Chapter 8 special senses

Special Senses
 Special Senses: smell, taste, sight,
hearing and equilibrium
 The special sense receptors are either
large, complex sensory organs
(eyes/ears) or localized clusters of
receptors (tastebuds/olfactory epithelium)

Special Senses
 Remember, these senses are overlapping!
 What you sense of “feel” is a blending of
stimulus effects

Anatomy of the Eye
 The adult eye is a sphere about 1 in. in
diameter
 Only 1/6 of the anterior surface can be
seen
 The rest of the eye is covered and
protected by fat and the walls of the bony
orbit

Anatomy of the Eye
 The accessory structures include:
◦ Extrinsic eye muscles
◦ Eyelids
◦ Conjunctiva
◦ Lacrimal apparatus

Anatomy of the Eye
 The eyelids offer anterior protection of the
eye
 Meet at the medial and lateral
commissure (canthus)

Anatomy of the Eye
 The palpebral fissure is the space between
the eyelids in an open eye
 The eyelashes protect the borders of each
eyelid
 The tarsal glands are modified sebaceous
glands associated with the eyelids

Anatomy of the Eye
 The conjunctiva is a delicate membrane
that lines the eyelids and covers part of
the outer surface of the eyeball

Anatomy of the Eye
 Conjunctivitis is an
irritation and
inflammation of the
conjunctiva
 Pinkeye is a
infectious form of
Conjunctivitis
caused by bacteria
or viruses

Anatomy of the Eye
 The lacrimal
apparatus consists
of the lacrimal gland
and a number of
ducts that drain the
lacrimal secretions
into the nasal cavity
 The lacrimal glands
are located above
the lateral end of
each eye

Anatomy of the Eye
 The lacrimal glands continually release a
salt solution (your tears!) onto the
anterior surface of the eye
 Tears flush across the eyeball into the
lacrimal canaliculi medially, then into the
lacrimal sac and finally into the
nasolacrimal duct which empties into the
nasal cavity

Anatomy of the Eye
 Lacrimal secretions are high in antibodies
and lysozyme to help cleanse and protect
the eye from foreign substances

Anatomy of the Eye
 There are six extrinsic or external eye
muscles
 Attached to the outer surface to each eye
 Produce gross eye movement
 Make it possible to follow objects
 Controlled by the cranial nerves
(abducens, oculomotor, trochlear)

Anatomy of the Eye
 The eye itself, commonly called the
eyeball, is a hollow sphere
 The wall is composed of three layers
 The interior is filled with fluids called
humors that help maintain its shape

Anatomy of the Eye
 The lens is the main focusing apparatus of
the eye and is supported upright within
the eye cavity, separating it into two
chambers

Anatomy of the Eye
 Three layers of the
eyeball wall:
 1. Fibrous
 2. Vascular
 3. Sensory

Anatomy of the Eye
 1. The Fibrous Layer
 Outermost layer
 Consists of the sclera (protective layer)
and cornea (transparent layer)

Anatomy of the Eye
 1. Fibrous Layer
 Sclera - the white of the eye
◦ Thick, connective tissue
 Cornea – the window of the eye, where
light enters the eye
◦ Many nerve endings (pain fibers)
◦ Most exposed part of the eye
◦ Self-repairing

Anatomy of the Eye
 Fun Fact: the cornea is the only tissue in
the body that can be transplanted from
one person to another without the worry
of rejection
 Because the cornea has no blood vessels,
it is beyond the reach of the immune
system

Anatomy of the Eye
 2. Vascular Layer
 3 distinguishable regions
◦ 1. Choroid (blood-rich, contains dark
pigment)
◦ Prevents light scattering inside the eye
◦ 2. Ciliary body (smooth muscle to which
the lens is attached by the ciliary
zonule)
◦ 3. Iris (pigmented part of the eye)
◦ Includes the pupil, where light passes

