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Biology 232
Human Anatomy and Physiology
Chapter 17 Lecture Outline
SPECIAL SENSES – sensory receptors in complex sensory organs
OLFACTION – sense of smell; chemoreceptors in olfactory epithelium
odorants – molecules that stimulate olfaction; hundreds of primary odors
Anatomy of Olfactory Epithelium
3 types of cells:
1) olfactory receptors – first-order neurons
bipolar neurons with 1 knob-shaped dendrite with olfactory cilia
cilia have receptors for odorants
olfactory nerve – bundled axons extending through cribriform plate
(olfactory receptors live about 1 month)
2) supporting cells – columnar epithelium; support, nourish, and
electrically insulate olfactory receptors
3) basal stem cells – divide and differentiate to produce new receptors
olfactory (Bowman’s) glands – in underlying connective tissue; secrete
mucus on surface – dissolves odorant molecules
(only odorants dissolved in mucus can stimulate receptors)
Physiology of Olfaction
odorants dissolve in mucus
odorants bind to receptors on olfactory cilia, which produce receptor
potentials
threshold potential produces an action potential, which propagates along
the olfactory nerve, through cribriform plate
(olfactory threshold is low – as little as 4 odorant molecules)
olfactory nerves (cranial nerve I) synapse in olfactory bulbs with
second-order neurons
olfactory tract – second-order neurons to primary olfactory area
in medial temporal lobe
axon collaterals also run to:
limbic system & hypothalamus – emotion and memory
frontal lobe – association area for odor identification
adaptation – occurs rapidly at first, then slows
olfactory receptors adapt little
central adaptation occurs in the olfactory bulbs
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GUSTATION – sense of taste; chemoreceptors in taste buds on papillae of tongue
(some on pharynx)
tastants – molecules that stimulate gustation (must dissolve in saliva)
5 Primary Tastes:
sour, sweet, salty, bitter
umami – meaty or savory
(watery)
taste is augmented by olfaction and tactile sensations
Papillae of Tongue:
circumvallate papillae – about 12 large, circular structures on proximal
tongue; each has up to100 taste buds
fungiform papillae – mushroom-shaped; scattered over tongue; each has
about 5 taste buds
filiform papillae – thread-like; scattered over tongue; no taste buds;
have tactile receptors – detect texture of foods
Anatomy of Taste Buds – oval structures in walls of papillae
3 types of cells:
gustatory (taste) cells – specialized receptor cells
gustatory hairs – long microvilli with tastant receptors
taste pore – opening in taste bud; gustatory hairs pass through
base of gustatory cell synapses with first-order neuron
(gustatory cells live about 10 days)
supporting (transitional) cells – surround gustatory receptors
differentiate to form new gustatory receptors
basal cells – divide and differentiate to form supporting cells
Physiology of Gustation
tastants dissolve in saliva
tastants stimulate gustatory hairs, producing receptor potentials
(gustatory receptors are more sensitive to some tastants)
receptor potentials trigger synaptic vesicles to release neurotransmitters
into synaptic cleft
neurotransmitters bind to receptors on first-order neuron
(first-order neurons have highly branched dendrites synapsing with
many gustatory receptors; different tastes activate different groups
of neurons)
first-order neurons project via cranial nerves VII, IX, X to medulla
second-order neurons to thalamus
third-order neurons to primary gustatory area
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VISION – sense of sight; photoreceptors in retina of eyes
ophthamology – branch of medicine dealing with diseases and disorders of eye
optometry – licensed practice for improvement of vision, and detection and
prevention of associated disorders
Accessory Structures of the Eye
eyelids (palpebrae) – skin flaps that protect eye
lacrimal caruncle – contains sebaceous & sweat glands
Meibomian glands – modified sebaceous glands of eyelids
lubricate lids, prevent sticking together
conjunctiva – protective mucous membrane on surface of eye and
inner eyelids
eyelashes and eyebrows – shade and protect eyes
ciliary glands – sebaceous glands of eyelashes
sty – infected ciliary gland
lacrimal apparatus – cleans, lubricates, and protects eye
lacrimal glands – produce tears (watery fluid containing salts,
mucus, lysozyme – bactericide)
nasolacrimal duct – drains tears into nasal cavity
