Anatomy & Physiology 34B
Chapter 15 – Special Senses
I. Overview
A. Properties & Types of Sensory Receptors
B. Eye Anatomy & Accessory Structures
B. Formation of an Image
C. Sensory Transduction in the Retina
D. Ear Anatomy
E. Physiology of Hearing
F. Equilibrium
G. Olfactory & Gustatory Senses
II. Properties & Types of Sensory Receptors
A. Sense organs are extensions of the nervous system that
1. respond to changes in the environment and
2. transmit nerve impulses to the CNS
B. To perceive a sensation, the following are necessary:
1. A stimulus (chemical, mechanical, or light) to initiate a
nervous system response
2. A receptor (sensory neuron dendrites or specialized
epithelial cell) must convert the stimulus to a nerve impulse
3. Conduction of the nerve impulse from the receptor to the
brain, via sensory (ascending) projection tracts in the spinal
cord
4. Interpretation of the perception in the brain’s cerebral
cortex, after passing through the medulla, pons, & thalamus
C. Sense receptors may be classified several ways
1. How they are stimulated
2. By their distribution in the body
3. By the origins of their stimuli
4. By the duration of their electrical impulse firing frequencies
D. The way sense receptors are stimulated
2
1. Chemoreceptors respond to chemicals; found in the tongue,
nose, blood vessels
2.
Thermoreceptors respond to heat and cold; in skin
3.
Nociceptors respond to pain; found throughout the body,
except in the brain
4.
Mechanoreceptors respond to physical deformation of the
plasma membrane caused by touch, pressure, stretch,
tension, or vibration; found in skin, viscera, and joints
5. Photoreceptors in the eyes respond to light
E. The distribution of sense receptors in the body
1. General (somesthetic) senses
a. Have receptors throughout the skin, muscles, tendons, joint
capsules, and viscera.
b. Include mechanoreceptors, thermoreceptors,
chemoreceptors, and nociceptors
2. Special senses
a. Are limited to the head and innervated by cranial nerves
b. Include vision, hearing, equilibrium, taste, and smell
F. The origins of stimuli for sense receptors
1. Interoceptors detect stimuli in internal organs and produce
feelings of visceral pain, nausea, stretch, and pressure
2. Proprioceptors sense the position and movements of body
parts; found in muscles, tendons, and joint capsules
3. Exteroceptors sense external stimuli; include receptors for
vision, hearing, taste, smell, touch, and cutaneous pain
G. The duration of their electrical impulse firing frequencies
1. Phasic receptors generate a burst of impulses when first
stimulated, adapt quickly, and stop even if stimuli continues
2. Example: touch, pressure, and smell receptors
3. Tonic receptors adapt slowly and generate impulses
continually
4. Example: nociceptors, proprioceptors
3
III. Eye Anatomy & Accessory Structures
A. The visual system consists of the eye and its accessory structures
B. Accessory structures of the eye either protect the eyeball or
enable eye movement and consist of the following
1. Orbit of the skull surrounds the eye and includes the frontal,
lacrimal, ethmoid, zygomatic, sphenoid, maxilla, & palatine
bones
2. Eyebrows, beneath which are the orbicularis oculi and
corrugator muscles
3. Eyelids & eyelashes
a. Eyelids (palpebrae) are composed of thin skin, CT, and
attached muscles
1) The corners of the eyelids are the medial & lateral canthi
2) The lacrimal caruncle, containing sebaceous &
sudoriferous glands, is found in the medial canthus
3) Conjunctiva - thin, transparent mucus secreting
epithelium that lines the interior eyelid and exposed white
of the eyeball surface.