Anatomy of the Eye
 2. Vascular Layer
 Circularly and radially arranged smooth
muscle fibers form the iris
 Regulates the amount of light entering the
eye to see clearly in available light
 In bright light/close vision the pupil
contracts, in dim light/distant vision the
pupil dilates

Anatomy of the Eye
 3. Sensory Layer
 Two-layered retina
 Extends anteriorly to the ciliary body

Anatomy of the Eye
 The outer pigmented layer of the retina is
full of pigmented cells that absorb light
and prevent light scattering
 Also act as phagocytes, and store vitamin
A

Anatomy of the Eye
 The inner neural layer of the retina
contains millions of photoreceptors – rods
and cones

Anatomy of the Eye
 Electrical signals
pass from the
photoreceptors via
a 2 neuron chain,
then leave via the
optic nerve
 This nerve impulse
is transmitted to
the optic cortex and
results in vision

Anatomy of the Eye
 Photoreceptors are over the entire retina
except at the optic disc, where the optic
nerve leaves the eyeball
 This is your blind spot!

Anatomy of the Eye
 Rods allow us
to see in gray
tones and dim
light
 Peripheral
vision
 More dense at
the edges or
periphery of the
retina

Anatomy of the Eye
 Night Blindness
(Nyctalopia)
 Interferes with rod
function and limits the
ability to see at night
 Usually results from
prolonged vitamin A
deficiency
◦ Causes neural retina
deterioration

Anatomy of the Eye
 Cones allow for details and color to be
seen in bright light
 Densest in the center of the retina

Anatomy of the Eye
 Lateral to each blind spot is the fovea
centralis, a tiny pit that contains only
cones
 Greatest visual acuity (sharpest vision)

Anatomy of the Eye
 There are three varieties of cones:
 “blue”
 “green”
 “red” (actually responds to green and red)

Anatomy of the Eye
 Impulses received at the same time from
more than one type of cone by the visual
cortex are interpreted as intermediate
colors
 Ex. both blue and red result in purple

Anatomy of the Eye
 When all three types of cones are
stimulated we see white
 “mixing” of colors occurs in the brain, not
the retina

Anatomy of the Eye
 Color blindness
 Lack of one type of
cone leads to
partial color
blindness
 Lack of all three
cones leads to
total color
blindness
 Sex-linked trait,
almost exclusively
in males

Anatomy of the Eye
 Lens: a flexible biconvex crystal-like
structure
 Light entering the eye is focused on the
retina by the lens
 Lens is held upright by the ciliary zonule

Anatomy of the Eye
 The lens divides the eye into two
segments:
 1. anterior (aqueous) segment
◦ Contains aqueous humor
◦ Provides nutrients for lens and cornea
 2. posterior (vitreous) segment
◦ Filled with vitreous humor
◦ Keeps eyeball from collapsing inward
 Both provide intraocular pressure

Anatomy of the Eye
 Aqueous humor is reabsorbed into the
venous blood through the scleral venous
sinus, or the canal of Schlemm

Anatomy of the Eye
 Cataracts
 In youth, the lens is transparent and a
hardened jelly-like texture
 As we age, it becomes increasingly hard
and opaque

Anatomy of the Eye
 Cataracts cause vision to become hazy
and distorted, and eventually cause
blindness
 Risk factors: Type II diabetes, exposure to
intense sunlight, heavy smoking
 Current treatments: surgical removal,
replacement lens implants or specialized
glasses

Anatomy of the Eye
 Glaucoma
 Occurs when the drainage of the aqueous
humor is blocked, and fluids back up
 Pressure on the eye increases,
compressing the retina and optic nerve

Anatomy of the Eye
 Glaucoma causes
pain and possible
blindness
 Progresses slowly
and painlessly until
the damage is done
 Tonometer is used
to test intraocular
pressure

Anatomy of the Eye
 Glaucoma is commonly treated with
eyedrops that increase the rate of
drainage
 Laser or surgical enlargement of the
drainage channel can also be used