crying – parasympathetic response to sadness or happiness
extrinsic eye muscles – move eyeball; innervated by cranial nerves III,
IV, VI
superior rectus – upward rotation
inferior rectus – downward rotation
lateral rectus – lateral rotation
medial rectus – medial rotation
superior and inferior obliques – stabilize eyeball
Anatomy of the Eye– 1 inch diameter; recessed in orbit, only anterior 1/6 visible
hollow ball composed of 3 layers
3 layers of eye:
1) fibrous tunic – superficial layer
cornea – anterior transparent portion
avascular, different curvature helps focus vision
3 layers of cornea:
outer - nonkeratinized stratified squamous epithelium
middle – mainly matrix, layers of collagen
inner - simple squamous epithelium
sclera – “white” of eye; dense connective tissue which gives rigid
shape to eyeball; attachment site for extrinsic muscles
2) vascular tunic (uvea) – middle layer
choroid – posterior portion; highly vascular - provides nutrients to
retina
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iris – anterior portion; regulates light entering eye; ANS controls
pupil – opening in iris where light enters eye
pupillary constrictor muscle – decrease pupil diameter
parasympathetic response
pupillary dilator muscles – increase pupil diameter
sympathetic response
ciliary body - between uvea and iris
ciliary processes – posterior epithelial folds which produce
aqueous humor
suspensory ligaments – attach ciliary processes to lens
ciliary muscle – focuses lens
3) neural tunic (retina) – inner layer on posterior 3/4 of eyeball
location of visual receptors
ophthalmoscope – instrument for viewing retina
optic disc – optic nerve (cranial nerve II) and central retinal
artery and vein pass through eye wall; blind spot
2 layers of retina:
pigmented layer – outer layer; epithelial cells contain melanin
which absorbs stray light
neural layer – inner surface layer; contains photoreceptors and
cells which process visual input before passing it to optic
nerve
3 cell layers of neural layer:
photoreceptor layer – outermost layer
rods (125mil) – very sensitive to low light
black and white vision only
found mainly in peripheral retina
disorders = night blindness
cones (6mil) – need more light for excitation
give sharper vision in color
main visual receptors
found mainly in center of retina
disorders = color blindness
macula lutea – center of retina; no rods
central fovea – central depression; 1 layer
highest concentratioin of cones
area of sharpest vision
bipolar cell layer – middle layer; adjusts contrast
of image
bipolar cells, horizontal cells, amacrine cells
ganglion cell layer – inner layer
cell bodies of first-order neurons (CN II)
axons cross retinal surface to optic disc
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Lens – transparent, avascular ball
composed of layers of transparent proteins (crystallins)
elastic connective tissue capsule rebounds to round shape
cataract – opacity of lens or its capsule which obscures vision
Lens Divides Eye Into 2 Cavities:
1) anterior cavity – anterior to lens
anterior chamber – between cornea and iris
posterior chamber – between lens and iris
aqueous humor – watery fluid in anterior cavity
nourishes lens and cornea
ciliary processes – capillaries which filter blood to
produce aqueous humor
scleral venous sinus – drains excess aqueous
humor from eye
2) vitreous chamber – posterior to lens
vitreous body – thick, jelly-like substance that fills
chamber and holds retina in place
forms in embryo, not replaced
intraocular pressure – pressure in eye maintained by aqueous
humor; maintains shape of eyeball
glaucoma – eye disease characterized by increased intraocular
pressure due to decreased drainage of aqueous humor
PHYSIOLOGY OF VISION
photoreceptors – specialized light receptor cells
rods and cones (named for shape of outer segment)
rods – detect presence or absence of light
cones – detect different wavelengths (colors) of light
photopigments – organic molecules that absorb light
2 components of photopigments:
retinal – vitamin A derivative that absorbs light and
changes shape
activates a chain of reactions that result in
production of receptor potentials
opsins (4 types) – glycoproteins that promote absorption of
different wavelengths of light
rhodopsin – rods; absorbs any visible light
3 cone photopigments – absorb blue, green, or red
light wavelengths
light and dark adaptation – period of time required for pigments to
shift between light and dark function; caused by differing
receptor sensitivities and regeneration times
