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THE SPECIAL SENSES
BIO 137 Anatomy & Physiology I
Special Senses
• Receptors for the special senses of smell, taste,
vision, hearing, and equilibrium are anatomically
distinct from one another and are concentrated in
specific locations in the head
• In addition to the stimuli and the receptors, there are
specific afferent pathways and translation sites in the brain
for information assembled from these special senses
Special Senses
 Comparing
the general senses and the special senses
General Senses
– Include somatic sensations
Special Senses
–Include smell, taste, vision,
(tactile, thermal, pain, and
hearing and equilibrium
proprioceptive) and
–Are concentrated in specific
visceral sensations
locations in the head
– Are scattered throughout
the body
– Are relatively simple
structures
–Are anatomically distinct
structures
–Form complex neural
pathways
Olfaction and Taste
• Olfaction is the process of perceiving smells. Smell and
taste are brought about through the interpretation of
chemicals present in the environment
• Olfactory and gustatory (taste) impulses travel not only to the
cerebral cortex, but also to the limbic system
• We can have emotional responses and strong memories to certain smells
and tastes
• gustation and olfaction work together. Olfaction is much stronger/more
sensitive (when someone has a cold it is difficult to taste food)
Olfaction
• The olfactory epithelium
is located in the superior
part of the nasal cavity
covering the surface of the
cribriform plate and
extending along the
superior nasal concha
Olfaction
• The olfactory apparatus can detect about 10,000 different
odors, often in concentrations as low as 1/25 billionth of a
milligram per milliliter of air
• When an odorant binds to the
receptor of an olfactory hair it
initiates a cascade of intracellular
events
Olfaction
• Once generated, nerve impulses travel through the two
olfactory nerves  olfactory bulbs  olfactory tract 
primary olfactory area in the temporal lobe of the cortex
• Olfaction is the only sensory
system that doesn’t go through the
relay stations in the thalamus
Olfaction
• Olfactory sensory pathways (centrally) are rapidly
adapting, decreasing activity by 50% in the first second, and
completely accommodating in 1–2 minutes
Olfactory Nerve Pathways
1. Odorant molecules enter as gases and
2.
3.
4.
5.
dissolve in liquid
Bind olfactory receptor cells
Nerve Impulse travels on fibers through
cribriform plate of ethmoid bone
Fibers Synapse with neurons of olfactory
bulbs (CNI)
Travel along olfactory tracts to limbic
system (amygdala) and cerebral cortex
Gustation
• Gustation, or taste, detects five primary tastes : sour,
sweet, bitter, salty, and umami (“meaty” or “savory”)
Sense of Taste
• Taste Buds are organs of taste
• Humans have 10,000 of these located on papillae of
tongue, roof of mouth, cheek linings, walls of pharynx
• Taste Receptors are chemoreceptors, called taste cells that
are located on taste buds
• Taste cells have receptive microvilli (taste hair) that project
through a taste pore
• Taste hair is sensitive portion
that detects chemicals
dissolved in saliva
• Undergo rapid
sensory adaptation
Gustation
• In addition, the entire surface of the tongue has filiform
papillae that contain tactile
receptors but no
taste buds
•They increase
friction between the
tongue and food,
making it easier to
move food in the oral cavity
Gustation
• Three cranial nerves contain axons gustatory neurons that
innervate the taste buds
• The facial (VII) nerve serves taste buds in the anterior 2/3 of the
tongue
• The glossopharyngeal (IX)
nerve serves taste buds in the
posterior 1/3 of the tongue
• The vagus (X) nerve serves taste
buds in the throat and epiglottis
Gustation
• Nerve impulses propagate along these cranial nerves to the
gustatory nucleus in the medulla oblongata. From there,
axons carrying taste signals project
to the hypothalamus, limbic
system, and thalamus
• Taste is perceived consciously as
signals from the thalamus arrive
at the primary gustatory area at
the base of the somatosensory
cortex in the parietal lobe
Taste Pathways
• Taste signals enter the brain at the medulla from 3 cranial
nerves
• Facial Nerve, VII, carries impulses from anterior 2/3 of
tongue
• Glossopharyngeal Nerve, IX, carries impulses from
posterior 1/3 of tongue
• Vagus Nerve, X, carries impulses from base of tongue &
pharynx
• Impulses then travel to the Thalamus and to the cerebral
cortex
• Taste impulse then travels to the limbic system for an
emotional tag
Vision
• Our visual perception is dependent on the eye, its
accessory structures, the optic tracts, and the 1o visual
cortex and it’s association areas
• Vision is possible because of
photoreceptors that
detect light
Accessory Eye Structures
• The upper and lower palpebrae are the eyelids
• CN III (Oculomotor) supplies 4 of the 6 extraocular
muscles, plus the muscles that raise the upper eyelid
• The conjunctiva is a clear
mucous membrane that
covers the white (avascular)
part of the eye
Accessory Eye Structures
• The lacrimal glands are each about the size an almond,
situated superolateral to the eyeball. Leading from the
lacrimal glands are 6 to 12 excretory lacrimal ducts
• Tears (lacrimal fluid) run from the lacrimal glands, into the excretory
lacrimal ducts, onto the surface of the conjunctiva, over the surface
of the eyeball
Accessory Eye Structures
• Tears drain into the lacrimal puncta, which are two openings
on the nasal side of the extreme edge of the eyeball.