4) Conjunctivitis is an inflammation of the conjunctiva.
b. Eyelashes have sebaceous glands at the base of the hair
follicles; infection of these glands causes a sty
4. Lacrimal Apparatus includes the lacrimal gland, sac, & duct
a. Lacrimal glands secrete lacrimal fluid (tears) and are found
in the superolateral orbit
b. Tears drain into two small openings (lacrimal puncta),
through the lacrimal canal, into the lacrimal sac, to the
nasolacrimal duct, to the nasal cavity
5. Ocular (eye) Muscles - move the eyeball and are controlled by
6 extrinsic muscles:
Muscle
Location
Innervated by
Action
4
1 Superior
rectus
2 Inferior
rectus
3 Lateral
rectus
4 Medial
rectus
5 Inferior
oblique
6 Superior
oblique
on superior eye
surface
on inferior eye
surface
on lateral eye surface
CN III
(oculomotor)
CN III
looking up
CN VI
(abducens)
CN III
lateral eye
movement
crossing eyes
on medial surface of
eye
circles bottom of eye CN III
looking down
looking up &
laterally
looking down
& laterally
attaches via tendon
CN IV
bet. super. & inf.
(trochlear)
lateral rectus
The rectus muscles all originate on the annular ring, posterior to
eye orbit, and insert on the anterior eyeball.
C. Eye Anatomy - consists of 3 basic layers: the fibrous tunic,
vascular tunic, & internal (sensory) tunic, and internal chambers
1. Fibrous tunic – tough outermost eyeball layer, consists of
a. Sclera - white of the eye composed of collagen & elastic
fibers; optic nerve exits from sclera at back of eye.
b. Cornea - convex, clear part of sclera on the anterior eyeball.
Corneal transplants & Lasix surgery may be performed.
c. Limbus – junction between the sclera & cornea; contains
epithelial stem cells that renew the cornea.
2. Vascular tunic - consists of the choroid, ciliary body, & iris
a. Choroid - thin, dark vascular layer that lines the posterior
5/6th of the internal sclera; its blood vessels nourish the tunics
b. Ciliary body thick, anterior portion that forms an internal
muscular ring toward the front of the eyeball; consists of:
5
1) Ciliary muscle, smooth muscle fibers, controlled by CN
III & parasympathetic nerves
2) Ciliary processes with capillaries that secrete aqueous
humor into the eye’s anterior cavity
3) Suspensory ligaments connect ciliary muscles to the
c. Lens - a thick, clear layer of protein fibers, which controls
eye focus (accomodation) via contraction & relaxation of
ciliary muscles.
1) Presbyopia is the loss of lens elasticity & accomodation.
2) Cataracts are a clouding of the lens.
d. Iris - colored, anterior part of the vascular tunic, continuous
with the choroid; consists of smooth muscle fibers (circular &
radial) that regulate the amount of light in through the
e. Pupil - an opening in the center of the iris that lets light in
3. Sensory tunic (Retina) - innermost eye layer; consists of an
outer pigmented layer and inner neural layer that contains
a. Rods - photoreceptor cells on the peripheral posterior retina;
respond to dim light for black & white vision
b. Cones - photoreceptor cells that provide color vision &
surround a central depression called the fovea centralis
c. Bipolar cells between photoreceptor cells and ganglionic
cells
d. Ganglionic cells between bipolar cells and the vitreous body;
these neurons converge to form the optic nerve
e. The optic disc, found where the optic nerve exits the back of
the eye, has no photoreceptors, thus is a blind spot.
f. The macula lutea, near the optic disk, contains mostly cones
and a pit called the fovea centralis, which has only cones.
g. The ora serrata is the posterior margin of the ciliary body,
where the retina’s neural layer ends.
