Connects with the transparent
Special Senses cornea
Secretes mucus to lubricate the
eye and keep it moist
Special senses include:
Smell
Taste
Sight
Hearing
Equilibrium
Special sense receptors
Large, complex sensory organs
Localized clusters of receptors
Part I: The Eye and Vision
70 percent of all sensory receptors are
in the eyes
Each eye has over 1 million nerve fibers
carrying information to the brain
Lacrimal apparatus = lacrimal
gland + ducts
Lacrimal gland—produces
lacrimal fluid (tears); situated on
lateral end of each eye
Tears drain across the eye into
the lacrimal canaliculi, then the
lacrimal sac, and into the
nasolacrimal duct, which
empties into the nasal cavity
Tears contain:
Dilute salt solution
Mucus
Antibodies
Lysozyme (enzyme that
destroys bacteria)
Anatomy of the Eye
Function of tears
Accessory structures include the: Cleanse, protect, moisten,
Extrinsic eye muscles lubricate the eye
Eyelids
Conjunctiva
Lacrimal apparatus Extrinsic eye muscles
Six muscles attach to the outer
. surface of the eye
Produce gross eye movements
External and Accessory Structures
Eyelids Internal Structures: The
Meet at the medial and lateral Eyeball
commissure (canthus)
Three layers, or tunics, form the wall of
the eyeball
Eyelashes 1. Fibrous layer: outside layer
Tarsal glands produce an oily 2. Vascular layer: middle layer
secretion that lubricates the eye 3. Sensory layer: inside layer
Ciliary glands are located
between the eyelashes Humors are fluids that fill the interior
of the eyeball
Conjunctiva Lens divides the eye into two chambers
Membrane that lines the eyelids
and eyeball
Fibrous layer = sclera + cornea 2. Cones
Sclera Allow for detailed color
White connective tissue vision
layer Densest in the center of
Seen anteriorly as the
the retina
―white of the eye‖
Cornea Fovea centralis–lateral to
Transparent, central blind spot
anterior portion Area of the retina
Allows for light to pass with only cones
through Visual acuity
Repairs itself easily (sharpest vision) is
The only human tissue here
that can be transplanted
No photoreceptor cells are at the
without fear of rejection
optic disc, or blind spot
Vascular layer
Choroid is a blood-rich nutritive Cone sensitivity
layer that contains a pigment Three types of cones
(prevents light from scattering) Each cone type is
Choroid is modified anteriorly sensitive to different
into two smooth muscle wavelengths of visible light
structures
Ciliary body
Iris—regulates amount of
light entering eye
Pigmented layer
that gives eye color
Pupil—rounded
opening in the iris
Sensory layer
Retina contains two layers
Outer pigmented layer
absorbs light and prevents
it from scattering
Inner neural layer
contains receptor cells
(photoreceptors)
1. Rods
2. Cones
Electrical signals pass from
photoreceptors via a two-neuron
chain
Bipolar neurons
Ganglion cells
Signals leave the retina toward
the brain through the optic nerve
Optic disc (blind spot) is where Lens
the optic nerve leaves the eyeball Flexible, biconvex crystal-like
Cannot see images structure
focused on the optic disc Held in place by a suspensory
ligament attached to the ciliary
1. Rods body
Most are found toward the
edges of the retina Lens divides the eye into two chambers
Allow vision in dim light Anterior (aqueous) segment
and peripheral vision Anterior to the lens
All perception is in gray Contains aqueous humor,
tones a clear, watery fluid
Posterior (vitreous) segment
Posterior to the lens
Contains vitreous humor, Location where the optic
a gel-like substance nerves cross
Fibers from the medial
Aqueous humor side of each eye cross over
Watery fluid found between lens to the opposite side of the
and cornea brain
Similar to blood plasma Optic tracts
Helps maintain intraocular Contain fibers from the
pressure lateral side of the eye on
Provides nutrients for the lens the same side and the
and cornea medial side of the opposite
Reabsorbed into venous blood eye
through the scleral venous Synapse with neurons in
sinus, or canal of Schlemm the thalamus
Optic radiation
Axons from the thalamus
Vitreous humor
run to the occipital lobe
Gel-like substance posterior to
Synapse with cortical
the lens
cells, and vision
Prevents the eye from collapsing
interpretation (seeing)
Helps maintain intraocular
occurs
pressure
Summary of the pathway of impulses
from the retina to the point of visual
Ophthalmoscope
interpretation
Instrument used to illuminate
Optic nerve
the interior of the eyeball and
Optic chiasma
