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The Special Senses of the Human Body

Special senses

-have receptors strategically placed in unique organs
-include: olfaction, taste, visual system, hearing & balance

Olfaction key points

-7 primary odors now recognized, but average person can recognize 4000 different odors. perceived by olfactory epithelium.
-dendrites of olfactory neurons have enlarged ends (olfactory vesicles)
-cilia (olfactory hairs) of olfactory neuron embedded in mucus. odorants dissolve in mucus.

Olfaction key points 2

-odorants attach to receptors, cilia depolarize & initiate action potentials in olfactory neurons. one receptor may respond to more than one type of odor.
-olfactory epithelium replaced as it wears down. olfactory neurons replaced by basal cells every 2 months. unique: most neurons are permanent cells (aren't replaced if they die).

Neuronal Pathways of Olfaction

-olfactory sensory pathway: olfactory neurons (bipolar) in the olfactory epithelium pass through cribiform plate to olfactory bulbs & synapse w/ tufted cells or mitral cells
-these extend to olfactory tract & synapse w/ association neurons
-association neurons also receive input from brain, so info can be modified before it reaches the brain.

Neuronal Pathways of Olfaction 2

-info goes to olfactory cortex of frontal lobe w/out going through thalamus (only major sense that does not go through thalamus).
-3 regions in frontal lobe affect conscious perception of smell & interact w/ limbic system
-lateral, medial, & intermediate olfactory areas

Lateral olfactory area

conscious perception of smell

Medial olfactory area

visceral & emotional reactions to odors

Intermediate olfactory area

effect modification of incoming info



Taste buds

-the sensory structures that detect taste
-includes supporting cells surrounding taste (gustatory) cells
-taste cells have microvilli (gustatory hairs) extending into taste pores
-replaced about every 10 days


-specialized regions on the tongue
-types: filiform, vallate, fungiform, & foliate


filament-shaped; provide a rough surface for food manipulation


largest, least numerous. 8-12 in V along border between anterior & posterior parts of tongue. have taste buds.


mushroom-shaped. scattered irregularly over the superior surface of tongue. look like small red dots interspersed among the filiform. have taste buds.


leaf-shaped. in folds on the sides of the tongue. contain most sensitive taste buds. decrease in number w/ age.


-substances which are dissolved in saliva, enter the taste pores.
-by various mechanism (depending on the taste), tastants cause the taste cells to depolarize.

Taste types

sour, salty, bitter, sweet, umami


most sensitive receptors on lateral aspects of the tongue. H+ ion of acids cause depolarization.


most sensitive receptors on tip of tongue. shares lowest sensitivity w/ sweet. anything w/ Na+ causes depolarization.


most sensitive receptors on posterior aspect. highest sensitivity. sensation produced by alkaloids, which are toxic.


most sensitive receptors on tip of tongue. shares lowest sensitivity w/ salty. sugars, some carbohydrates, & some proteins (NutraSweet: aspartame).


"Savory"; scattered sensitivity. caused by amino acids such as glutamate, binding to receptors

Taste 2

-texture affects the perception of taste
-temperature affects taste perception
-very rapid adaptation, both at level of taste bud & within CNS
-taste influenced by olfaction
-different tastes have different thresholds w/ bitter being the taste to which we are most sensitive. many alkaloids (bitter) are poisonous

The Vertebrate Eye

cornea, choroid, iris, aqueous humor, vitreous humor, lens, retina, macula lutea, fovea, conjunctiva


transparent, admits light


is the vascular layer of the eye, provides oxygen & nourishment to the outer layers of the retina


-behind the cornea
-controls diameter of pupil
-regulates amount of light that strikes the lens

aqueous humor

a clear, gelatinous fluid in front chamber of eye. maintains the intraocular pressure & inflates the globe of the eye.

