreceptor
receptor cell specialized to respond to light stimuli
premotor cortex
cortical area anterior to the primary motor cortex that is responsible for planning movements
primary sensory cortex
region of the cerebral cortex that initially receives sensory input from an ascending pathway from the thalamus and begins the processing that will result in conscious perception of that modality
proprioception
sense of position and movement of the body
proprioceptor
receptor cell that senses changes in the position and kinesthetic aspects of the body
pupil
open hole at the center of the iris that light passes through into the eye
pyramidal decussation
location at which corticospinal tract fibers cross the midline and segregate into the anterior and lateral divisions of the pathway
pyramids
segment of the descending motor pathway that travels in the anterior position of the medulla
receptor cell
cell that transduces environmental stimuli into neural signals
red nucleus
midbrain nucleus that sends corrective commands to the spinal cord along the rubrospinal tract, based on disparity between an original command and the sensory feedback from movement
reticulospinal tract
extrapyramidal connections between the brain stem and spinal cord that modulate movement, contribute to posture, and regulate muscle tone
retina
nervous tissue of the eye at which phototransduction takes place
retinal
cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable)
retinal ganglion cell (RGC)
neuron of the retina that projects along the second cranial nerve
rhodopsin
photopigment molecule found in the rod photoreceptors
rod photoreceptor
one of the two types of retinal receptor cell that is specialized for low-light vision
round window
membrane that marks the end of the scala tympani
rubrospinal tract
descending motor control pathway, originating in the red nucleus, that mediates control of the limbs on the basis of cerebellar processing
saccule
structure of the inner ear responsible for transducing linear acceleration in the vertical plane
scala tympani
portion of the cochlea that extends from the apex to the round window
scala vestibuli
portion of the cochlea that extends from the oval window to the apex
sclera
white of the eye
semicircular canals
structures within the inner ear responsible for transducing rotational movement information
sensory homunculus
topographic representation of the body within the somatosensory cortex demonstrating the correspondence between neurons processing stimuli and sensitivity
sensory modality
a particular system for interpreting and perceiving environmental stimuli by the nervous system
solitary nucleus
medullar nucleus that receives taste information from the facial and glossopharyngeal nerves
somatosensation
general sense associated with modalities lumped together as touch
special sense
any sensory system associated with a specific organ structure, namely smell, taste, sight, hearing, and balance
spinal trigeminal nucleus
component of the trigeminal nuclei that is found in the medulla
spinothalamic tract
ascending tract of the spinal cord associated with pain and temperature sensations
spiral ganglion
location of neuronal cell bodies that transmit auditory information along the eighth cranial nerve
stapes
(also, stirrup) ossicle of the middle ear that is attached to the inner ear
stereocilia
array of apical membrane extensions in a hair cell that transduce movements when they are bent
stretch reflex
response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length
submodality
specific sense within a broader major sense such as sweet as a part of the sense of taste, or color as a part of vision
superior colliculus
structure in the midbrain that combines visual, auditory, and somatosensory input to coordinate spatial and topographic representations of the three sensory systems
superior oblique
extraocular muscle responsible for medial rotation of the eye
superior rectus
extraocular muscle responsible for looking up
supplemental motor area
cortical area anterior to the primary motor cortex that is responsible for planning movements
suprachiasmatic nucleus
hypothalamic target of the retina that helps to establish the circadian rhythm of the body on the basis of the presence or absence of daylight
taste buds
structures within a papilla on the tongue that contain gustatory receptor cells
tectorial membrane
component of the organ of Corti that lays over the hair cells, into which the stereocilia are embedded
tectospinal tract
extrapyramidal connections between the superior colliculus and spinal cord
thermoreceptor
sensory receptor specialized for temperature stimuli
topographical
relating to positional information
transduction
process of changing an environmental stimulus into the electrochemical signals of the nervous system
trochlea
cartilaginous structure that acts like a pulley for the superior oblique muscle
tympanic membrane
ear drum
umami
taste submodality for sensitivity to the concentration of amino acids; also called the savory sense
utricle
structure of the inner ear responsible for transducing linear acceleration in the horizontal plane
vascular tunic
middle layer of the eye primarily composed of connective tissue with a rich blood supply
ventral posterior nucleus
nucleus in the thalamus that is the target of gustatory sensations and projects to the cerebral cortex
ventral stream
connections between cortical areas from the occipital lobe to the temporal lobe that are responsible for identification of visual stimuli
vestibular ganglion
location of neuronal cell bodies that transmit equilibrium information along the eighth cranial nerve
vestibular nuclei
targets of the vestibular component of the eighth cranial nerve
vestibule
in the ear, the portion of the inner ear responsible for the sense of equilibrium
vestibulo-ocular reflex (VOR)
reflex based on connections between the vestibular system and the cranial nerves of eye movements that ensures images are stabilized on the retina as the head and body move
vestibulospinal tract
extrapyramidal connections between the vestibular nuclei in the brain stem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium
visceral sense
sense associated with the internal organs
vision
special sense of sight based on transduction of light stimuli
visual acuity
property of vision related to the sharpness of focus, which varies in relation to retinal position
vitreous humor
viscous fluid that fills the posterior chamber of the eye
working memory
function of the prefrontal cortex to maintain a representation of information that is not in the immediate environment
zonule fibers
fibrous connections between the ciliary body and the lens
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A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.

Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the 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, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.
Stimuli in the environment activate specialized receptor cells in the peripheral nervous system. Different types of stimuli are sensed by different types of receptor cells. Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. They can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.

A. Structural Receptor Types

The cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus (Figure 1). The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings. Also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch. The cells in the retina that respond to light stimuli are an example of a specialized receptor, a photoreceptor.

Another way that receptors can be classified is based on their location relative to the stimuli. An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin. An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. Finally, a proprioceptor is a receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move.

B. Functional Receptor Types

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

Receptor cells can be further categorized on the basis of the type of stimuli they transduce. Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell. Osmoreceptors respond to solute concentrations of body fluids. Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor. Physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. Another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.
Ask anyone what the senses are, and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Also, what is referred to simply as touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position, on the basis of the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.

Within the realm of physiology, senses can be classified as either general or specific. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.

Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Hearing and balance are also sensed by mechanoreceptors. Finally, vision involves the activation of photoreceptors.

Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus. For example, the general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.

For detailed information about sensory modalities, see the article “Sensory Modalities.”

Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.

A. Spinal Nerves

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

B. Cranial Nerves

Cranial nerves convey specific sensory information from the head and neck directly to the brain. Whereas spinal information is contralateral, cranial nerve systems, with some exceptions, are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

Receptor cell types can be classified on the basis of their structure. Sensory neurons can have either (a) free nerve endings or (b) encapsulated endings. Photoreceptors in the eyes, such as rod cells, are examples of (c) specialized receptor cells. These cells release neurotransmitters onto a bipolar cell, which then synapses with the optic nerve neurons.

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Script:
  1. The senses are olfaction (which means smell), gustation (which means taste), somatosensation (which means sensations associated with the skin and body), audition (which means hearing), equilibrium (which means balance), and vision.
  2. With the exception of somatosensation, this list represents the special senses, or those systems of the body that are associated with specific organs such as the tongue or eye.
  3. Somatosensation belongs to the general senses, which are those sensory structures that are distributed throughout the body and in the walls of various organs.
  4. The special senses are all primarily part of the somatic nervous system in that they are consciously perceived through cerebral processes, though some special senses contribute to autonomic function.
  5. The general senses can be divided into somatosensation, which is commonly considered touch, but includes tactile, pressure, vibration, temperature, and pain perception.
  6. The general senses also include the visceral senses, which are separate from the somatic nervous system function in that they do not normally rise to the level of conscious perception.
  7. The cells that transduce sensory stimuli into the electrochemical signals of the nervous system are classified on the basis of structural or functional aspects of the cells.
  8. The structural classifications are based on the anatomy of the cell that is interacting with the stimulus (which are free nerve endings, encapsulated endings, or specialized receptor cell).
  9. The classifications can also be based on where the cell is located relative to the stimulus (which are interoceptor, exteroceptor, proprioceptor).
  10. Thirdly, the functional classification is based on how the cell transduces the stimulus into a neural signal.
  11. Chemoreceptors respond to chemical stimuli and are the basis for olfaction and gustation.
  12. Related to chemoreceptors are osmoreceptors and nociceptors for fluid balance and pain reception, respectively.
  13. Mechanoreceptors respond to mechanical stimuli and are the basis for most aspects of somatosensation, as well as being the basis of audition and equilibrium in the inner ear.
  14. Thermoreceptors are sensitive to temperature changes, and photoreceptors are sensitive to light energy.
  15. The nerves that convey sensory information from the periphery to the central nervous system are either spinal nerves, connected to the spinal cord, or cranial nerves, connected to the brain.
  16. Spinal nerves have mixed populations of fibers; some are motor fibers and some are sensory.
  17. The sensory fibers connect to the spinal cord through the dorsal root, which is attached to the dorsal root ganglion.
  18. Sensory information from the body that is conveyed through spinal nerves will project to the opposite side of the brain to be processed by the cerebral cortex.
  19. The cranial nerves can be strictly 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.
  20. The cranial nerves are connected to the same side of the brain from which the sensory information originates.
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