4. Cavities & Chambers of the Eyeball – the interior eye is
separated by the lens into posterior & anterior segments:
a. Posterior segment is filled with a gel-like vitreous humor
6
b. Anterior segment is filled with watery aqueous humor
secreted by ciliary processes. The anterior segment is further
subdivided into:
1) Anterior chamber - between the cornea & iris
2) Posterior chamber - between the iris & lens
c. Excess aqueous humor drains through the canal of Schlemm
(scleral venous sinus), a ring-like blood vessel between the
iris and ciliary body
d. Glaucoma is an elevated pressure within the eye
1) Often caused by a blockage in the canal of Schlemm, so
aqueous humor builds up
2) Increasing pressure compresses retinal blood vessels and
the optic nerve, leading to blindness if untreated
3) Symptoms occur in late stages and include dimness of
vision, reduced visual field, and colored halos around lights
4) Early detection is through regular eye checks, including
examinations of the field of vision, optic nerve, and
intraocular pressure measured with a tonometer
7
III. Formation of an Image
A. Light entry through the pupil is controlled by two contractile
elements in the iris
1. Pupillary constrictor
a. Circular smooth muscle around the pupil
b. Parasympathetic nerves narrow the pupil in bright light
conditions, and when eye focus changes
c. Both pupils constrict when even one eye is exposed to light –
called the photopupillary reflex
2. Pupillary dilator
a. Myoepithelial cells radially arranged around the pupil
b. Sympathetic nerves widen the pupil in low light conditions
and during stressful conditions
B. Refraction (bending) of light through the eye is due to 4
substances (from outside to inside the eye)
1. Cornea – clear, convex structure on the anterior eye; most light
refraction occurs here
2. Aqueous humor – watery fluid in the anterior segment
3. Lens – clear, biconvex structure between the aqueous and
vitreous humor; fine tunes (focuses) the incoming image
4. Vitreous humor – clear, gel-like material in the posterior
segment
5. Astigmatism – involves an uneven surface on the lens and/or
cornea, which causes light to bend unevenly, causing blurred
vision
C. Emmetropia & the Near Response
1. Emmetropia is the relaxed state of a normal eye when it is
focused on an object 20 ft. or more away; incoming light rays
are parallel and focused on the retina
2. The near response is required to focus the eye on objects closer
than 20 ft.; it consists of the following
a. Convergence of the eyes – pupils begin to move medially
8
b. Pupil constriction to filter out peripheral light rays and
minimize spherical aberrations from the lens
c. Lens accommodation – a change in the curvature of the lens
via the following
1) Ciliary muscle around the lens contracts, narrowing its
diameter
2) Suspensory ligaments that connect the ciliary muscle to the
lens relax, allowing the stretched lens to become more
convex
3) A more convex lens refracts light more and focuses
divergent light rays onto the retina
3. Presbyopia is a loss of lens accommodation due to a gradual
lessening of lens elasticity as we age
4. Myopia (nearsightedness) results from an eye that is too long,
thus distant objects are focused in front of the retina’s focal point
5. Hyperopia (farsightedness) results from an eye that is too short,
thus near objects are focused behind the retina’s focal point
IV. Sensory Transduction in the Retina
A. The retina consists of 2 major layers
1. Pigment layer – posterior layer composed of dark cuboidal
epithelial cells; absorbs light that is not absorbed by receptor
cells in the
2. Neural layer – inner layer near the vitreous humor, composed of
three main layers of neural cells (from pigment layer to vitreous
humor):
a. Photoreceptor cells – first to receive incoming light rays
1) Rods – cylindrical cells containing rhodopsin pigment that
absorbs light; responsible for night vision in black & white
2) Cones – cone-shaped cells that include red, green, and blue
photoreceptors that function in bright light and are
responsible for color vision
3) Rods and cones receive and transmit nerve impulses to
9
b. Bipolar cells – neuron dendrites receive nerve impulses from
rods and cones, then transmit the impulses along axons to
c. Ganglionic cells – neuron dendrites receive nerve impulses
from bipolar cells; axons converge to form the optic nerve that
exits the back of the eye and carries impulses to the brain
B. Visual Pigments in the rods and cones include
1. Rhodopsin (visual purple) in rods consists of two parts
a. Opsin protein
b. Retinal – a light-absorbing molecule made from vitamin A
2. Photopsin (iodopsin) in cones also consist of 2 parts
a. Retinal - the same as that found in rhodopsin
b. Opsin proteins with different amino acid sequences that allow
for 3 types of cones that absorb light of 3 different wavelengths
C. Photochemical reaction of retinal
1. In the dark, retinal has a bent shape called cis-retinal
2. Light converts the cis-retinal to a straighter shape called transretinal, which has a violet color
3. The conversion of cis-retinal to trans-retinal leads to the
production of a visual nerve signal in the rods and cones
4. Trans-retinal loses its color quickly (bleaching), then is
transported to the pigment epithelium, reconverted to cis-retinal,
then transported back to the opsin in the photoreceptors
D. Light & Dark Adaptation
1. Light adaptation occurs when you go from darkness into light
a. Pupils constrict to reduce incoming light
b. Rods bleach quickly, and cones take over in about 5-10 min.
