14.4: Key Terms

Example and Directions

Words (or words that have the same definition)	The definition is case sensitive	(Optional) Image to display with the definition [Not displayed in Glossary, only in pop-up on pages]	(Optional) Caption for Image	(Optional) External or Internal Link	(Optional) Source for Definition
(Eg. "Genetic, Hereditary, DNA ...")	(Eg. "Relating to genes or heredity")		The infamous double helix	https://bio.libretexts.org/	CC-BY-SA; Delmar Larsen

Glossary Entries

Word(s)	Definition	Image	Caption	Link	Source
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