Module 9: The Somatic Nervous System

Lesson 2: Somatic Nervous System: Sensory Modalities

Hệ Thần Kinh Bản Thể: Các Phương Thức Của 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
Nội dung này đang được cập nhật.
Dưới đây là các bài văn nằm ở bên trái. Ở bên phải là các bài luyện tập (practice) để đánh giá khả năng đọc hiểu của bạn. Sẽ khó khăn trong thời gian đầu nếu vốn từ vựng của bạn còn hạn chế, đặc biệt là từ vựng Y khoa. Hãy kiên nhẫn và đọc nhiều nhất có kể, lượng kiến thức tích tụ dần sẽ giúp bạn đọc thoải mái hơn.
Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to recognition of the fifth taste, umami, during the mid-1980s. Umami is a Japanese word that means “delicious taste,” and is often translated to mean savory. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.

Gustation 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. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance (Figure 1): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.

Salty taste is simply the perception of sodium ions (Na+) in the saliva. When you eat something salty, the salt crystals dissociate into the component ions Na+ and Cl–, which dissolve into the saliva in your mouth. The Na+ concentration becomes high outside the gustatory cells, creating a strong concentration gradient that drives the diffusion of the ion into the cells. The entry of Na+ into these cells results in the depolarization of the cell membrane and the generation of a receptor potential.

Sour taste is the perception of H+ concentration. Just as with sodium ions in salty flavors, these hydrogen ions enter the cell and trigger depolarization. Sour flavors are, essentially, the perception of acids in our food. Increasing hydrogen ion concentrations in the saliva (lowering saliva pH) triggers progressively stronger graded potentials in the gustatory cells. For example, orange juice—which contains citric acid—will taste sour because it has a pH value of approximately 3. Of course, it is often sweetened so that the sour taste is masked.

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

Bitter taste is similar to sweet in that food molecules bind to G protein–coupled receptors. However, there are a number of different ways in which this can happen because there are a large diversity of bitter-tasting molecules. Some bitter molecules depolarize gustatory cells, whereas others hyperpolarize gustatory cells. Likewise, some bitter molecules increase G protein activation within the gustatory cells, whereas other bitter molecules decrease G protein activation. The specific response depends on which molecule is binding to the receptor.

One major group of bitter-tasting molecules are alkaloids. Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin. By containing toxic alkaloids, the plant is less susceptible to microbe infection and less attractive to herbivores.

Therefore, the function of bitter taste may primarily be related to stimulating the gag reflex to avoid ingesting poisons. Because of this, many bitter foods that are normally ingested are often combined with a sweet component to make them more palatable (cream and sugar in coffee, for example). The highest concentration of bitter receptors appear to be in the posterior tongue, where a gag reflex could still spit out poisonous food.

The taste known as umami is often referred to as the savory taste. Like sweet and bitter, 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 flavor is often perceived while eating protein-rich foods. Not surprisingly, dishes that contain meat are often described as savory.

Once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. These neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness.
Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 2). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the 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 will produce a graded membrane potential in the olfactory neurons.

The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.
Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure 3). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.

The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window.

The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time travelling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves (Figure 4). The membrane covering the round window will bulge out or pucker in with the movement of the fluid within the scala tympani.

A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 5). The cochlear duct contains several organs of Corti, which transduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.

The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces (Figure 6). The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. This bends the stereocilia either toward or away from the tallest member of each array. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close. When no sound is present, and the stereocilia are standing straight, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized.

As stated above, a given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency. Therefore, as the frequency of a sound changes, different hair cells are activated all along the basilar membrane. The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. The unit of Hertz measures the frequency of sound waves in terms of cycles produced per second. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows (Figure 8). Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave). The hair cells along the length of the cochlear duct, which are each sensitive to a particular frequency, allow the cochlea to separate auditory stimuli by frequency, just as a prism separates visible light into its component colors.
Along with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance. A similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. These cells are located within the vestibule of the inner ear. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.

The utricle and saccule are both largely composed of macula tissue (plural = maculae). The macula is composed of hair cells surrounded by support cells. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane (Figure 9). On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The otoliths essentially make the otolithic membrane top-heavy. The otolithic membrane moves separately from the macula in response to head movements. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The moving otolithic membrane, in turn, bends the stereocilia, causing some hair cells to depolarize as others hyperpolarize. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization.

The semicircular canals are three ring-like extensions of the vestibule. One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane (Figure 10). The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space.
Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.

Two types of somatosensory signals that are transduced 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 temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. For example, the sensation of heat associated with spicy foods involves 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 with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.

If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low frequency vibrations are sensed by mechanoreceptors called Merkel cells, also known as type I cutaneous mechanoreceptors. Merkel cells are located in the stratum basale of the epidermis. Deep pressure and vibration is transduced by lamellated (Pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. Light touch is transduced by the encapsulated endings known as tactile (Meissner) corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. Bulbous corpuscles are also known as Ruffini corpuscles, or type II cutaneous mechanoreceptors.

Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints. For example, have you ever stretched your muscles before or after exercise and noticed that you can only stretch so far before your muscles spasm back to a less stretched state? This spasm is a reflex that is initiated by stretch receptors to avoid muscle tearing. Such stretch receptors can also prevent over-contraction of a muscle. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in Table 14.1.
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 11). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located just inside the orbit, superior and lateral to the eyeball. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 12). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye laterally. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 11).

The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.

The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible (Figure 13). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.

The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor.

The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see Figure 13). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.

Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 13). As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea.

Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 14). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.

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

Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans- conformation, forming all-trans-retinal, which has a straight hydrocarbon chain (Figure 15).

The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.

The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue (Figure 16). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.

The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, 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, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.

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.

The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

(a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively.

A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined by the changes in amplitude and frequency of the sound waves entering the ear.

The three major spaces within the cochlea are highlighted. The scala tympani and scala vestibuli lie on either side of the cochlear duct. The organ of Corti, containing the mechanoreceptor hair cells, is adjacent to the scala tympani, where it sits atop the basilar membrane.

The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array.

LM × 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

The standing sound wave generated in the cochlea by the movement of the oval window deflects the basilar membrane on the basis of the frequency of sound. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.

The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration.

Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions.

NameHistorical (eponymous) nameLocation(s)Stimuli
Free nerve endings*Dermis, cornea, tongue, joint capsules, visceral organsPain, temperature, mechanical deformation
MechanoreceptorsMerkel’s discsEpidermal–dermal junction, mucosal membranesLow frequency vibration (5–15 Hz)
Bulbous corpuscleRuffini’s corpuscleDermis, joint capsulesStretch
Tactile corpuscleMeissner’s corpusclePapillary dermis, especially in the fingertips and lipsLight touch, vibrations below 50 Hz
Lamellated corpusclePacinian corpuscleDeep dermis, subcutaneous tissueDeep pressure, high-frequency vibration (around 250 Hz)
Hair follicle plexus*Wrapped around hair follicles in the dermisMovement of hair
Muscle spindle*In line with skeletal muscle fibersMuscle contraction and stretch
Tendon stretch organGolgi tendon organIn line with tendonsStretch of tendons

The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

The extraocular muscles move the eye within the orbit.

The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.

(a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.

Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.

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  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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