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Name ____________________________________________________________ Period ______
SPECIAL SENSES
The Senses
General senses of touch
o Temperature
o Pressure
o Pain
Special senses
o Smell
o Taste
o Sight
o Hearing
o Equilibrium
The Eye and Vision
70 percent of all sensory receptors are in the eyes
Each eye has over a million nerve fibers
Protection for the eye
o Most of the eye is enclosed in a bony orbit
o A cushion of fat surrounds most of the eye
Accessory Structures of the Eye
Eyelids
Eyelashes
Meibomian or tarsal glands – modified sebacious glands of the eyelid produce an oily
secretion to lubricate the eye
Ciliary glands – modified sweat glands between the eyelashes
Conjunctiva
o Membrane that lines the eyelids
o Connects to the surface of the eye
o Secretes mucus to lubricate the eye
Lacrimal apparatus
o Lacrimal gland – produces
lacrimal fluid – located
above the lateral end of each
eye
o Lacrimal canals – drains
lacrimal fluid from eyes into
the lacrimal sac – medial
location
Lacrimal sac – provides passage of
lacrimal fluid towards nasal cavity
Nasolacrimal duct – empties
lacrimal fluid into the nasal cavity
Function of the Lacrimal Apparatus
Properties of lacrimal fluid
o Dilute salt solution (tears)
o Contains antibodies and lysozyme, an enzyme that destroys bacteria
Protects, moistens, and lubricates the eye
Empties into the nasal cavity (sniffles!)
Six Extrinsic Eye Muscles
Muscles attach to the outer surface of the eye
Produce eye movements
Structure of the Eye
The wall is composed of three tunics
o Fibrous tunic or sclera– outside layer
o Vascular tunic or choroid – middle layer
o Sensory tunic or retina– inside layer
The Fibrous Tunic
Sclera
o White connective tissue layer
o Seen anteriorly as the “white of the eye”
Cornea
o Transparent, central anterior portion
o Allows for light to pass through
o Repairs itself easily
o The only human tissue that can be transplanted without fear of rejection (no blood
vessels)
Vascular Tunic or Choroid Layer
Blood-rich nutritive tunic w/ dark pigment
Pigment prevents light from scattering
Modified interiorly into two structures
o Cilliary body – smooth muscle
Attaches to lens by ciliary zonule, a ligament
o Iris – smooth muscle
Pigmented layer that gives eye color
Pupil – rounded opening in the iris
Sensory Tunic (Retina)
Contains receptor cells (photoreceptors)
o Rods (B/W)
o Cones (Color)
Signals pass from photoreceptors via a two-neuron chain
o Bipolar neurons
o Ganglion cells
Signals leave the retina toward the brain through the optic nerve
Neurons of the Retina and Vision
Rods
o Most are found towards the edges of the retina
o Allow dim light vision and peripheral vision
o Perception is all in gray tones
Cones
o Allow for detailed color vision
o Densest in the center of the retina
o Fovea centralis – area of the retina with only cones – lateral to each blind spot
No photoreceptor cells are at the optic disk, or blind spot – where the optic nerve leaves
the eyeball
Cone Sensitivity
There are three types of cones
Different cones are sensitive to different
wavelengths
o One blue, another green and the
3rd a range including green and
red (“red cones”)
Color blindness is the result of lack of
one cone type
Lens
Biconvex crystal-like structure
Held in place by a suspensory ligament attached to the ciliary body
Internal Eye Chamber Fluids
Aqueous humor
o Watery fluid found in chamber between the lens and cornea
o Similar to blood plasma; secreted by a special area of choroid
o Helps maintain intraocular pressure
o Provides nutrients for the lens and cornea
o Reabsorbed into venous blood through the scleral venous sinus or canal of
Schlemm
Vitreous humor
o Gel-like substance behind the lens
o Keeps the eye from collapsing
o Lasts a lifetime and is not replaced
Lens Accommodation
Light must be focused to a
point on the retina for optimal
vision
The eye is set for distance
vision
(over 20 ft away)
The lens must change shape to
focus for closer objects
Images Formed on the Retina
Visual Pathway
Photoreceptors of the retina
Optic nerve– bundle of axons
carrying retinal impulses
