Glossary: The Brain and Cranial Nerves

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

Betz cells: output cells of the primary motor cortex that cause musculature to move through synapses on cranial and spinal motor neurons

Broca’s area: region of the frontal lobe associated with the motor commands necessary for speech production

basilar membrane: in the ear, the floor of the cochlear duct on which the organ of Corti sits

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

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: 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

retinal ganglion cell (RGC): neuron of the retina that projects along the second cranial nerve

retinal: cofactor in an opsin molecule that undergoes a biochemical change when struck by a photon (pronounced with a stress on the last syllable)

retina: nervous tissue of the eye at which phototransduction takes place

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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