



Special senses are those organs and receptors
that are associated with touch (sensory
receptors), vision, hearing, equilibrium, smell,
and taste
Sight, hearing, smelling are distance senses.
They bring in information from far away
Touch and taste can only reveal information
about things you come in direct contact with.
These special senses receive stimuli from the
sensory receptors and transmit these impulses
to the brain for interpretation





Sensory receptors are structures that are stimulated by
changes in the environment.
Sensory receptors for pain, touch, temperature, and
pressure are found all over the body in the skin,
connective tissue, and muscle.
Special sensory receptors including the taste buds of
tongue, special cells in nose, retina of eye, special cells in
inner ear (organ of Corti)
When a sense organ is stimulated, the impulse travels
along nerve pathways to the brain where it is registered.
Sensation actually takes place in brain but is mentally
referred back to the sense organ. This is called projection
of the sensation.
Sensory receptors become less sensitive with age
(decrease in number of receptors make it difficult for
elderly to feel pain or cope with changes in temperature)
Human eye is a tender sphere about 1 inch in
diameter.
 Protected by orbital socket of skull, eyebrows,
eyelids, and eyelashes
 When we blink the eyes are bathed in fluid by
tears secreted from lacrimal glands (underside
of upper lid of each eye with duct at inner corner
of each eye *also connected to nasal lacrimal
duct*)

 Lacrimal secretions contain lysozymes which help
combat bacterial infections. Tears cleanse and
moisten the eyes on a continuous basis




Canthus is the angle where the upper and lower
eyelids meet
Conjunctiva is the thin membrane that lines the
eyelids and covers part of the eye. It secretes
mucus that helps lubricate the eye
Location of eye allows for superimposition of
images from each eye allowing us to see
stereoscopically in three dimensions (length,
width, and depth)
Wall of eye is made of three concentric layers, or
coats (sclera, choroid, retina)



Outer layer, or white of the eye
Tough, unyielding, fibrous capsule that
maintains the shape of the eye and protects
the delicate structures within
Muscles responsible for moving the eye
within the orbital socket are attached to the
outside of the sclera
 Muscles are referred to as extrinstic muscles





Referred to as the “window” of the eye
Transparent front of sclera to permit passage
of light rays
Cornea consists of 5 layers of flat cells
Possesses pain and touch receptors so it’s
sensitive to foreign particles
No blood vessels so transplantation can occur
without rejection



Middle layer of the eye
Contains blood vessels to nourish the eye and a
non-reflective pigment rendering it dark and
opaque
 Pigment darkens eye chamber preventing light
reflection within eye
In front, choroid coat has a circular opening called
the pupil
 Colored, muscular layer surrounds the pupil
called the iris (may be blue, green, gray, brown,
or black…related to number and size of melanin
pigments cells in the iris

Within iris are two sets of antagonistic smooth
muscles, the sphincter and dilator pupillae.
These intrinsic muscles help iris to control
amount of light entering the pupil.
 Sphincter muscles contract making pupil
smaller
 Dilator muscle contracts making pupil larger


Lens is crystalline structure located behind iris
and pupil
Elastic, disc-shaped structure with anterior and
posterior convex surfaces that form a biconvex
lens (posterior more curved than anterior)
 Curvature of each structure alters with age

Lens is held in place behind the pupil by
suspensory ligaments from ciliary body of
choroid layer
Innermost, or third coat of the eye
Located between posterior chamber and choroid
coat
 Does not extend around the front portion of eye
 Light-sensitive layer that light rays from an object
form an image
 Contains pigments and specialized cells known as
rods and cones which are sensitive to light


 Rod cells are sensitive to dim light and cones sensitive to
bright light
 Cones also responsible for color vision
 Three varieties of cone cells and each type is sensitive to
a special color




Part of retina where the nerve fibers enter the
optic nerve to go to the brain is called the
optic disc
Contains no rods or cones
Unable to convert images into nerve impulses
Aka blind spot


Focus point for light rays for best visual acuity
Composed mostly of cone cells which
specialize in bright light

Images in Light  cornea pupil lens
(where the light rays are bent or refracted)
retina (rods and cones pick up the stimulus)
optic nerve occipital lobe (cerebrum) of the
brain for interpretation