Anatomy of the Eye
 An ophthalmoscope is
an instrument used to
illuminate the interior of
the eyeball
 Conditions such as
diabetes,
arteriosclerosis, and
degeneration of the
optic nerve and retina,
can be detected by
examination with an
ophthalmoscope

Anatomy of the Eye
 Ophthalmascope

Pathway of Light/Light Refraction

Pathway of Light
 When light passes from one substance to
another substance of a different density,
it’s speed changes and the rays are bent
or refracted
 Light rays are refracted in the eye when
they encounter the cornea, aqueous
humor lens and vitreous humor

Pathway of Light
 The refractive powers of the corneas and
humors are constant
 The refractive power of the lens changes
by changing its shape

Pathway of Light
 The greater the lens convexity (bulge) the
more it bends the light
 The flatter the lens, the less it bends light

Pathway of Light
 The resting eye is set for distant vision
 Light from a distant source (+20ft)
approaches the eye in parallel rays
 The lens does not need to change shape
to focus

Pathway of Light
 Light from a near source tends to scatter
or diverge
 The lens must bulge to make clear vision
possible
 In order to bulge, the ciliary body
contracts, and the lens becomes more
convex

Pathway of Light
 Accommodation: the ability of the eye to
focus specifically on close objects

Pathway of Light
 The image formed on the retina as a
result of the light-bending activity of the
lens is a real image – it is reversed from
left to right, upside down, and smaller
than the object
 The farther away the object is, the smaller
its image on the retina

Pathway of Light
 The normal eye can accommodate
properly
 Vision problems occur when the lens is
too strong or too weak, or from structural
problems

Pathway of Light
 The eye that focuses images correctly on
the retina is said to have emmetropia

Pathway of Light
 Myopia is nearsightedness
 It occurs when the parallel light rays from
distant objects fail to reach the retina and
instead are focused in front of it
 Results from eyeball that is too long, a
lens that is too strong, or a cornea that is
too curved
 Requires concave corrective lenses

Pathway of Light
 Hyperopia is farsightedness
 It occurs when the parallel light rays from
distant objects are focused behind the
retina
 Lens is usually short or “lazy”
 Often subject to eyestrain
 Requires convex corrective lenses

Pathway of Light
 Unequal curvatures in
different parts of the
cornea or lens cause
astigmatism
 In this condition, images
occur because points of
light are focused not on
points on the retina but as
lines
 Special cylindrically ground
lenses or contacts are used
to correct this problem

Visual Fields and Visual Pathways
to the Brain

Visual Fields and Pathways
 The optic nerve is
located in back of
the eye, and
carries impulses
form the retina to
the brain

 At the optic chiasma the fibers from the
medial side of each eye cross over to the
opposite site of the brain

 The fiber tracts
that result are
called the optic
tracts
 Each optic tract
contains fibers
from the lateral
side of the eye on
the same side and
the medial side of
the opposite eye

 The optic tract
fibers synapse with
neurons in the
thalamus, whose
axons form the
optic radiation.
 This runs to the
occipital lobe of the
brain

 There they
synapse with the
cortical cells, and
visual
interpretation, or
seeing, occurs.

 Each side of the brain
receives visual input
from both eyes
 Lateral of the same
side eye, medial form
the other eye

 Each eye “sees” a slightly different view
 The visual fields of each eye overlap
 Humans have binocular vision, “two-eyed”
vision
 Allows for depth perception as the visual
cortex fuses the two images

 Hemianopia: the loss of the same side of
the visual field in both eyes
 Results from damage to the visual cortex

 People who suffer from
hemianopia are not
able to see things past
the middle of the
visual field on either
the left or right side

Eye Reflexes
 Both the internal and external eye
muscles are needed for proper eye
function
 The external muscles, as mentioned
earlier, are responsible for following
moving objects
 They are also responsible for convergence

Eye Reflexes
 Convergence: the
reflexive movement of
the eyes medially
when viewing close
objects
 Both eyes are aimed
toward the near
object being viewed

Eye Reflexes
 When the eyes are suddenly exposed to
bright light, the pupils instantly constrict
 This is called the photopupillary reflex