vitreous humor

clear gel that fills the space between the lens & the retina of the eyeball; helps hold the eye in place


focuses image on the retina


-lines the back of the eye
-photoreceptors and neurons integrate info detected by photoreceptors

macula lutea

is an oval-shaped highly pigmented yellow spot near the center of the retina


found in the macula lutea; provides highest resolution vision; contains a lot of cone cells (for color vision)


thin transparent mucous membrane

three tunics

fibrous: sclera & cornea (outer layer)
vascular: choroid, ciliary body, iris (middle)
nervous: retina

fibrous tunic

contains sclera & cornea


white outer layer. maintains shape, protects internal structures, provides muscle attachment point, continuous w/ cornea


connective tissue matrix containing collagen, elastic fibers, & proteoglycans
-avascular, transparent, allows light to enter eye; bends & refracts light

vascular tunic

middle layer. contains most of blood vessels of eye: branches off the internal carotid arteries. contains melanin.
-contains iris, ciliary body, & choroid


colored part of eye. controls light entering pupil. smooth muscle determines size of pupil.
-sphincter pupillae: circular smooth muscle; contraction=decrease pupil size (or constricts)
-dilator pupillae: radial smooth muscle; contraction=dilation of pupil

ciliary body

produces aqueous humor that fills anterior chamber
-ciliary muscles: smooth muscle that controls lens shape


associated w/ sclera. very thin, pigmented. is vascularized.

nervous tunic

2 layers: pigmented & sensory retina

pigmented retina

outer, pigmented layer. pigment of this layer & choroid help to separate sensory cells & reduce light scattering.

sensory retina

inner layer of rod & cone cells sensitive to light

anterior compartment

-anterior to lens; filled w/ aqueous humor
-contains 3 chambers of eye

3 chambers of eye

anterior, posterior & vitreous

anterior chamber

between cornea & iris

posterior chamber

-between iris & lens
-helps maintain intraocular pressure; supplies nutrients to structures bathed by it; contributes to refraction of light


abnormal increase in intraocular pressure

vitreous chamber

posterior to lens. filled w/ jelly-like vitreous humor. helps maintain intraocular pressure, holds lens & retina in place, refracts light.

the lens

-held by suspensory ligaments attached to ciliary muscles. changes shape as ciliary muscles contract & relax
-transparent, biconvex

visible light

portion of electromagnetic spectrum detected by human eye


bending of light


light striking a convex surface

focal point

-point where light rays converge & cross
-lens changes shape causing adjustment of focal point on the retina


causing light to converge

near point of vision

closer than 20 feet. changes occur in lens, size of pupil, & distance between pupils.


ciliary muscles contract due to parasympathetic input via cranial nerve III. pulls choroid toward lens reducing tension on suspensory ligaments. lens becomes more spherical, greater refraction of light.

pupil constriction

varies depth of focus


as objects move close to the eye, eyes are rotated medially. reflex contraction of the medial rectus muscles.

how the eye processes light

involves several cells, especially photoreceptor cells called rods & cones

Amacrine & horizontal cells

help integrate & regulate the input from multiple photoreceptor cells (eg. horizontal cells are responsible for allowing eyes to adjust to see well under both bright & dim light conditions)


specialized for detection of low-intensity light


specialized for detecting light of different wavelengths (colors)

The Photoreceptor cells of the Eye

(called rods & cones)...
-contain photoreceptive molecules that
-absorb energy of light
-generate changes in membrane potential
-in rods, photopigment is rhodopsin
-in cones, photopigment is iodopsin

Photoreceptive Molecules

(such as rhodopsin)
-absorb light in photoreceptor cells
-consist of retinal combined w/ an opsin protein
-there are 3 types of photopigments in cones (called photopsins)
-photopigments are found in the discs of photoreceptor cells (that is, the rods & cones)


retinal+opsin=photoreceptive molecule found in rods; is a G-protein coupled receptor

In the rod photoreceptor cell: steps 1-4

1. cis-Retinal absorbs light; is converted to trans-retinal
2. the protein segment (opsin) of rhodopsin is activated, triggering activation of G protein transducin
3. the activated G protein activates phosphodiesterase
4. activated phosphodiesterase breaks down cGMP to 5'-GMP, which then detaches from the Na+ channel

In rod photoreceptor cell: steps 5-7

5. loss of cGMP closes Na channel
6. membrane hyperpolarizes & reduces neurotransmitter release
7. when glutamate is not released (or reduced) from the rod cell, the bipolar cell is no longer inhibited & can now stimulate ganglion cells, produce APs, send signal to brain

Processing visual info

-in dark, rods & cones are depolarized & continually release glutamate
-some bipolar cells depolarize in response to glutamate, others hyperpolarize (depending on type of receptor present on bipolar cell)
-when light hits rods & cones, they hyperpolarize (b/c Na+ channel that was originally open in dark, that allowed Na+ to enter & depolarize cell, has been closed)
-this shuts off release of glutamate
-some bipolar cells will now be no longer inhibited by glutamate. they release neurotransmitters which stimulate ganglionic cells to generate APs to the brain

