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Title: Special Senses


1
Chapter 8
  • Special Senses

2
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.

3
Introduction
  • However, we still have 5 SPECIAL SENSES. These
    include
  • Smell
  • Taste
  • Sight
  • Hearing
  • EQUILIBRIUM (Not Touch)

4
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.

5
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.

6
Anatomy of the Eye
  • The adult eye is a sphere that measures about 1
    inch in diameter.
  • Only 1/6 of the eyes 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

7
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

8
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

9
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.

10
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.

11
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.

12
Internal Structures
13
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

14
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

15
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.

16
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

17
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

18
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.

19
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.

20
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

21
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).

22
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.

23
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

24
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

25
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

26
Chemical Senses Taste and Smell
  • Chemical substances MUST be dissolved in water to
    excite the receptors for smell and taste.

27
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

28
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!!

29
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.

30
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.
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