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bleaching and regeneration cycles – after absorbing light,
photopigments breakdown and are rebuilt using
energy from ATP
rhodopsin regenerates slowly, while cone photopigments
regenerate quickly
light adaptation – going from dark to light
vision due mainly to cones; fairly rapid adaptation
dark adaptation – going from light to dark
vision due mainly to rods; slow adaptation
receptor potentials processed by bipolar cells and horizontal and amacrine
cells; increase sensitivity and contrast, and assist in differentiating
colors
ganglion cells (first-order neurons)receive inhibitory and excitatory
neurotransmitter signals from bipolar and amacrine cells; when
threshold stimulus is reached, action potentials are triggered and
conducted via axons to the optic disc
optic nerves formed by bundled axons of ganglion cells
optic chiasm – optic nerves converge and half of axons cross over
optic tracts run to thalamus and synapse with second-order neurons
second-order neurons project to primary visual area of occipital lobes
cerebral hemispheres process information from opposite visual field
binocular vision – depth perception due to overlap of visual fields
Image Formation – 3 processes involved
1) Refraction – bending of light rays as they pass through substances of
differing densities
cornea – 75% of refraction
lens – 25% ; varies due to focusing (accommodation)
produces inverted image on retina (upside-down and left-right
reversal); brain interprets correct orientation
2) Accommodation – focusing light rays on central fovea of retina by
changing curvature of lens = changing refraction
ciliary muscle – circular muscle
contraction reduces tension on lens = lens more round
relaxation increases tension on lens = flattens lens
distant vision (20 ft or more) – parallel light rays; focus on retina
with relaxed ciliary muscle = flattened lens (no work)
near vision (less than 20 ft) – divergent light rays; require more
refraction to focus image on retina
contracted ciliary muscle = rounder lens (work required)
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near point of vision – closest point that you can focus; depends on
elasticity of lens
presbyopia – loss of elasticity of lens with age
children – 3-4 inches
age 40 – 8 inches
age 60 – 31 inches
constriction of pupil aids in focusing near vision – decreases
number of divergent light rays entering pupil
3) Convergence – associated with binocular vision; as object moves
closer eyes must rotate medially to focus it centrally
Refraction Abnormalities – due to misshaped eyeball or lens
emmetropic eye (normal) – distant objects focused on retina
myopic eye (near-sighted) – eyeball too long, unable to focus on
distant objects; correct with less refraction (diverging lens)
hyperopic eye (far-sighted) – eyeball too short, unable to focus
on close objects; correct with more refraction (converging
lens)
astigmatism – irregular curvature of cornea or lens
HEARING AND EQUILIBRIUM
otoscope – instrument for inspecting the ears
Anatomy of the Ear – 3 main regions
1) external ear – collects and channels sound waves into ear
auricle – funnel of elastic cartilage
external acoustic canal – leads from auricle to eardrum; 1 inch canal
in temporal bone (external auditory meatus)
ceruminous glands – specialized sebaceous glands; associated
hairs near external opening; protect ear from foreign
material
cerumen – ear wax
tympanic membrane (eardrum) – partition between external auditory
canal and middle ear; thin, semitransparent membrane composed
of 3 layers:
outer – epidermis
middle – connective tissue with collagen & elastic fibers
inner – simple cuboidal epithelium
2) middle ear – air-filled cavity in temporal bone; lined with epithelium
auditory ossicles – 3 smallest bones in body; articulate by synovial joints
suspended across middle ear by ligaments
malleus (hammer) – attached to inner tympanic membrane;
articulates with incus
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incus (anvil) – middle bone; articulates with malleus and stapes
stapes (stirrup) – articulates with incus; periosteum of bone
attaches to the oval window (opening to inner ear)
2 skeletal muscles – protect ear from loud noises
tensor tympani muscle – attached to malleus
stapedius muscle – attached to stapes
round window – membrane-covered opening to inner ear
Eustachian (auditory) tube – middle ear to nasopharynx
opens to equalize air pressure in middle ear
otitis media – middle ear infection; bacteria enter through