2. Dark adaptation occurs when you go from light to dark
a. Pupils dilate to admit more light into the eye
b. Rhodopsin regenerates faster than it bleaches; rods reach
maximum sensitivity in about 20-30 min.
10
E. Rod & Cone locations are relative to their functions
1. Cones are found predominantly in the retina’s macula lutea and
are the only photoreceptors found in the fovea centralis (focal
point)
a. Each cone synapses with one bipolar cell, which synapses
with one ganglionic cell that sends the image to the brain
b. The one-to-one relationship between the cells produces a
sharp color image in bright light
2. Rods are scattered throughout the peripheral retina
a. One ganglionic cell can receive input from several rods
b. This results in a less clear image in dim light
F. Color Vision is produced by three types of cones
1. Red cone photopsins absorb orangish light wavelengths (558
nm)
2. Green cone photopsins absorb greenish light wavelengths (531
nm)
3. Blue cone photopsins absorb bluish light wavelengths (420 nm)
4. Perception of different colors is based on a mixture of nerve
signals from the three cone types
5. Color blindness is a hereditary condition in which a person
lacks one or more photopsins in their cones, resulting in the
inability to see certain colors
G. Stereoscopic vision (depth perception) occurs because the visual
fields of our two eyes overlap, allowing each eye to see an object
from slightly different angles
11
H. The visual projection pathway from photoreceptors to brain
1. Photoreceptors transmit visual impulses to bipolar cells
2. Bipolar cells transmit impulses to ganglionic cells, the axons of
which form the optic nerves (CN II)
3. Optic nerves partially cross over at the optic chiasma, then
enter the brain via the optic tracts
4. Most optic tract nerve fibers synapse with neurons in the lateral
geniculate nuclei of the thalamus
5. Lateral geniculate neurons form the optic radiation of nerve
fibers that transmit impulses to the visual cortex in the occipital
lobes of the brain for interpretation
6. A few optic tract nerve fibers go to the midbrain to synapse with
the
a. Superior colliculi, which control visual reflexes of the
extrinsic eye muscles
b. Pretectal nuclei, which are involved in the photopupillary
and accommodation reflexes
12
V. Ear Anatomy & Equilibrium
A. Structures of the outer, middle, & inner ear are involved in
hearing. The inner ear also has structures for sense of balance
(equilibrium)
B. Outer (external) ear consists of the:
1. Auricle (pinna) composed of elastic cartilage & skin
2. Auditory canal - a fleshy tube within the external auditory
meatus of the skull, that ends with the
3. Tympanic membrane (eardrum), a thin epithelial partition
between the auditory canal and the middle ear tympanic cavity
a. The TM vibrates in response to incoming sound waves
b. Excess external or internal pressure can cause a perforated
eardrum.
