Special Senses

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Chapter 8

• Special Senses

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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.

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Introduction

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

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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.

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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.

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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

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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

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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

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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.

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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.

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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.

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Internal Structures
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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

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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

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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.

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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

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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

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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.

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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.

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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

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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).

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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.

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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

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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

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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

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Chemical Senses Taste and Smell

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

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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

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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!!

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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.

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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.