Clearness/sharpness of visual
perception recorded as two
numbers
 First number represents the distance
in feet between the subject and the
test chart (Snellen Chart)
 Second number represents the
number of feet a person with normal
acuity would stand to see clearly
 20/20 is considered normal acuity
 20/200 or worse is considered legally
blind











Myopia: nearsighted (focus falls short of retina)
Hyperopia: farsighted (focus falls behind retina)
Astigmatism: focused image is distorted (cornea is not spherical)
Color blindness: congenital lack of one or more types of cones; sex-linked
Conjunctivitis: pink eye (inflammation of conjunctival membranes)
Glaucoma: destruction of retina and atrophy of optic nerve (excessive
intraocular pressure)
Cataract: gradual blurring and loss of vision (lens becomes cloudy)
Macular degeneration: dimming or distortion of vision that is most
obvious when reading (gradual thinning of retina)
Detached retina: loss of peripheral vision and then loss of central vision
(tear in retina)
Sty: eye is red, swollen, and painful (tiny abscess at base of eyelash
caused by inflammation of sebaceous gland of eyelid)
Strabismus: crossed eyes (muscles of eyeball do not coordinate their
action)

Adapted to pick up sounds waves and send these
impulses to the auditory center of the brain
 Auditory center is located in the temporal area just above
the ears
 Receptor for hearing is the delicate organ of Corti which is
located within the cochlea of the inner ear

Also involved with equilibrium
 Receptors in inner ear send message to cerebellum in the
brain about head position, to help maintain balance
 Other receptors in our eyes and around our joints pick up
information and it’s processed in the cerebellum and
cerebral cortex to enable the body to cope with changes in
equilibrium

Three parts: external ear, middle ear, and inner ear

Auricle/Pinna: the flap that funnels sound waves
 helix=rim
 lobule=lobe



External auditory meatus: opening to auditory
canal; lined with sebaceous or ceruminous
glands that secrete a wax-like or oily substance
called cerumen which protects the ear
External auditory canal: short, narrow chamber
extends from auricle to tympanic membrane
Tympanic membrane/Eardrum: stretches across
canal; vibrates in response to sound waves and
transmits them to middle ear


Cavity in the temporal bone
Connects with pharynx (throat) by means of
the eustachian tube
 Tube serves to equalize air pressure in middle ear
with that of outside atmosphere

Chain of three tiny bones: malleus (hammer),
incus (anvil), and stapes (stirrups)
 Transmit sound waves from eardrum to inner ear
by vibration
Labyrinth: hollowed out structure of inner ear
Vestibule: central egg-shaped cavity of labyrinth
Oval window: separates middle ear from inner ear; located
just under the base of the stapes; vibrations reach the inner
ear through this structure
 Cochlea: snail-shaped structure where sound vibrations are
converted into nerve impulses



 Organ of Corti: pick up nerve impulses and transmit them
through auditory nerve to the hearing center of the cerebrum
 Semicircular Canals: three canals that contains a liquid, and
delicate hairlike cells that bend when the liquid is set in motion
by head and body movement; these impulses are sent to the
cerebellum, helping to maintain body balance or equilibrium
(NOTHING to do with the sense of hearing)

Sound waves  pinna, or outer ear
auditory canal  tympanic membrane ear
ossicles (hammer, anvil and stirrup stimulate
the receptors in the cochlea) cochlear
nerve (part of vestibulocochlear nerve)
temporal lobe of the brain for interpretation

Movement of head equilibrium receptors in
the semicircular and vestibule areas of the
inner ear  vestibular nerve (part of
vestibulocochlear nerve) cerebellum of
brain for interpretation
If the delicate hair cells in
the organ of Corti become
overstimulated, they will
become damaged
 When the same sound keeps
reaching the ears, the
auditory receptors adapt to
the sound and we do not
hear it
 Sound is measured in
decibels (dB). The scale runs
from the faintest sound the
human ear can hear at 10 dB
to over 165 dB.