Eye Reflexes
 The photopupillary reflex prevents
excessively bright light from damaging
the photoreceptors

Eye Reflexes
 The accomodation
pupillary reflex also
constricts the pupil,
but occurs when
viewing close
objects
 It allows for more
acute vision

Eye Reflexes
 Reading requires almost continuous work
by both sets of muscles
 Ciliary fibers cause the lens to bulge, the
iris constricts the pupil, convergence
occurs
 This is why eyestrain often occurs

The Ear
 Ears are key to the senses of hearing and
balance
 Sound vibrations move fluids to stimulate
hearing receptors
 Gross movements of the head move fluids
in the balance organs

The Ear
 Receptors that respond to physical forces
are called mechanoreceptors
 These two sense organs are housed in the
ear
 They respond to different stimuli and are
activated independently of one another

Anatomy of the Ear
 Anatomically, the ear is divided into 3
major sections:
 1. external (outer) ear - hearing
 2. middle ear - hearing
 3. internal (inner) ear – hearing and
equilibrium

External Ear
 The external (outer) ear is composed of
the auricle and the external acoustic
meatus

External Ear
 The auricle (pinna) is
“the ear,” the shell-
shaped structure
surrounding the
auditory canal opening
 It collects and directs
sound waves into the
auditory canal
 Function is weak in
humans

External Ear
 The external acoustic meatus (auditory
canal) is a short, narrow chamber (1x.25
in) carved into the temporal bone of the
skull

External Ear
 The auditory
external acoustic
meatus has skin-
lined walls with
ceruminous glands
which secrete
cerumen (earwax)
 It provides a sticky
trap for foreign
bodies and repels
insects

External Ear
 Sound waves entering the auditory canal
eventually hit the tympanic membrane
(eardrum) and cause it to vibrate
 The canal ends at the eardrum, which
separate the external from the middle ear

Middle Ear
 The tympanic cavity or middle ear is a
small, air-filled, mucosa-lined cavity
within the temporal bone

Middle Ear
 It is flanked laterally
by the eardrum and
medially by a bony
wall with two
openings, the oval
window and the
inferior, membrane-
covered round
window

Middle Ear
 The pharyngotympanic (auditory) tube
also called the eustachian tube runs
obliquely downward to link the middle ear
cavity with the throat, and the mucosae
lining the two regions are continuous

Middle Ear
 Normally the auditory tube is flattened
and closed
 Swallowing and yawning opens this tube
briefly to equalize the pressure in the
middle ear cavity
 The eardrum does not vibrate freely
unless the pressure on both surfaces is
the same

Middle Ear
 When the pressures are
unequal, the eardrum
bulges inward or
outward making hearing
difficult or causes
earaches
 The sensation of ear-
popping is the equalizing
of pressures

Middle Ear
 An inflammation of the otitis media is
common in children with sore throats
 The eardrum becomes inflamed, bulges
and often the cavity fills with fluid or pus
 A myringotomy is sometimes required to
relieve the pressure
 A tiny tube can also be used for drainage

Middle Ear
 The tympanic membrane is spanned by
the three smallest bones in the body, the
ossicles
 The ossicles transmit the vibratory motion
of the eardrum to the fluids of the inner
ear

Middle Ear
 The ossicles are named for their shape:
 Hammer (malleus)
 Anvil (incus)
 Stirrup (stapes)

Middle Ear
 When the eardrum moves, the hammer
moves as well, and transfers the vibration
to the anvil
 The anvil passes the vibration to the
stirrup

Middle Ear
 The stirrup which presses on the oval
window of the inner ear
 The movement at the oval window sets
the fluids of the inner ear into motion,
eventually exciting the hearing receptors
 How Hearing Works

Inner Ear
 The internal ear is a
maze of bony
chambers called the
bony, or osseous,
labyrinth, located
deep within the
temporal bone
behind the eye
socket