fundus (posterior wall)
Optic tract
Can detect diabetes,
Thalamus
arteriosclerosis, degeneration of
Optic radiation
the optic nerve and retina
Optic cortex in occipital lobe of
brain
Physiology of Vision Visual fields
Pathway of light through the eye and
Each eye ―sees‖ a slightly
light refraction
different view
Light must be focused to a point
Field of view overlaps for each
on the retina for optimal vision
eye
Light is bent, or refracted, by the
cornea, aqueous humor, lens, Binocular vision results and provides:
and vitreous humor Depth perception (three-
The eye is set for distant vision dimensional vision)
(over 20 feet away)
Accommodation—the lens must
change shape to focus on closer
A Closer Look
objects (less than 20 feet away)
Emmetropia—eye focuses images
Pathway of light through the eye and
correctly on the retina
light refraction (continued)
Image formed on the retina is a
Myopia (nearsightedness)
real image
Distant objects appear blurry
Real images are:
Light from those objects fails to
Reversed from left to right
reach the retina and are focused
Upside down
in front of it
Smaller than the object
Results from an eyeball that is
Visual fields and visual pathways to the
too long
brain
Optic nerve
Hyperopia (farsightedness)
Bundle of axons that exit
Near objects are blurry, whereas
the back of the eye
distant objects are clear
carrying impulses from
the retina Distant objects are focused
Optic chiasma behind the retina
Results from an eyeball that is 2. Middle ear
too short or from a ―lazy lens‖ 3. Internal (inner) ear
Astigmatism
Images are blurry External (outer) ear
Results from light focusing as Auricle (pinna)
lines, not points, on the retina External acoustic meatus
because of unequal curvatures of (auditory canal)
the cornea or lens Narrow chamber in the
temporal bone
Lined with skin and
ceruminous (earwax)
Physiology of Vision glands
Ends at the tympanic
Eye reflexes membrane (eardrum)
Convergence: reflexive External ear is involved only in
movement of the eyes medially collecting sound waves
when we focus on a close object Middle ear cavity (tympanic
Photopupillary reflex: bright cavity)
light causes pupils to constrict Air-filled, mucosa-lined cavity
Accommodation pupillary reflex: within the temporal bone
viewing close objects causes Involved only in the sense of
pupils to constrict hearing
The accommodation reflex (or Located between tympanic
near response) is a three- membrane and oval window and
part reflex that brings near round window
objects into focus through lens Pharyngotympanic tube
thickening, pupillary (auditory tube)
constriction, and inward rotation Links middle ear cavity
of the eyes—eye convergence. ... with the throat
This brings the near object into Equalizes pressure in the
focus. middle ear cavity so the
eardrum can vibrate
Three bones (ossicles) span the
cavity
1. Malleus (hammer)
2. Incus (anvil)
3. Stapes (stirrup)
Function
Transmit vibrations from
tympanic membrane to
the fluids of the inner ear
Vibrations travel from the
hammer → anvil → stirrup
→ oval window of inner
ear
Part II: The Ear: Hearing and Internal (inner) ear
Balance Includes sense organs for
hearing and balance
Ear houses two senses Bony labyrinth (osseous
1. Hearing labyrinth) consists of:
2. Equilibrium (balance) Cochlea
Vestibule
Receptors are mechanoreceptors Semicircular canals
Different organs house receptors for Bony labyrinth is filled with
each sense perilymph
Membranous labyrinth is
Anatomy of the Ear suspended in perilymph
The ear is divided into three areas and contains endolymph
1. External (outer) ear
Cochlear nerve attached to hair
cells transmits nerve impulses to
auditory cortex on temporal lobe
Equilibrium
Pathway of vibrations from sound
Equilibrium receptors of the inner ear waves
are called the vestibular apparatus Move by the ossicles from the
Vestibular apparatus has two eardrum to the oval window
functional parts Sound is amplified by the
Static equilibrium ossicles
Dynamic equilibrium Pressure waves cause vibrations
in the basilar membrane in the
spiral organ of Corti
1. Static Equilibrium Hair cells of the tectorial
membrane are bent when the
Maculae—receptors in the vestibule basilar membrane vibrates
against it
Report on the position of the An action potential starts in the
head cochlear nerve (cranial nerve
Help us keep our head erect VIII), and the impulse travels to
Send information via the the temporal lobe
vestibular nerve (division of
