Chapter 8 The Special Senses
Biology 112
Tri-County Technical College
Pendleton, SC
You’re special…I’m special
• Usually told we have five senses: touch,
taste, smell, sight, and hearing
• TOUCH actually mixture of general senses:
temperature, pressure, and pain receptors of
skin and proprioceptors of muscles and
joints
• Smell, taste, sight, and hearing are called
special senses
Special Senses, cont.
• Receptors for 5th sense, equilibrium, are
housed in ear along with organ of hearing
• Special sense receptors are either large,
complex sensory organs (such as eyes and
ears) or localized clusters of receptors (such
as taste buds and olfactory epithelium)
The Human Eye
• Eyelids meet at medial and lateral canthus
(protection)
• Modified sebaceous glands at eyelid edges called
meibomian glands produce oily secretion that
lubricates eyes
• Ciliary glands (modified sweat glands) lie
between eyelashes and help lubricate eyeball
• Sty is inflammation of one of these ciliary glands
Human Eye, cont.
• Conjunctiva is delicate membrane that lines
eyeball and covers parts of outer surface of eyeball
• Ends at edge of cornea by fusing with corneal
epithelium
• Secretes mucus to lubricate and keep eyeball moist
• Conjunctivitis (inflammation of conjunctive)
results in reddened, irritated eyes
• Pinkeye (conjunctiva infection caused by
bacteria/virus) is highly contagious
Human Eye III
• Lacrimal apparatus consists of number of
glands and ducts that drain lacrimal
secretions into nasal cavity
• Lacrimal glands located above lateral end
of each eye release dilute salt solution
(tears) onto anterior surface of eyeball
through lacrimal gland ducts
Human Eye IV
• Tears flush across eyeball into lacrimal canals
medially, then into lacrimal sac, and finally into
nasolacrimal duct which empties into nasal cavity
• Tears contain antibodies and lysozyme
• Cleanses and protects as moistens and lubricates
• Sclera (thick, white connective tissue) is
outermost protective tunic (also called fibrous
tunic
– Seen anteriorly as “white of the eye”
Human Eye V
• Central anterior portion (sclera) modified to
crystal clear
• This transparent window is called the cornea
allows light to enter the eye
• Iris is pigmented part of eye and contains the
pupil
• Six extrinsic (external) eye muscles attached to
outer surface of each eye
• Produce gross eye movements and make it
possible for eye to follow a moving object
Human Eye Visual
Human Eye Visual II
Eye Tunics
• Eye (eyeball) is hollow sphere whose wall is
composed of three tunics (coats)
• Its interior is filled with fluids called humors that
help maintain its shape
• Outermost tunic (sclera or fibrous tunic) is thick,
white connective tissue seen anteriorly as the
“white of the eye”
• Central anterior portion modified to crystal clear
structure called the cornea through which light
enters eye
Eye Tunics, cont.
• Cornea well supplied with nerve endings,
has ability to repair itself
• **Cornea has NO blood vessels and is
ONLY tissue that can be transplanted from
one person to another without fear of
rejection
• Choroid is middle coat of eyeball and is
blood-rich nutritive tunic containing dark
pigment
Eye Tunics III
• Anteriorly, choroid modified to form two smooth
muscle structures called ciliary body to which
lens is attached and the iris
• Pigmented iris has rounded opening (pupil)
through which light passes
• Circularly/radially arranged smooth muscle fibers
form iris which acts like diaphragm of camera
regulating amount of light entering eye
Eye Tunics IV
• Innermost sensory tunic of eye is delicate white
retina
• Retina contains millions of receptors cells (rods
and cones)
• Rods and cones called photoreceptors because
they respond to light
• Photoreceptor cells distributed over entire retina
except where optic nerve leaves eyeball
• This site is called the optic disk (blind spot)
Eye Tunics Visual
Rods and Cones
• Rods most dense at periphery of retina and
decrease in number as center of retina
approaches
• Rods allow vision in gray tones in dim light
and provide for peripheral vision
• Night blindness caused by interference with
rod function
– Most common cause prolonged vitamin A
deficiency
Rods and Cones, cont.