Inner Ear
 The three subdivisions of the bony
labyrinth are the:
 1. cochlea (spiral, pea size)
 2. vestibule
 3. semicircular canals

Inner Ear
 The inner ear is a
cavity filled with a
plasmalike fluid called
perilymph
 Suspended in the
perilymph is a
membraneous
labyrinth, a system of
membrane sacs that
more or less follow the
shape of the bony
labyrinth

Inner Ear
 Suspended in the
perilymph is a
membranous
labyrinth, a system of
membrane sacs that
more or less follow
the shape of the bony
labyrinth
 The membranous
labyrinth itself
contains a thicker
fluid called endolymph
 How Hearing Works

Mechanisms of Equilibrium
 Equilibrium is not an easy sense to
describe
 It responds to various head movements
 The equilibrium receptors of the inner ear,
are collectively called the vestibular
apparatus

Mechanisms of Equilibrium
 The vestibular apparatus can be divided
into two functional aspects:
 1. static equilibrium
 2. dynamic equilibrium

Static Equilibrium
 Within the membrane
sacs of the vestibule
are receptors called
maculae
 The maculae are
essential to the sense
of static equilibrium:
balance concerned
with changes in the
position of the head

Static Equilibrium
 The maculae report on changes in the
position of the head in space with respect
to the pull of gravity when the body is not
moving
 Give information on which way is up or
down
 Help keep the head erect

Static Equilibrium
 Each macula is a patch of receptor (hair)
cells with their “hairs” embedded in the
otolithic hair membrane, a jellylike mass
studded with otoliths or “earstones,” tiny
stones made of calcium salts

Static Equilibrium
 As the head
moves, the
otoliths roll in
response to
changes in
gravitational pull
 The surrounding
gel pulls, slides
over the hair
cells, bending
the hairs

Static Equilibrium
 This movement activates the hair cells,
which send impulses along the vestibular
nerve to the cerebellum of the brain,
giving information on head position in
space

Dynamic Equilibrium
 Dynamic equilibrium receptors, found in
the semicircular canals, respond to
angular or rotary movements of the head

Dynamic Equilibrium
 Within the ampulla, a swollen region at
the base of each membranous
semicircular canal is a receptor region
called a crista ampulliaris or crista

Dynamic Equilibrium
 The crista consists of a tuft of hair cells
covered with a gelatinous cap called
cupula
 When your head moves in arclike/angular
motion, the endolymph lags behind

Dynamic Equilibrium
 As the cupula drags against the stationary
endolymph the cupula bends with the
body’s motion

Dynamic Equilibrium
 This motion
stimulates the hair
cells, and impulses
are transmitted up
the vestibular
nerve to the
cerebellum
 Bending the
cupula in the
opposite direction
reduces impulse
generation

Dynamic Equilibrium
 When you are
moving at a
constant rate, the
receptors gradually
stop sending
impulses, and you
no longer have the
sensation of
motion until your
speed or direction
of movement
changes

Dynamic Equilibrium
 The receptors of the semicircular canal
and vestibule are responsible for dynamic
and static equilibrium separately, they
usually act together

Dynamic Equilibrium
 Besides these equilibrium senses, sight
and proprioceptors of the muscles and
tendons are also important in providing
information used to control balance to the
cerebellum

Mechanism of Hearing
 Within the cochlear duct, the endolymph-
containing membranous labyrinth of the
cochlea is the spiral organ of Corti
 This contains the hearing receptors, or
hair cells

 The scalae, chambers, above and below
the cochlear duct contain perilymph
 Sound waves that reach the cochlea
through vibrations of the eardrum,
ossicles and oval window set the cochlear
fluids in motion

 As the sound waves are transmitted by
the ossicles, their force is increased by
the lever activity of the ossicles
 The total force exerted in the large
eardrum reaches the oval window, which
sets the inner ear fluids in motion

 The pressure waves set up vibrations in
the basilar membrane
 The receptor cells are stimulated when
the hairs are bent or tweaked by the
movement of the tectorial membrane

