Chapter 8

Special Senses

Introduction:

We are usually told that we have 5 senses that keep us in touch with what is going on in the external world: Touch, Taste, Smell, Sight, and Hearing However, TOUCH is a mixture of the

general senses of temperature, pressure, and receptors of the skin, muscles, and joints.

Introduction:

However, we still have 5 SPECIAL SENSES. These include: Smell Taste Sight Hearing EQUILIBRIUM (Not Touch)

Introduction: These special senses contain SPECIAL

SENSE RECEPTORS. Special sense receptors are either large,

complex sensory organs (eyes and ears) or localized cluster of receptors (taste buds).

This chapter focuses on the functional anatomy of each of the special sense organs individually, but keep in mind that sensory inputs are overlapping.

The Eye and Vision:

Vision is the special sense that has been studied most.

Of all the sensory receptors in the body, 70% are in the eyes.

Vision is also the sense that requires the most “learning.”

Anatomy of the Eye The adult eye is a sphere that measures

about 1 inch in diameter. Only 1/6 of the eye’s surface can be

seen while the rest of it is enclosed and protected by fat.

External and Accessory Structures: Extrinsic Eye Muscles Eyelids Conjunctiva Lacrimal Apparatus

EXTRINSIC EYE MUSCLES

(External Eye Muscles) External eye muscles that are

attached to the outer surface of each eye

These muscles produce eye movements and make it possible for the eyes to follow a moving object

EYELIDS

They meet at the medial and lateral corners of the eye

They protect the eyes anteriorly. Eyelashes - project from the

border of each eyelid

CONJUNCTIVA It is a delicate membrane that lines the

eyelids and covers part of the outer surface of the eyeball

It secretes mucus, which helps to lubricate the eyeball and keep it moist.

Inflammation of the conjunctiva is known as conjunctivitis or pinkeye - infection caused by bacteria or viruses and is highly contagious.

LACRIMAL APPARATUS It consists of the lacrimal gland and many

ducts that drain the lacrimal secretions into the nasal cavity

Lacrimal glands - they continually release a salt solution (TEARS) onto the anterior surface of the eyeball through several small ducts

Lacrimal secretion also contains antibodies and lysozyme - an enzyme that destroys bacteria. It cleanses and protects the eye surface as it moistens and lubricates it.

LACRIMAL APPARATUS If lacrimal secretion increases, tears spill

over the eyelids and fill the nasal cavities. This causes congestion and the “sniffles.” This can occur if the eyes are irritated by foreign

objects or chemicals. The enhanced tearing acts to wash away or dilute the irritating substance.

This can also occur when we are emotionally upset. Some scientists say that the importance of “emotional tears” is that it reduces stress.

Internal Structures

The Eyeball Eyeball - known as the eye itself - a hollow sphere The wall of the eyeball is composed of 3 layers

(1) Sclera - outermost layer that protects the eye This is the “white” of the eye Consists of the cornea - central part of the sclera where light

enters the eye (2) Choroid - middle layer

Contains a dark pigment. This pigment prevents light from scattering inside the eye.

Iris - contains a rounded opening called the pupil - where light passes

(3) Retina - innermost layer The retina contains millions of receptor cells called

photoreceptors A photoreceptor responds to light. There are 2 types of photoreceptors: rods and cones

Rods and Cones:

Rods - allow us to see in gray tones in dim light, and provide for our peripheral vision

Cones - allow us to see the details of our world in color under bright light

Rods and Cones: There are 3 varieties of cones - Each type is

most sensitive to particular wavelengths of visible light

One responds to blue light One responds to green light One responds to a range including both green and red

light **NOT THE 3 PRIMARY COLORS: YELLOW, RED,

AND BLUE A lack of all 3 cone types results in total COLOR

BLINDNESS. But, a lack in just one cone is partial color blindness. Color blindness occurs almost exclusively in males.

Lens

Focuses the light entering the eye on the retina

Cataracts - a condition that results from the hardening of the lens and causes vision to become hazy. Treatment = lens implant

Vision problems: Myopia - also known as NEARSIGHTEDNESS

It occurs when the light rays from distant objects fail to reach the retina and instead are focused in front of it

Distant objects appear blurry - able to see close up Hyperopia - also known as FARSIGHTEDNESS

It occurs when the light rays from distant objects are focused BEHIND the retina.