auditory tube
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3) inner ear – location of organs for hearing and equilibrium
bony labyrinth – series of cavities in temporal bone that enclose
membranous organs of hearing and equilibrium
perilymph – fluid similar to CSF within bony labyrinth
3 regions of bony labyrinth:
1) vestibule – central portion
2) 3 semicircular canals – 90 degrees to each other
3) cochlea (snail-shaped) – cochlear duct (part of membranous
labyrinth) divides into 2 channels
vestibular duct – begins at oval window, ends at tip of
cochlea
tympanic duct – begins at tip of cochlea, ends at round
window
membranous labyrinth – series of membranous sacs and tubes in bony
labyrinth
endolymph – fluid within membranous labyrinth
3 regions of membranous labyrinth:
1) saccule & utricle – 2 sacs in vestibule; function in equilibrium
2) 3 semicircular ducts – in semicircular canals; function in
equilibrium
3) cochlear duct – in cochlea; functions in hearing
vestibular membrane – between cochlear duct and
vestibular duct
tectorial membrane – stiff membrane in cochlear duct
basilar membrane – between cochlear duct and tympanic
duct
organ of Corti – sensory organ on basilar membrane
hair cells – mechanoreceptors
stereocilia – hairs which produce receptor
potentials when bent against tectorial
membrane
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PHYSIOLOGY OF HEARING
sound waves – alternating ripples of high and low pressure air originating
from a vibrating object
frequency – number of waves/second (Hz)
high frequency = high pitch
audible range 20-20,000 Hz
amplitude (size) – loudness; measured in decibels (dB)
>90dB hearing protection is required; damages hair cells
Pathway for Hearing:
sound waves funneled into external ear and strike tympanic membrane
producing vibrations varying in frequency and amplitude
malleus transmits vibrations to incus, incus to stapes, and stapes to oval
window (mechanically amplified)
vibrations at oval window membrane produce pressure waves in
perilymph of cochlea
perilymph pressure waves travel from vestibular duct to tympanic duct
to round window (round window prevents echo)
pressure waves produce vibrations of basilar membrane
hair cells bend against tectorial membrane
each segment of basilar membrane vibrates at a different
wavelength – brain interprets as pitch
hair cells (specialized receptor cells) produce receptor potentials
which trigger release of neurotransmitter
(number of hair cells stimulated determines loudness)
neurotransmitter triggers action potential in first-order neurons of
cochlear branch of c.n.VIII, which project to medulla
integration occurs in the inferior colliculi for reflexes
impulses project to the thalamus, then to the auditory cortex
in the temporal lobe
EQUILIBRIUM (Balance)
vestibular apparatus – organs of equilibrium (saccule, utricle,
semicircular ducts)
Static Equilibrium – detect body position relative to gravity
saccule and utricle (otolithic organs)
maculae – thickened regions in walls of each which are
perpendicular to each other
hair cells with stereocilia and 1 large kinocilium
otolithic membrane – glycoprotein layer resting on
hair cells
otoliths – calcium carbonate crystals
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as head tilts, otoliths (due to gravity) pull otolithic
membrane downhill, bending stereocilia
bending stereocilia causes receptor potentials in hair cell
bending towards kinocilium excites hair cell
bending away from kinocilium inhibits hair cell
neurotransmitter triggers action potential in first-order
neurons of vestibular branch of c.n.VIII
Dynamic Equilibrium – detect rotational head movements
semicircular ducts – lie at right angles to each other in 3 planes
anterior and posterior ducts – sagittal and frontal planes
lateral duct – transverse plane
ampulla - dilated portion; contains sensory organ
crista – elevation with hair cells
cupula – gelatinous material coating hair cells
as head moves, ducts and their structures move, but endolymph &
cupula inside lag (due to inertia) and bend hair cells
bending hair cells produces receptor potentials
action potentials in vestibular branch of c.n.VIII
(saccule and utricle also involved in dynamic equilibrium)
vestibular branch of c.n. VIII projects to brain stem and cerebellum
complex integration pathways monitor position and movements and
use input to coordinate movements of eyes, head & neck, and
adjust muscle tone to maintain balance
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