• Cones are discriminatory receptors that allow
color vision under bright conditions
• Cones are densest in center of retina and < in
number toward retinal edge
• Fovea centralis is tiny pit that contains only cones
and is area of greatest visual acuity (point of
sharpest vision)
• Three varieties of cones and each type is more
sensitive to particular wavelength of light
Rods and Cones III
• Blue, green and range including both green and
red
• Impulses received at same time from more than
one type of cone by visual cortex are interpreted
as intermediate colors
• Lack of all three cone types = total color blindness
whereas lack of one type = partial color blindness
• Color blindness is sex-linked disorder
Rods and Cones Visual
Image Formation
• Light passes from one substance to another
substance of different density, its speed
changes and its rays are bent (refracted)
• Light rays are bent in eye as they encounter
cornea, aqueous humor, lens, and vitreous
humor
• Refractive power of cornea and humors
constant
Image Formation, cont.
• Refractive power of lens changed by
changing its shape
• Greater lens convexity (bulge) the more
light is bent
• Flatter the lens, less light is bent
• Image formed on retina is a real image
• Reversed from right to left, inverted (upside
down) and smaller than the object
Pathway of Light
• Corneaaqueous humorthrough pupil
through pupillensvitreous humor
retina (rods and cones)
Coming to term with terms…
• Accommodation: ability of eye to focus
specifically for close objects (< 20 feet)
• Astigmatism: results from unequal curvatures in
different parts of cornea or lens & causes blurry
images because points of light are focused not
points on retina but as lines
• Blind spot: (optic disc) where optic nerve exits
eyeball—no rods or cones here so light from
object focused on optic disc, the object disappears
Terms, cont.
• Cataract: Caused by lens becoming >ingly hard
and opague
– Vision becomes hazy and eventually blindness
– Lens implant or special cataract glasses
• Emmetropia: term given eye that focuses images
correctly on retina (harmonious vision)
• Glaucoma: Drainage of aqueous humor is
blocked causing pressure within eye to increase—
results in compression of delicate retina and optic
nerve
Terms III
• Causes pain and eventual blindness
– Tonometer used to measure intraocular pressure (> 40
should be annual exam)
– Eyedrops (miotics) or surgical enlargement of drainage
channel
• Hyperopia: “farsightedness” occurs when light
rays from distant object focused behind retina
• Myopia: “nearsightedness” occurs when light rays
from distant object fail to reach retina and are
focused in front of retina
Pathway to Optic Cortex
• Axons carrying impulses from retina
bundled together as posterior aspect of
eyeball and exit as optic nerve
• Fibers from medial side of each eye cross
over to opposite sides at optic chiasma
• Fiber tracts that result are the optic tracts
• Each tract contains fibers from lateral side
of eye on same side and medial side of eye
of opposite eye
Optic Cortex, cont.
• Optic tract fibers synapse with neurons in
thalamus whose axons form optic radiation
which runs to occipital lobe of brain
• There they synapse with cortical cells and
visual interpretation occurs
Pathway Visual
Visual Reflexes
• Convergence is reflexive movement of eye
medially when viewing close objects
– Convergence occurs both eyes aimed toward near
object being viewed
• Photopupillary reflex: occurs when eyes are
suddenly exposed to bright light and pupils
immediately constrict
– Protective reflex prevents damage to photoreceptors
• Accommodation pupillary reflex: occurs when
pupils constrict when close objects viewed
– Provides for more acute vision
Ear Structures
• Outer and middle ear involved with hearing
ONLY
• Inner ear functions in both equilibrium and
hearing
• Outer ear composed of pinna and external
auditory canal
• Pinna (auricle) what most would call “ear”
– Shell shaped structure surrounding auditory
canal opening
Ear Structures, cont.