Oititis media: infection of the middle ear; causes earache
 Treatment--Myringotomy: opening made in tympanic
membrane…tubes may be placed in the ear to allow fluids to
drain off)
Otosclerosis: stapes bone of middle ear becomes
spongelike and then hardens so stapes becomes
immovable; common cause of deafness in young adults
 Meniere’s Disease: affects semicircular canals of inner ear,
causing vertigo (dizziness)
 Conductive hearing loss: sounds of inner ear blocked by
earwax or fluid in middle ear
 Sensorineural damage: damage to parts of inner ear or
auditory nerve resulting in partial or complete deafness




Human nose can detect about 10,000
different smells
Smells account for about 90% of what we
think of as taste
Sense of smell can alert us to environmental
dangers

Sensors responsible for smell are located in
dime-sized area called the olfactory region, on
the top side of each nasal cavity
 Sensors called olfactory receptor cells (neurons)
 When nasal cavity becomes congested with mucus
from a cold, the olfactory receptors are covered or
partially blocked so sense of smell affected
 Olfactory hairs extend from these neurons into the
nasal cavity, where they are covered by a thin,
protective layer of mucus
When you inhale an odor, the chemicals that caused the
odor dissolve in the mucous layer surrounding the
olfactory hairs
 This dissolving action stimulates the olfactory receptor
cells, which send impulses through the olfactory
filaments that make up the olfactory nerve
 Olfactory nerve sends impulses to the olfactory cortex of
the brain
 Nerve pathway between the nose and the brain travels
through limbic system, the part of the brain responsible
for emotion

 Smell may trigger positive or negative emotion because we
have associated a particular experience with that scent

Rhinitis: inflammation of mucous membranes that line the nasal
passage; most frequent cause is common cold, but other causes
include infection, allergies, strong chemical odors, and certain illegal
drugs
 Releases histamines that trigger a reaction that produces nasal congestion
and drainage
 Treatment is taking antihistamines to curb activity of histamines

Deviated septum: large shift in position of septum away from the
center; can be surgically repaired
 Caused by injury or from birth

Perforated septum: one or more holes in septum; can be surgically
treated
 Caused by injury, an ulcer, long-term exposure to toxic fumes, or illegal
drug use

Nasal polyps: soft, noncancerous growths in lining of nasal passages or
sinuses; may result from chronic inflammation of nasal cavity
Human mouth contains ~10,000 sensory
receptors for the sense of taste
 These taste buds scattered throughout the
interior of the mouth, including the lips and
sides, top and back of the mouth
 Most taste buds, however, reside on tiny bumps
on tongue known as papillae
 Within each taste bud, gustatory cells send tiny
gustatory hairs up through the taste pores, very
small openings in the top of the taste bud



Chemical molecules from food dissolve in
saliva to produce compounds called tastants.
The tastants stimulate the gustatory hairs to
send nerve impulses to the brain
Three of the cranial nerves– facial nerve,
glossopharyngeal nerve, and vagus nerve—
are responsible for transmitting taste
sensations to the brain
Sweet
Salty
Sour
Bitter
Unami: the taste of beef as well as the taste of monosodium
glutamate, a seasoning commonly added to processed foods to
enhance their taste
*A single gustatory cell responds to only one of the five taste sensations.
*Individual taste buds contain 50 to 100 gustatory cells which typically
include all five taste sensations
*Average person is able to distinguish approximately 10,000 different
flavors
*Flavor is a combination of taste, smell, texture or consistency, and
temperature










Discoloration: may appear black when taking bismuth preparations for
upset stomach; may appear pale due to iron-deficiency anemia; white
patches may accompany fever, dehydration or mouth breathing
Infection: may result from tongue piercing, or traumatic accident
causing a severe bite of tongue; antibiotics are used to treat
infections; tongue heals more quickly than any other part of body
Hairy tongue: unnatural growth of gustatory hairs of the tongue; most
common cause is inadequate oral hygiene; other causes include
certain medications, excessive drinking of coffee or tea, frequent
tobacco use, or radiation treatment to head or neck region
Burning mouth syndrome: sensation of moderate to severe burning in
the mouth, tingling, numbness, or dryness of the mouth and a bitter or
metallic taste; caused by damage to taste and pain receptors, chronic
dry mouth, nutritional deficiencies, hormonal changes, acid reflux, and
infections; treatment is based on underlying cause
Cancer: unexplained red or white areas, sores, or hard lumps
(especially if painless); most oral cancers grow on the sides of tongue
or on the floor of the mouth