C. Middle ear
1. Tympanic cavity - air-filled cavity in the petrous portion of the
temporal bone
2. Posterior wall has an recessed area called the mastoid antrum
that leads to the mastoid air cells
3. Anterior wall has an opening called the eustachian (auditory or
pharyngotympanic) tube (meatus) that leads to the nasopharynx
a. The eustachian tube is normally flattened
b. Swallowing or yawning opens the tube, allowing air to enter
or leave the tympanic cavity, which equalizes pressure on
both sides of the tympanic membrane
c. Throat infections can also spread to the mid. ear via this tube
d. Otitis media is an inflammation of the middle ear due to
infection; can sometimes be alleviated by a myringotomy, in
which tiny tubes are inserted into the eardrum to drain excess
fluid
4. Medial wall has a bony partition with the oval and round
window that separates the middle ear from the inner ear
13
5. Three auditory ossicles extend from the tympanic membrane to
the oval window
a. Malleus (hammer) - articulates with tympanic membrane
b. Incus (anvil) - articulates with malleus & stapes
c. Stapes (stirrup) - articulates with incus & oval window
6. Skeletal muscles of the middle ear include the
a. Stapedius, which originates on the posterior tympanic cavity
and inserts on the stapes
b. Tensor tympani, which originates on the eustachian tube and
inserts on the malleus
D. Inner Ear - the labyrinth, consists of an outer bony labyrinth
that surrounds and protects the inner membranous labyrinth.
1. Perilymph fluid circulates between the bony & membranous
labyrinths
2. Endolymph circulates within the membranous labyrinth
3. Both fluids conduct vibrations involved in hearing &
equilibrium
4. The bony labyrinth is divided into 3 main areas; the vestibule,
semicircular canals, and cochlea
5. Vestibule - central part of the bony labyrinth; controls balance
& equilibrium and contains the:
a. Vestibular (oval) window, into which the stapes fits,
b. Cochlear (round) window on the cochlear end.
c. The membranous labyrinth within consists of 2 connected
sacs, the
1) Utricle - larger sac, in the upper back of the vestibule,
2) Saccule - smaller sac
6. Macula in both sacs contain:
a. Epithelial support cells
b. Receptor hair cells are embedded in an overlying otolithic
membrane containing calcium carbonate otoliths; together
14
they sense equilibrium and linear acceleration and send
impulses to
c. The vestibular nerve, which joins the cochlear nerve to form
the vestibulocochlear nerve (CN VIII)
7. Semicircular canals - 3 bony canals posterior to the vestibule
and positioned at right angles to each other. The membranous
labyrinth contains the
a. Semicircular ducts, each of which has a
b. Membranous ampulla at one end and connects with the
utricle and houses the cristae ampullaris, which contains:
1) Supporting cells – epithelial cells that surround the
2) Receptor hair cells embedded in a gel-like cupula; these
sense head rotation and send impulses to the vestibular
nerve
8. Cochlea (snail) - coiled 2½ times around bone, contains 3
chambers
a. Upper scala vestibuli (vestibular duct) begins at the
vestibular window, extends to the end (apex) of the cochlea,
and contains perilymph fluid
b. Lower scala tympani (tympanic duct) begins at the apex,
ends at the cochlear window (secondary tympanic
membrane), also contains perilymph
c. Both scalas are separated except at the cochlear apex, where
they join via a canal called the helicotrema
d. Between the vestibular and tympanic ducts is the cochlear