 The length of the fibers spanning the
basilar membrane “tunes” specific regions
to vibrate at specific frequencies
 Shorter fibers are disturbed by high-
pitched sounds
 Longer fibers are disturbed by low-pitched
sounds

 Once stimulated, the hair cells transmit
impulses along the cochlear nerve to the
auditory cortex

 Sound usually reaches the two ears at
different times, or “in stereo”
 This helps us determine the location of
sounds

 We are able to habituate, or adapt, to
constant, repetitive sounds

 Hearing is also the last sense to leave us
when we fall asleep and the first to return
when we wake up

Hearing and Equilibrium Deficits

 Deafness: hearing loss of any degree
 Two main kinds:
 1. conduction
 2. sensorineural

 Temporary or
permanent
conduction
deafness results
when something
interferes with
the conduction
of sound
vibrations to the
fluids of the
inner ear

 Conduction
deafness can be
caused by
 Earwax
 Otosclerosis, the
fusion of the
ossicles
 Ruptured eardrum

 Sensorineural deafness occurs when there
is degeneration or damage to the receptor
cells in the spiral organ of Corti, cochlear
nerve or neurons in the auditory cortex

 Hearing aids use the
skull bones to conduct
sound vibrations to
the inner ear
 They are generally
very successful in
helping people with
conduction deafness to
hear
 They are less helpful
for people with
sensorineural deafness

 Equilibrium problems are usually obvious;
nausea, dizziness, and problems in
maintaining balance
 There are often strange, jerky eye
movements

 Meniere’s Syndrome
causes progressive
deafness of the inner
ear
 Sufferers become
nauseated and often
have howling or
ringing sounds in
their ears and vertigo

 Vertigo: a sense of spinning so severe
that they cannot stand up without
extreme discomfort

Chemical Senses - Taste and Smell

Taste and Smell
 Both senses use chemoreceptors
Stimulated by chemicals in solutions
Taste has four types of receptors
Smell can differentiate a large range of
chemicals
 Both senses complement each other and
respond to many of the same stimuli

Olfaction - Smell
 Olfactory
receptors are in
the roof of the
nasal cavity
Neurons with
long cilia
Chemicals must
be dissolved in
mucus for
detection

Olfaction - Smell
 Impulses are transmitted via the olfactory
nerve
 Interpretation of smells is made in the cortex
(olfactory area of temporal lobe)

http://asb.aecom.yu.edu/histology/labs/images/slides/A74_OlfactoryEpith_40X.jpg

Taste
 Taste buds house the receptor
organs
 Location of taste buds
Most on the tongue
Soft palate
Cheeks

Tongue and Taste
 The tongue is covered
with projections called
papillae
 Filiform papillae –
sharp with no taste
buds
 Fungiform papillae –
rounded with taste
buds
 Circumvallate papillae
– large papillae with
taste buds
 Taste buds are found on
the sides of papillae

http://neuromedia.neurobio.ucla.edu/campbell/oral_cavity/wp_images/96_fungiform.gif

http://www.esg.montana.edu/esg/kla/ta/vallate.jpg

Structure of Taste Buds
 Gustatory cells
are the
receptors
Have gustatory
hairs (long
microvilli)
Hairs are
stimulated by
chemicals
dissolved in
saliva

Structure of taste buds
 Impulses are carried to the gustatory
complex (parietal lobe) by several
cranial nerves because taste buds are
found in different areas
Facial nerve
Glossopharyngeal nerve
Vagus nerve

http://www.biosci.ohiou.edu/introbioslab/Bios171/images/lab6/Tastebuds.JPG

Taste Sensations
 Sweet receptors
Sugars
Saccharine
Some amino acids
 Sour receptors
Acids
 Bitter receptors
Alkaloids
 Salty receptors
Metal ions
 Umami
Glutamate, aspartate (MSG, meats)

Development of the Special
Senses
 Formed early in embryonic development
 Eyes are outgrowths of the brain
 All special senses are functional at birth

Chapter 8 special senses

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