cranial nerve VIII) to the High-pitched sounds disturb the short,
cerebellum of the brain stiff fibers of the basilar membrane
Receptor cells close to the oval
Anatomy of the maculae window are stimulated
Hair cells are embedded in the Low-pitched sounds disturb the long,
otolithic membrane floppy fibers of the basilar membrane
Otoliths (tiny stones) float in a Specific hair cells further along
gel around hair cells the cochlea are affected
Movements cause otoliths to roll
and bend hair cells
Hearing and Equilibrium
2. Dynamic Equilibrium Deficits
Crista ampullaris Deafness is any degree of hearing loss
Responds to angular or Conduction deafness results
rotational movements of the when the transmission of sound
head vibrations through the external
Located in the ampulla of each and middle ears is hindered
semicircular canal Sensorineural deafness results
Tuft of hair cells covered with from damage to the nervous
cupula (gelatinous cap) system structures involved in
If the head moves, the cupula hearing
drags against the endolymph Ménière’s syndrome affects the
Hair cells are stimulated, and the inner ear and causes progressive
impulse travels the vestibular deafness and perhaps vertigo
nerve to the cerebellum (sensation of spinning)
Hearing
Spiral organ of Corti
Located within the cochlear duct
Receptors = hair cells on the
basilar membrane
Gel-like tectorial membrane is
capable of bending hair cells
The tongue is covered with projections
called papillae that contain taste buds
Part III: Chemical Senses: Vallate (circumvallate) papillae
Smell and Taste Fungiform papillae
Filiform papillae
Chemoreceptors
Stimulated by chemicals in Gustatory cells are the taste receptors
solution Possess gustatory hairs (long
Taste has five types of receptors microvilli)
Smell can differentiate a wider Gustatory hairs protrude
range of chemicals through a taste pore
Both senses complement each other Hairs are stimulated by
and respond to many of the same chemicals dissolved in saliva
stimuli
Impulses are carried to the gustatory
complex by several cranial nerves
because taste buds are found in
Olfactory Receptors and the different areas
Facial nerve (cranial nerve VII)
Sense of Smell Glossopharyngeal nerve (cranial
nerve IX)
Olfactory receptors are in roof of nasal
Vagus nerve (cranial nerve X)
cavity
Taste buds are replaced frequently by
Olfactory receptor cells (neurons)
basal cells
with long cilia known as olfactory
hairs detect chemicals
Five basic taste sensations
Chemicals must be dissolved in
1. Sweet receptors respond to
mucus for detection by
sugars, saccharine, some amino
chemoreceptors called olfactory
acids
receptors
2. Sour receptors respond to H+
Impulses are transmitted via the
ions or acids
olfactory filaments to the olfactory
3. Bitter receptors respond to
nerve (cranial nerve I)
alkaloids
Smells are interpreted in the olfactory
4. Salty receptors respond to metal
cortex
ions
5. Umami receptors respond to the
amino acid glutamate or the
beefy taste of meat
Part IV: Developmental
Aspects of the Special Senses
Special sense organs are formed early
in embryonic development
Maternal infections during the first 5 or
6 weeks of pregnancy may cause visual
abnormalities as well as sensorineural
deafness in the developing child
Vision requires the most learning
Taste Buds and the Sense of The infant has poor visual acuity (is
farsighted) and lacks color vision and
Taste depth perception at birth
The eye continues to grow and mature
Taste buds house the receptor organs
until age
Locations of taste buds 8 or 9
Most are on the tongue
Soft palate Age-related eye issues
Superior part of the pharynx Presbyopia—―old vision‖ results
Cheeks from decreasing lens elasticity
that accompanies aging
Causes difficulty to focus
for close vision
Lacrimal glands become less
active
Lens becomes discolored
Dilator muscles of iris become
less efficient, causing pupils to
remain constricted
The newborn infant can hear sounds,
but initial responses are reflexive
By the toddler stage, the child is
listening critically and beginning to
imitate sounds as language
development begins
Age-related ear problems
Presbycusis—type of
sensorineural deafness that may
result from otosclerosis
Otosclerosis—ear ossicles
fuse
Congenital ear problems usually
result from missing pinnas and
closed or missing external
acoustic meatuses
Taste and smell are most acute at
birth and decrease in sensitivity after
age 40 as the number of olfactory and
gustatory receptors decreases