duct (scala media), a triangular middle chamber that ends
where the other two ducts join; it contains the
1) Vestibular membrane - roof of the duct
2) Basilar membrane - floor of duct
3) Endolymph fluid
4) Organ of Corti (spiral organ) - sound receptors that
transform mechanical vibrations into nerve impulses are
found in the basilar membrane here
15
a) The epithelium consists of supporting cells & hair cells
b) The base of the hair cells are anchored in the basilar
membrane and their stereocilia tips are embedded in the
gelatinous tectorial membrane
c) Sound induces perilymph movement, which causes the
hair cells in the tectorial membrane to bend, exciting
sensory cells, which release neurotransmitter to the
cochlear nerve
9. Cochlear sensory neurons in the vestibulocochlear nerve (CN
VIII) send impulses to the brain’s medulla, to the inferior
colliculi, to the thalamus, to the auditory cortex in the temporal
lobe, where the impulses are perceived as sound
VI. The Physiology of Hearing
A. Auditory ossicles in the middle ear
1. Incoming sound waves vibrate the tympanic membrane
2. Tympanic vibrations move the malleus, incus, and stapes
4. The stapes moves against the oval (vestibular) window
5. The tympanic reflex helps to protect the ear from loud sounds
a. Tensor tympani tenses the tympanic membrane
b. Stapedius muscle limits stapes movement
c. Sudden loud noises can still damage hearing by fracturing the
stereocilia of the cochlear hair cells
B. Stimulation of cochlear hair cells
1. Stapes movement causes waves in the scala vestibuli perilymph
2. Perilymph movement pushes the vestibular membrane down
3. Vestibular membrane movement pushes on endolymph in the
cochlear duct
4. Endolymph movement pushes the basilar membrane down & up,
bending the hair cells’ stereocilia in the tectorial membrane,
sending auditory nerve impulses to the cochlear nerve
5. Basilar movement pushes on the perilymph of the scala tympani,
causing the round window membrane to bulge out and in
16
C. Sensory Coding – how we distinguish loudness (amplitude) and
pitch (frequency)
1. Loud sounds cause the organ of Corti to vibrate more
vigorously
a. More hair cells are excited over a larger area of the basilar
membrane
b. More nerve impulses are sent to the cochlear nerve
c. The brain detects intense activity from a large region of the
organ of Corti, and interprets it as loud sound
2. Sound frequency (pitch) is distinguished by the region of the
basilar membrane that is vibrated
a. Collagen fibers span the length of the basilar membrane,
increasing in length from the base (oval window) to the apex
b. Audible sound waves with frequencies between 20,000 – 20
Hz are detected at different areas along the basilar membrane
c. Audible sound waves set up perilymph waves that travel
through the scala vestibuli, flexing the vestibular membrane,
transferring waves to the cochlea, which flexes the basilar
membrane
d. Higher frequency waves stimulate fibers near the base of the
basilar membrane, vibrating the membrane at the high
frequency
e. Lower frequency waves stimulate fibers near the basilar
membrane apex, vibrating the membrane at the low
frequency
f. The stimulation of specific hair cells along the basilar
membrane length is interpreted in the brain as sound of a
certain pitch
D. The Auditory Projection Pathway – sound impulses are sent to
the brain in the following sequence: cochlear hair cells  bipolar
17
neurons  cochlear nerve  vestibulocochlear nerve  pons 
inferior colliculi  thalamus  auditory cortex (temporal lobe)