Superior and inferior lacrimal canals empty the tears into
the nasolacrimal sac and nasolacrimal duct
• The right and left sided nasolacrimal ducts empty into each side of
the nose
Accessory Eye Structures
• Watery eyes occur when lacrimal fluid builds up, as when
something obstructs the nasolacrimal ducts
• Blocked nasolacrimal ducts can be caused by an inflammation of
the nasal mucosa, such as a cold
• Over production of lacrimal fluid occurs in response to
parasympathetic stimulation, caused by an emotional response
(crying), and tears spill over the edges of the eyelids and drain into the
nasal cavity (causing nasal stuffiness)
Anatomy of the Eye
The wall of the eyeball consists of three layers or tunics:
1. The fibrous tunic is the outer layer and is composed of the
sclera (“white” of the eye) and the cornea (the transparent
epithelium the protects the front of the eye)
2. The vascular tunic is the middle layer and is composed of
the choroid, the ciliary body and the iris
3. The nervous tunic is the
inner retinal layer
Middle Tunic
• The choroid forms the major vascular portion that lines
the internal surface of the sclera
• The ciliary body consists of two parts:
• The ciliary processes that secrete aqueous humor
• The ciliary muscle that changes the shape of the lens to adapt to near
and far vision
• The iris is the colored portion of the eyeball consisting of circular and
radial smooth muscle fibers
Middle Tunic
• Suspensory ligaments extend inward from the ciliary
processes and hold the lens of the eye in place
• Lens is a clear, membrane like structure that divides the
eyeball into an anterior and posterior chamber
• Lens is very elastic and can change shape due to changes in ciliary
muscles and suspensory ligaments = Accommodation
Aqueous Humor
• fluid in anterior cavity of eye
• secreted by epithelium on inner surface of the ciliary body
• provides nutrients for cornea/lens
• maintains shape of anterior portion of eye
• Normally drains back to bloodstream
• What is glaucoma?
•Vitreous humor – thick gel of posterior cavity that holds
retina flat against choroid coat & maintains round
shape of eyeball
Structure of Retina
• Retina contains visual receptors called photoreceptors (rods
and cones)
• 3 layers
• Outer
• Photoreceptors (rods & cones)
• Middle
• Bipolar neurons
• Inner
• Ganglion cells (axons join & form optic nerve)
• Light passes through ganglion and bipolar neurons before
reaching photoreceptors
Retinal Layers
Anatomy of the Eye
• The retina consists of two
types of photoreceptor cells,
rods and cones
• Rods are abundant in the
periphery of the retina whereas
cones are found more frequently
in the central areas
Anatomy of the Eye
• Each eye contains ≈ 120 million rod-
shaped photoreceptors that are
adapted for a low light threshold
(high sensitivity) - they produce low
resolution, black and white images
• a loss of rods with age makes it difficult to
drive at night
Anatomy of the Eye
• Cone-shaped photoreceptors function in bright light to
produce high resolution color images
• They exists in three varieties,
corresponding to the type of
pigment they contain: red, green or blue
• The photopigments are concentrated in
the outer segment of the receptor, while the
inner segment contains the
nucleus and organelles
Retina
•
The exact center of the retina is called the macula lutea, and
in its center is a small depression called the central fovea (or
fovea centralis)
• There are no rods or nerve cells in the fovea, only a high
concentration of cones - this gives us the sharp central
vision
• Optic disc - back of retina, site where nerve fibers leave
retina
•Central retinal artery
enters and central
retinal vein exits
• BLIND SPOT
•CONTAINS NO
PHOTORECEPTORS
The Pupillary Response
• The pupil is an opening in the center of the iris. It is
composed of a radial muscle that “radiates” away from the
center, and a circular muscle that is in the center
• Contraction of the inner circular muscle fibers (bright light) cause the
pupil to constrict
• Contraction of the radial
Fibers (dim light) causes
it to dilate
Accommodation
• Ability to adjust strength (shape) of lens in order to focus
both near and far sources on retina
• Controlled by ciliary muscles and suspensory ligaments
• Normal image formation depends on refraction of light
waves, accommodation of the lens, constriction of the
pupil, and convergence of the two eyes
Refraction : Focusing Light
•Refraction is the bending of light rays in order for
them to be focused on the retina
•Refraction occurs when light reaches the cornea
and again when it reaches the lens
•This is the basis for corrective measures for vision
Refraction
• If the shape of the eye is normal, light waves are focused
sharply on the retina
• Image detected on retina is upside down & backward
because of bending of light rays
• Image is interpreted correctly in visual cortex
Refraction Disorders
• Myopia
• Nearsightedness
• Light focused in front of
retina
• Corrected with concave
lens
• Hyperopia
• Farsightedness
• Light focused behind
retina
• Corrected with convex
lens
HOW DOES THE BRAIN
INTERPRET WHAT WE
SEE?