G-protein-linked receptors

-rhodopsin is a G-protein linked receptor found on membrane of rod photoreceptor cells
-utilizes cyclic GMP:
-concentration of GMP controlled by:
-synthesis by guanylyl cyclase,
-degradation by cyclic GMP phosphodiesterase
-light activates rhodopsin, causing phosphodiesterase to convert to cGMP to 5' GMP
-this closes Na+ channel on rod cell
-glutamate is no longer released from rod cell
-bipolar cell no longer inhibited & can send APs to ganglionic cells to brain

Rods light & dark adaptation

-in bright light, more rhodopsin broken down to Vitamin A, protecting eye & making it less sensitive to light
-in darker conditions, more rhodopsin produced so eye is more sensitive to light
-takes eyes a while to accommodate when going from dark to light & vice versa b/c of these chemical changes that must occur

Pupils light & dark adaptation

constriction in bright light; dilation in dim light


responsible for color vision & visual acuity
-numerous in fovea & macula lutea; fewer over rest of retina
-visual pigment is iodopsin: 3 types that respond to blue, red & green light
-overlap in response to light, thus interpretations of gradation of color possible: several millions

Receptive Fields

-area from which a ganglion cell receives input
-roughly circular w/ receptive field center
-2 types of receptive fields: on-center & off-center cells
-interneurons present in inner layers & modify signal b/f signal leaves retina. enhance borders & contours, increasing intensity at borders

on-center ganglion cells

generate more action potentials when light is directed onto the receptive field. respond to intensity of light

off-center cells

more action potentials when light is off or when light does not hit center of field. respond to contrasts in light.

How is location of a visual stimulus encoded in nervous system?

I. Comparing input:
-from 2 eyes gives: left/right location, & distance (or depth perception).
-when eyes move up & down gives: up/down location
II. Map-like projection from retina to cortex relays the left/right, up/down location, & distance (depth perception) info

Visual Fields

-close 1 eye. everything you can see w/ your open eye=the visual field for that eye
-visual fields of each eye partially overlap
-region of overlap=area of binocular vision
-see same object w/ both eyes. thus image of object reaches retina of one eye at a slightly different angle from that of the other- results in depth perception.


info in a visual field (eg, "right" visual field or "left" visual field) is processed in the opposite side of the brain

Processed signal...

is sent via optic nerve through lateral geniculate nuclei to visual cortex.
Note: visual field to left is viewed by right side of both eyes & is processed in right visual cortex (& vice versa)


-focal point too near lens, image focused in front of retina


-image focused behind retina


degeneration of accommodation, corrected by reading glasses


cornea or lens not uniformly curved


lack of parallelism of light paths through eyes

Retinal detachment

can result in complete blindness


increased intraocular pressure by aqueous humor buildup


clouding of lens

Macular degeneration

common in older people, loss in acute vision


dysfunction of peripheral circulation

3 divisions of ear

external, middle, & inner ear

external ear

hearing. terminates at eardrum (tympanic membrane). includes auricle & external auditory canal

middle ear

hearing. air-filled space containing auditory ossicles.

inner ear

hearing & balance. interconnecting fluid-filled tunnels & chambers within the temporal bone

external ear 2

-auricle or pinna: elastic cartilage covered w/ skin
-external auditory canal: lined w/ hairs & ceruminous glands. produce cerumen.
-tympanic membrane:
-thin membrane of 2 layers of epithelium w/ connective tissue btwn
-sound waves cause it to vibrate
-border btwn external & middle ear

middle ear 2

-separated from inner air by oval & round windows
-2 passages for air
-auditory or eustachian tube: opens into pharynx, equalizes pressure
-passage to mastoid air cells in mastoid process
-ossicles: malleus, incus, stapes: transmit vibrations from eardrum to oval window
-oval window: connection btwn middle & inner ear. foot of the stapes rests here & is held in place by annular ligament


hearing relies on sensory hair cells in organs (the ear) that respond to the vibrations of sound waves

sound waves

-exist as variations of pressure in a medium such as air.
-are created by the vibration of an object, which causes the air surrounding it to vibrate
-the vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound

vibrations produce...