• External auditory canal is short, narrow
chamber carved into temporal bone of skull
• Ceruminous glands (secrete earwax)
located in skin lined walls of external
auditory canal
• Sound waves entering canal eventually hit
tympanic membrane (eardrum) and cause
it to vibrate
Ear Structures III
• Tympanic membrane separates outer from
middle ear
• Middle ear (tympanic cavity) is small, airfilled cavity within temporal bone
• Flanked laterally by eardrum and medially
by bony wall with two openings: oval
window and inferior, membrane-covered
round window
Ear Structures IV
• Auditory tube links middle ear with throat
• Normally flattened and closed but
swallowing/yawning can open it briefly to
equalize pressure in middle ear with external
pressure
• **Important because eardrum does NOT vibrate
freely unless pressure on both sides of its surfaces
is the same
• Bulge outward or inward otherwise = hearing
difficulty
Ear Structures V
• Tympanic cavity spanned by three smallest
bones in body called ossicles which
transmit vibratory motion of eardrum to
fluids in inner ear
• Malleus (hammer), incus (anvil), and
stapes (stirrup)
• When eardrum moves, hammer moves with
itvibration to anvilvibration to stirrup
presses on oval window of inner ear
Ear Structures VI
• Movement of oval window sets fluids of
inner ear into motion…eventually exciting
hearing receptors
• Inner ear is maze of bony chambers called
osseous or bony labyrinth located deep in
temporal bone just behind eye socket
• Three subdivisions called cochlea,
vestibule, and semicircular canals
Ear Structures VII
• Bony labyrinth filled with plasma-like fluid
called perilymph
• Suspended in perilymph is membranous
labyrinth which is system of membrane
sacs that more or less follow shape of bony
labyrinth
• Membranous labyrinth contains thicker
fluid called endolymph
Ear Structures Visual I
Ear Structures Visual II
Organ of Corti
• Within membranes of snail-like cochlea is Organ
of Corti which contains hearing receptors called
hair cells
• Sound waves reach cochlea through vibrations of
eardrum, ossicles, and oval window & set cochlear
fluids into motion
• Receptor cells on basilar membrane in organ of
Corti are stimulated when their “hairs” are bent or
“tweaked” by movement of gel-like tectorial
membrane that lies over them
Organ of Corti, and more
• Once stimulated, hair cells transmit impulses along
cochlear nerve (division of 8th cranialvestibulocochlear) to the auditory cortex in
temporal lobe = hearing
• Sensorineural deafness occurs when degeneration
or damage to receptor cells in organ of Corti, to
cochlear nerve, or to neurons of auditory cortex
• Often occurs from extending listening to
excessively loud noises
• Is a problem of nervous system structures
More, cont.
• Conduction deafness results when
something interferes with conduction of
sound vibrations to fluids of inner ear
• Earwax buildup, fusion of ossicles, ruptured
eardrum, or otitis media (inner ear
infection)
• Is a problem of mechanical factors
Sound Location
• Sound usually reaches the two ears at
different times so we have stereo hearing
– Well, least ways, the lucky ones among us do…
• Functionally, this slight difference assists in
helping determine where sounds are coming
from in the environment
Equilibrium and Balance
• Equilibrium sense “responds” to various
movements of head
• Equilibrium receptors of inner ear (often called
vestibular apparatus) can be divided into two
functional parts
• One is responsible for monitoring static
equilibrium (report on position of head with
respect to gravity when body is not moving)
• The other monitors dynamic equilibrium (respond
to angular or rotatory movements of head rather
than to straight-line movements)
E and B, cont.