Optic nerve crosses at the optic
chiasma
Optic tracts – contain fibers
from the lateral side of the eye
on the same side and the
medial side of the opposite eye
Optic tract fibers synapse w/
neurons in the thalamus
Thalamus (axons form optic
radiation)
Visual cortex of the occipital
lobe
Eye Reflexes
Internal muscles are controlled
by the autonomic nervous
system
o Bright light causes
pupils to constrict
through action of radial
and ciliary muscles
o Viewing close objects causes accommodation
External muscles control eye movement to follow objects
Viewing close objects causes convergence (eyes moving medially)
The Ear
Houses two senses
o Hearing
o Equilibrium (balance)
Receptors are mechanoreceptors
Different organs house receptors for each sense
Anatomy of the Ear
The ear is divided into three areas
o Outer (external) ear
o Middle ear
o Inner ear
The External Ear
Involved in hearing only
Structures of the external ear
o Pinna (auricle)
o External auditory canal
The External Auditory Canal
Narrow chamber in the temporal bone
Lined with skin
Ceruminous (wax) glands are present
Ends at the tympanic membrane
The Middle Ear or Tympanic Cavity
Air-filled cavity within the temporal bone
o Flanked laterally by eardrum and medially by a bony wall with 2 openings, the
oval window & the inferior, membrane-covered round window
Only involved in the sense of hearing
Two tubes are associated with the inner ear
o The opening from the auditory canal is covered by the tympanic membrane
o The auditory tube connecting the middle ear with the throat
Allows for equalizing pressure during yawning or swallowing
This tube is otherwise collapsed
Bones of the Tympanic Cavity
Three bones (ossicles) span the cavity
o Malleus (hammer)
o Incus (anvil)
o Stapes (stirrip)
Vibrations from eardrum move the malleus,which moves the anvil, which moves the
stirrup and presses on the oval window of the inner ear
These bones transfer sound to the inner ear
Inner Ear or Bony Labyrinth
Includes sense organs for hearing and balance
Filled with perilymph
A maze of bony chambers within the temporal bone
o Cochlea
o Vestibule
o Semicircular canals
Chemical Senses – Taste and Smell
Both senses use chemoreceptors
o Stimulated by chemicals in solution
o Taste has four types of receptors
o Smell can differentiate a large range of chemicals
Both senses complement each other and respond to many of the same stimuli
Olfaction – The Sense of Smell
Olfactory receptors (neurons with long cilia) are in the roof of the nasal cavity
o Chemicals must be dissolved in mucus for detection and impulse transmission by
the hairs (cilia)
Impulses are transmitted via the olfactory nerve
Interpretation of smells is made in the cortex
Cortex stores “snapshots” of smells & is tied to the limbic system (emotions)
Olfactory Epithelium
The Sense of Taste
Taste buds (10,000+) house the receptor organs
Location of taste buds
o Most are on the tongue
o Soft palate
o Cheeks
The Tongue and Taste
The tongue is covered with projections called papillae
o Filiform papillae – sharp with no taste buds
o Fungifiorm papillae – rounded with taste buds
o Circumvallate papillae – large papillae with taste buds
Taste buds are found on the sides of papillae
Structure of Taste Buds
Gustatory cells are the receptors
o Have gustatory hairs (long microvilli)
o Hairs are stimulated by chemicals dissolved in saliva
Depolarizes the nerve and the impulse is carried to the brain
Impulses are carried to the gustatory complex by several cranial nerves because taste
buds are found in different areas
o Facial nerve
o Glossopharyngeal nerve
o Vagus nerve
Anatomy of Taste Buds
Taste Sensations
Sweet receptors
o Sugars
o Saccharine
o Some amino acids
Sour receptors
o Acids
Bitter receptors
o Alkaloids
Salty receptors
o Metal ions
Developmental Aspects of the Special Senses
Formed early in embryonic development
Eyes are outgrowths of the brain
o Developing by the 4th week
o Eyeballs enlarge until age 8 or 9 (lenses throughout life)
o Babies can’t see near at birth (hyperopic)
All special senses are functional at birth
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