volume or loudness

function of wave amplitude


function of wave frequency


resonance quality or overtones of sound


tympanic membrane

In the cochlea...

vibrations transmitted from the eardrum through the fluid in the inner ear make the basilar membrane vibrate, bending the hair cells against the tectorial membrane and generating action potentials in afferent neurons that lead to auditory regions of the brain


a spiraled, hollow, conical chamber of bone
-structures include: scala vestibuli, scala tympani, and scala media

scala vestibuli

(containing perilymph), lies superior to the cochlear duct; abuts the oval window (is on outer side of cochlea)

scala tympani

(containing perilymph), lies inferior to the scala media; terminates at the round window (is on outer side of cochlea)

scala media

(containing endolymph), which is the membranous cochlear duct containing the organ of Corti

Endolymph & perilymph

both contain electrolytes & proteins


is rich in potassium (which produces an ionic, electrical potential)


is rich in Na+

Basilar Membrane

-forms part of floor of cochlear duct
-anchors sensory hair cells in Organ of Corti
-vibrates in response to vibrations transmitted through inner ear
-15,000 hair cells located along basilar membrane
-synapse to afferent neurons
-afferent neurons=auditory nerve -> auditory center in temporal lobe of brain

Organ of Corti

-located in the cochlear duct & contains sensory hair cells that detect the sound vibrations transmitted to the inner ear.
-is the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea
-is the sensory epithelium, a cellular layer on the basilar membrane

Detection of sound vibrations

-hair cells arranged in rows. of inner hair cells (responsible for hearing) & outer hair cells (regulate tension on basilar membrane)
-hair bundle: stereocilia of one inner hair cell
-tip link (gating spring) attaches tip of each stereocilium in a hair bundle to the side of the next longer stereocilium.
-as stereocilia bend, they open K+ gates (mechanically gated ion channel)

Basilar membrane

-narrow near the oval window; wider at the outer end of cochlear duct
-high pitched sounds (which are comprised of high frequency vibrations) vibrate the basilar membrane at its narrow, beginning end
-low pitched sounds vibrate the basilar membrane near the wider, farther end

Basilar membrane 2

-hair cells in the organ of Corti are "tuned" to certain sound frequencies by way of their location in the cochlea due to the degree of stiffness in the basilar membrane
-stiffness is due to the thickness & width of the basilar membrane
-basilar membrane is stiffest & narrowest near its beginning (at the oval window, where the ossicle bones transmit the vibrations coming from the eardrum)
-here, it allows only high-frequency vibrations to move the basilar membrane, and thus the hair cells

Basilar membrane 3

-the farther from oval window, the less stiff the basilar membrane is, & thus the less sensitive to high frequencies
-low frequencies travel down the tube, & the less-stiff membrane is moved most easily by them where the reduced stiffness allows
-as the basilar membrane gets less & less stiff, it responds better to lower frequencies

Sounds we hear

-combinations of vibrations at different frequencies & intensities occurring simultaneously w/ different degrees of force along the basilar membrane of the cochlea
-specific action potentials generated in specific neurons are transmitted to auditory centers in the brain, integrated, result in our perception of a specific sound (voice, motor, bird)


mechanoreceptors in humans: vestibular apparatus
-3 semicircular canals
-2 fluid filled chambers: utricle & saccule
-perceives position & motion of head

Semicircular canals

filled w/ endolymph, detects rotational motion. oriented perpendicular

Ampulla of a semicircular canal

Ampulla (region at base of a semicircular canal) detects rotational movement of the head/body & causes movement of the endolymph which displaces the cupula & bends the sensory hair cell -> generates action potentials in afferent neurons that synapse w/ hair cells

Utricle & Saccule

(fluid filled chambers)
-oriented 30 degrees to each other
-info about head position (up/down) & changes in rate of linear motion of body
-contain sensory hair cells w/ stereocilia
-hair cells have a membrane that contains otoliths: calcium carbonate crystals
-when head tilts or body moves non-linearly, otolithic membrane moves -> bends hair cells -> neurotransmitter release -> action potential -> brain perceives movement

Balance: In sum

-gelatinous mass moves in response to gravity, bending hair cells & initiating action potentials
-otoliths stimulate hair cells w/ varying frequencies
-patterns of stimulation translated by brain into specific info about head position or acceleration

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