• Within membrane sacs of vestibule are receptors
called maculae that are essential to static
equilibrium
• Maculae report on position of head with respect to
pull of gravity when body NOT moving
• Provide info on which way is up or down and help
keep head erect
• Each macula is patch of receptor cells with “hairs”
embedded in otolithic membrane (gel or jellylike
material containing otoliths (tiny stones of
calcium salts)
E and B III
• As head moves, otoliths roll in response to
changes in pull of gravity
• This movement creates pull on the gel
which slides over hair cells, bending their
hairs
• This activates hair cells which send
impulses along vestibular nerve to
cerebellum of brain, informing it of position
of head in space
E and B IV
• Dynamic equilibrium receptors found in
semicircular canals and respond to angular
or rotatory movments of head rather than to
straight-line movements
• Semicircular canals oriented in three planes
of space
• Regardless of plane one moves in, there will
be receptors to detect that movement
E and B V
• Within each SC canal is receptor region called
crista ampullaris—turf of hair cells covered with
gelatinous cap called capula
• Head moves in arclike/angular direction,
endolymph in canal lags behind and moves in
opposite direction pushing capula in direction
opposite to body’s motion
• This stimulates hair cells and impulses transmitted
up vestibular nerve to cerebellum
• When moving at constant rate, receptors stop
sending and one no longer has sense of motion
until speed or direction of movement changes
Taste and Olfaction
• Receptors for taste and olfaction are
chemoreceptors because they respond to chemicals
in solution
• Thousands of olfactory receptors occupy postage
stamp size area in roof of each nasal cavity
• Olfactory receptor cells are neurons with olfactory
hairs
– Long cilia on nasal epithelium that are continually
bathed by layer of mucus secreted by underlying glands
Olfaction, cont.
• When receptors stimulated by chemicals
dissolved in mucus, they transmit impulses
along olfactory nerve (1st cranial nerve) to
olfactory cortex of brain
• Interpretation of odor occurs and “odor
snapshot” is made
• Olfactory pathways closely tied to limbic
system (emotional-visceral part of brain)
Olfaction III
• Olfactory impressions are long-lasting and
part of one’s memories and emotions
• OMG, Ms. Pennington and My Sin®
perfume…Beam me up Scotty….I can’t
Captain, I ain’t got the power
• Olfactory receptors are very sensitive—just
a few molecules can activate them
• Olfactory neurons adapt very quickly
Taste
• Taste buds (specific receptors for sense of
taste) are widely scattered in oral cavity
– Most of 10,000 or so located on tongue
– Few found on soft palate & inner surface of
cheeks
• Three kinds of papillae: filiform (sharp),
fungiform (rounded), and circumvallate
• Taste buds found on sides of circumvallate
and more numerous fungiform papillae
Taste, cont.
• Specific cells that respond to chemicals dissolved
in saliva are epithelial cells called gustatory cells
which are surrounded by supporting cells in taste
bud
• Gustatory cell’s long microvilli (gustatory hairs)
protrude through taste pores and when
depolarized, they send impulses to the brain
• Three cranial nerves (7th, 9th, and 10th) carry
impulses from various taste buds to gustatory
cortex
Taste Sensations
• Four basic taste sensations, each corresponding to
stimulation of one of four major types of taste
buds
• Sweet (may be the OH- group)
• Sour (H+)
• Bitter (alkaloids)
• Salty (metal ions in solution)
• Taste heavily impacted by sense of olfaction
• Dislike for bitterness is protective (many natural
poisons and spoiled foods are bitter)
Happens to all of us…SOL
• Presbyopia or “old vision” results from
decreasing lens elasticity that accompanies
aging
• Lacrimal glands become less active and
eyes tend to be dry
• Lens loses it clarity, dilator muscles of iris
become less efficient, and photoreceptors
begin to die die to lack of oxygen and
nutrients (poor circulation)
But considering the alternatives,..
• Presbycusis begins by age 60 and results from
gradual deterioration and atrophy of organ of Corti
• Leads to loss of hearing in high tones/speec
sounds
• By mid-40s, ability to taste and smell diminishes
• By 80, half cannot smell at all and sense of taste is
very poor