Light must reach photoreceptors in retina
and be transformed into electrical (neural)
signals in the CNS
Visual Transduction
• The first step in visual transduction is absorption of light by
a photopigment, a colored protein that undergoes
structural changes when it
absorbs light in the outer
segment of a photoreceptor
•Light absorption initiates
a series of events that
lead to the production
of a receptor potential
(number 4 in the diagram)
Phototransduction
• Conversion of light stimuli into neural signals
• Mechanism same for all photoreceptors
• Occurs in bright light
1. Photopigment absorbs light & breaks down to 2
components
2. Hyperpolarizes rods/cones
3. Decrease in release of inhibitory neurotransmitter
• What is the effect in dim light?
HOW DOES THE RETINA SIGNAL
THE BRAIN IF
PHOTOTRANSDUCTION INVOLVES
AN INHIBITORY
NEUROTRANSMITTER?
• Prevent release of inhibitory neurotransmitter =
Excitation
• Light: remove inhibitory NT = Excite
• Dark: presence of inhibitory NT = Inhibition
Visual Pathway
• Fibers partially cross
over in optic chiasm
• Right half of brain
interprets image from left
eye and vice versa
• How does this affect
damage to a particular
area of optic pathway?
Structure of Eye: Anterior to Posterior
• Cornea
• Aqueous Humor
• Iris
• Pupil
• Lens
• Vitreous Humor in Posterior Cavity
• Retina
• Choroid (continues anteriorly to form ciliary body)
• Sclera
PATHWAY OF LIGHT
• OPTIC NERVE
• CORNEA
• OPTIC CHIASM
• AQUEOUS HUMOR
• OPTIC TRACT
• LENS
• THALAMUS
• VITREOUS HUMOR
• PRIMARY VISUAL
• PHOTORECEPTORS
OF RETINA
CORTEX OF
OCCIPITAL CORTEX
Sense of Hearing and Equilibrium
• The ear is the organ of hearing
•The ear also contains receptors for equilibrium
The Ear
• The ear has 3 principle regions
• The external ear, which uses air to collect and channel sound waves
• The middle ear, which uses a
bony system to amplify
sound vibrations
• The internal ear, which
generates action potentials to transmit
sound and balance information to the brain
Auditory & Vestibular Sense
External Ear:
Middle Ear:
Inner Ear:
auricle (pinna)
external auditory meatus
tympanic membrane
ossicles (malleus, incus, stapes)
tympanic cavity
Eustachian tube
oval window
cochlea
vestibule
semicircular canals
Fig 12.9
External Ear
• auricle (pinna)
• collects sounds waves
• external auditory meatus
• directs sound waves
toward tympanic
membrane
• lined with ceruminous
glands & hairs
• terminates with
tympanic
membrane
12-25
Middle Ear or Tympanic Cavity
• air-filled space in temporal bone
• tympanic membrane (eardrum)
• vibrates in response to sound
waves
• auditory ossicles
• vibrate in response to
tympanic membrane
• vibrations are concentrated
and amplified by ossicles
• malleus, incus, and stapes
• Vibration of ossicles causes
movement of fluid in inner ear
Eustachian (Auditory) Tube
• connects tympanic
cavity (middle ear
cavity) to nasopharynx
• helps maintain equal
pressure on both sides
of tympanic membrane
• usually closed by
valve-like flaps in
throat
12-27
Eustachian Tubes: Air Pressure Changes
• Traveling from high altitude to low altitude causes an ↑
in air pressure outside tympanic membrane
• Pushes tympanic membrane inward
• Swallowing, yawning or chewing allow valves in throat to open,
air enters tube and equalizes pressure on inside of eardrum
• “popping sound”
Middle Ear Infections
• Mucous membranes lining eustachian tube are
continuous with lining of middle ear
• Passageway for bacteria from throat or nasal passages
to middle ear
• Children have shorter eustachian tubes, so
infection more frequent in children
• Insertion of ‘tubes’ through tympanic membrane
into eustachian tube allows ears to drain
Inner Ear
• Complex system of intercommunicating chambers and
tubes called a labyrinth
•
• Osseous labyrinth
• bony canal in temporal
bone filled with fluid perilymph
• Membranous labyrinth
• tube that lies within osseous labyrinth
• filled with fluid endolymph
Inner Ear:
Labyrinths
Inner Ear
Oval window – entrance to
inner ear, covered by stapes
3 Parts of Labyrinths
• cochlea
• functions in hearing
• semicircular canals
• functions in
equilibrium
• vestibule
• functions in hearing
and equilibrium
12-29
Cochlea
• Coiled tubular system
• 3 fluid filled longitudinal
compartments
• Scala vestibuli
• Cochlear Duct
• Scala tympani
• Helicotrema – apex
where fluid in top and
bottom is continuous
Sense of Hearing: Cochlea
Scala vestibuli
• upper bony compartment of labyrinth
• leads from oval window to apex of spiral
• filled with perilymph
Scala tympani
• lower bony compartment of labyrinth
• extends from apex of the cochlea to round window
• filled with perilymph
These are separated by cochlear duct filled with endolymph
Inner Ear: Cochlea
Sense of Hearing: Cochlea
Cochlear duct
• portion of membranous
labyrinth within cochlea
• filled with endolymph
Vestibular membrane
• forms roof of cochlear
duct
Basilar membrane
• forms floor of cochlear
duct
• Sense organ for
hearing located here
12-31
Cochlear Anatomy
• Scala vestibuli
• Vestibular membrane
• Cochlear duct
• Basilar membrane
• Floor of cochlear duct
• Organ of Corti located
here
• Scala tympani
Sense of Hearing: Organ of Corti
• Organ of Corti is located
on the upper surface of the
basilar membrane
• Hearing receptor cells are
located on the organ of
Corti
• Hair cells covered by a
tectorial membrane
• These receptors are
mechanoreceptors
Movement of Sound
• Vibrations of ossicles in middle ear cause movement of
fluid in inner ear
• Results in vibrations of basilar membrane, and bending of
hair cells on Organ of Corti
• Hair cells bend against tectorial membrane
• Nerve impulse initiated on cochlear branch of CN VIII
Auditory Pathway
• Vibrations to basilar membrane
• Hair receptor cells bend against tectorial membrane
• Ca++ enters receptor cells
• Neurotransmitter from hair cells released to outside
• Stimulates nearby sensory fibers
• NI carried along cochlear branch of Vestibulocochlear
nerve (CNVIII)
• Medulla Oblongata (fibers cross-over here)
• Thalamus
• Temporal Cortex
• Impulses from each ear interpreted on both sides of
brain
The Sense of Hearing:
Auditory Nerve Pathways
12-34
Sound Pathway
• Auricle, External Auditory Meatus
• Tympanic Membrane, Malleus, Incus, Stapes
(through oval window to)
• Perilymph of scala vestibuli, vestibular membrane
• Endolymph of cochlear duct
• Hair cells of Organ of Corti. NT released &
stimulates sensory neuron
• Travels along Cochlear branch of
vestibulocochlear nerve (CNVIII)
• Medulla to Thalamus to
• Primary auditory cortex of temporal lobe
Hearing Loss
• Conductive
• Interference with transmission of vibrations to inner ear
• Physical blockage, eardrum rupture, middle ear
infections, restriction of ossicle movement
• Can be treated with hearing aids
• Sensorineural
• Sound waves transmitted, no neural processing
• Damage to Vestibulocochlear Nerve or cochlea
• Can sometimes be treated with cochlear implants
Sense of Equilibrium
• Feeling of equilibrium is derived from two senses
• Involves semicircular canals & vestibule
• Vestibule connects semicircular canals to cochlea
Static Equilibrium
• senses
movement of
head when body
is still
• vestibule
Dynamic Equilibrium
• senses rotation and
movement of head when
they suddenly move or
rotate
• Semicircular canals
12-36
Sense of Static Equilibrium: Vestibule
• Receptor cells send
info along vestibular
branch of CN VIII
•Motor impulses are
then sent out to help
maintain balance
Vestibular Sense: How the body handles
movement
• Sensory receptors in inner ear give you information about
movement, gravity and vibration
• Keeps you upright when you bend or rotate the head (postural
adjustments)
• Quick start and stop movements give very intense
vestibular stimulation
Vestibular Input
• Jumping up and down
• Running
• Swinging
• Spinning
• Vestibular system is the primary organizer of sensory
input