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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