1. Vibration of the cochlear basilar membrane bends hair cell
stereocilia in the tectorial membrane, sending a nerve impulse to
2. Bipolar neurons in the spiral ganglion that curves around the
modiolus, which sends the impulse to the
3. Cochlear nerve that joins with the vestibular nerve to form the
4. Vestibulocochlear nerve (CN VIII), which sends the impulse to
the
5. Pons in the brain stem, which transmits the impulse to the
6. Inferior colliculi (auditory reflex center) in the midbrain to the
7. Thalamus (medial geniculate nuclei), which relays the impulse
to the
8. Primary auditory cortex in the temporal lobe where it is
interpreted as sound
E. Deafness (hearing loss) may be caused by two different sources
1. Conduction deafness – results from damage to structures that
transmit sound vibrations to the inner ear
2. Nerve (sensorineural) deafness – results from damage to
cochlear hair cells or other nerve tissue involved in hearing
IX. Equilibrium (coordination and balance)
A. Two types of equilibrium are
1. Static – the perception of head orientation when the body is
stationary
2. Dynamic – the perception of motion or acceleration.
B. Two types of acceleration include
1. Linear – a change in velocity in a straight line
2. Angular – a change in the rate of rotation
C. Inner ear organs responsible for equilibrium are the
1. Utricle & Saccule in the vestibule sense static equilibrium and
linear acceleration
18
2. Semicircular ducts sense angular acceleration (head rotation)
D. Utricle & Saccule sensory structures include the
1. Macula – a patch of sensory hair cells surrounded by epithelial
support cells
2. Each hair cell has many stereocilia and one kinocilium
embedded in an otolithic membrane
3. The membrane also contains tiny calcium carbonate “stones”
called otoliths
4. When the head moves, the otolithic membrane slides with
gravity and bends the hair cells, which send a nerve impulse to
the vestibular nerve
5. The brain compares input from the utricle & saccule to
determine head orientation, particularly in respect to linear and
vertical acceleration
E. Semicircular Ducts sensory structures within include the
1. Ampulla – expanded end of the ducts where they join the
vestibule, contain the
2. Crista ampullaris, which houses the receptor hair cells,
epithelial support cells, and cupula
a. Hair cells have stereocilia and a kinocilium embedded in a
b. Cupula – a gelatinous membrane that extends from the crista
to the roof of the ampulla
c. When the head turns the duct rotates, but the endolymph lags
behind and pushes the cupula, which bends the stereocilia and
stimulates the hair cells
d. The hair cells transmit nerve impulses to the vestibular nerve,
which joins the cochlear nerve to send the impulses to the
brain, which interprets the head rotation
F. Equilibrium Projection Pathways to the Brain
1. Stimulated hair cells in the macula and the ampulla send nerve
impulses to the
19
2. Vestibular nerve, which transmits the impulses to the
a. Cerebellum, which is controls skeletal muscle coordination
and balance
b. Pons vestibular nucleus, which sends the impulses to the
c. Cervical spinal cord, which sends the impulses to the cranial
nerves that control eye movement (CN III, IV, VI) and the
muscles of the head and neck (CN XI)
X. Olfactory & Gustatory Senses
A. Olfactory Sense (sense of smell)
A. Olfactory receptors are dendritic endings of the olfactory nerve
(CN I) that respond to chemical stimuli
B. Odor impulses are transmitted it to the olfactory cortex via the
following pathway:
1. Olfactory receptor cells in nasal epithelium receive the
stimulus
2. The impulse is transmitted via olfactory nerves (CN I) that
extend through the cribriform plate of the ethmoid bone to the
3. Olfactory bulbs on both sides of the crista galli, beneath the
frontal lobes. Neurons here convey the impulse to neurons of
4. Olfactory tract, which transmits the impulse to the
5. Olfactory cortex within the temporal and frontal lobes, where it
is interpreted as odor
B. Gustatory Sense (sense of taste)
1. Gustatory receptors are specialized epithelial cells, clustered
in taste buds, that respond to chemical stimuli
2. Taste buds are lemon shaped structures composed of gustatory
cells surrounded by supporting cells and basal cells
3. Each gustatory cell has a microvilli taste hairs that extend
through a taste pore on the taste bud surface
4. There are 3 major types of tongue papillae:
a. Vallate (circumvallate) papillae - largest but least
numerous, arranged in an inverted V on the back of tongue
20
b. Fungiform papillae - knoblike papilla on the tip & sides of
tongue; both fungiform & vallate papillae contain taste buds
c. Filiform papillae - short, thick, threadlike; on the anterior
2/3 of tongue; contains no taste buds
5. Five basic tastes are sensed in different parts of the tongue:
a. Sweet - tip of tongue
b. Sour - sides of tongue
c. Bitter - back of tongue
d. Salty - most of tongue, esp. on edges
e. Umami – a meaty taste produced by amino acids
6. Taste impulses are transmitted via the glossopharyngeal (CN
IX) and facial (CN VII) nerves to the gustatory cortex in the
parietal lobes for interpretation