Nearby objects appear blurry - able to see distances Astigmatism - caused by unequal curvatures

in different parts of the cornea or lens

The Ear: Hearing and Balance

Our hearing apparatus allows us to hear an extraordinary range of sound, and it also allows us to maintain our balance.

Anatomy of the Ear The ear is divided into three major

areas: outer or external middle inner or internal

The outer and middle ear structures are involved with HEARING ONLY.

The inner ear functions in BOTH EQUILIBRIUM and HEARING.

Outer (External) Ear: Composed of the pinna and the external

auditory canal. Pinna - also known as the auricle - the shell-

shaped structure surrounding the auditory canal opening.

External Auditory Canal - a short, narrow chamber located in the temporal bone of the skull

Composed of ceruminous glands - secretes a waxy yellow substance called earwax or cerumen

Sound waves that enter the external auditory canal eventually hit the tympanic membrane or the eardrum and cause it to vibrate

Middle Ear: Also known as the tympanic cavity - a small,

air-filled cavity within the temporal bone Two openings in the middle ear:

Oval Window Round Window

Auditory Tube - links the middle ear with the throat.

Normally this tube is flat and closed, but swallowing or yawning can open it briefly to equalize the pressure in the middle ear cavity (forces air into auditory tube)

If the pressure is unequal, it causes hearing difficulty (voices sound far away).

Middle Ear The tympanic cavity contains 3 small bones

which are known as the OSSICLES: Hammer - also known as the malleus Anvil - also known as the incus Stirrup - also know as the stapes

When the ear drum moves, the hammer moves with it and transfers the vibration to the anvil. The anvil passes it on to the stirrup, which presses on the oval window of the inner ear. The movement of the oval window sets the fluids of the inner ear into motion which excites the hearing receptors.

Inner (Internal) Ear:

3 subdivisions of the inner ear: Cochlea - resembles a snail shell and

houses the hearing receptors Vestibule - located between the

cochlea and the semicircular canals Semicircular Canals - appears as a

maze of circles

Mechanism of Hearing:

Within the cochlea is the ORGAN OF CORTI. It contains the hearing receptors or HAIR CELLS.

The hair cells are stimulated by sound vibrations

Hearing Deficits or Problems: DEAFNESS - hearing loss of any degree - from

a slight loss to a total inability to hear sound CONDUCTION DEAFNESS - occurs when

something interferes with the conduction of sound vibrations to the fluids of the inner ear

Causes = buildup of earwax, a ruptured eardrum, or fusion of the ossicles

SENSORINEURAL DEAFNESS - occurs when there is degeneration or damage to the receptor cells in the organ of Corti

Causes = listening to excessively loud sounds

Chemical Senses: Taste and Smell

Chemical substances MUST be dissolved in water to excite the receptors for smell and taste.

Olfactory Receptors and the Sense of Smell:

OLFACTORY RECEPTORS - receptors for the sense of smell and are located in the superior part of the nasal cavity

Sniffing causes more air to flow across the olfactory receptors which intensifies the sense of smell

Taste Buds and the Sense of Taste: TASTE BUDS - receptors for the sense

of taste and are scattered in the oral cavity (most are located on the tongue)

There are 4 basic taste sensations: Sweet receptors - responds to sugars Sour receptors - responds to acidic

solutions Bitter receptors - responds to alkaloids Salty receptors - responds to metal ions

Location of the taste sensations - DRAW LOCATIONS!!

Developmental Aspects of the Special Senses Special sense organs are formed early

in embryonic development. The eyes begin to develop by the fourth

week Maternal infections during the first five

or six weeks of pregnancy may cause visual abnormalities as well as deafness in the developing child.

An infant is farsighted and lacks color vision and depth perception at birth.

Developmental Aspects of the Special Senses The eye continues to grow and mature

until the eighth or ninth year of life but the lens grows throughout life.

The newborn infant can hear sounds, but initial responses are reflexive.

By the toddler stage, the child is listening critically, begins to imitate sounds, and language development begins.

Taste and smell are sharp at birth and decrease in sensitivity after the age of 40.