Chapter 16 The Special Senses

1
Chapter 16The Special Senses

• Smell, taste, vision, hearing and equilibrium
• Housed in complex sensory organs
• Ophthalmology is science of the eye
• Otolaryngology is science of the ear

2
Chemical Senses

• Interaction of molecules with receptor cells
• Olfaction (smell) and gustation (taste)
• Both project to cerebral cortex limbic system
• evokes strong emotional reactions

3
Olfactory Epithelium

• 1 square inch of membrane holding 10-100 million
  receptors
• Covers superior nasal cavity and cribriform plate
• 3 types of receptor cells

4
Olfaction Sense of Smell

• Odorants bind to receptors
• Na channels open
• Depolarization occurs
• Nerve impulse is triggered

5
Adaptation Odor Thresholds

• Adaptation decreasing sensitivity
• Olfactory adaptation is rapid
• 50 in 1 second
• complete in 1 minute
• Low threshold
• only a few molecules need to be present
• methyl mercaptan added to natural gas as warning

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Gustatory Sensation Taste

• Taste requires dissolving of substances
• Four classes of stimuli--sour, bitter, sweet, and
  salty
• 10,000 taste buds found on tongue, soft palate
  larynx

7
Anatomy of Taste Buds

• An oval body consisting of 50 receptor cells
  surrounded by supporting cells
• A single gustatory hair projects upward through
  the taste pore
• Basal cells develop into new receptor cells every
  10 days.

8
Physiology of Taste

• Complete adaptation in 1 to 5 minutes
• Thresholds for tastes vary among the 4 primary
  tastes
• most sensitive to bitter (poisons)
• least sensitive to salty and sweet

9
Accessory Structures of Eye

• Eyelids or palpebrae
• protect lubricate
• Tarsal glands
• oily secretions keep lids from sticking together
• Conjunctiva
• stops at corneal edge
• dilated BV--bloodshot

10
Eyelashes Eyebrows
Eyeball 1 inch diameter
5/6 of Eyeball inside orbit protected

• Eyelashes eyebrows help protect from foreign
  objects, perspiration sunlight
• Sebaceous glands are found at base of eyelashes
  (sty)
• Palpebral fissure is gap between the eyelids

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Lacrimal Apparatus

• About 1 ml of tears produced per day. Spread over
  eye by blinking. Contains bactericidal enzyme
  called lysozyme.

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Extraocular Muscles

• Six muscles that insert on the exterior surface
  of the eyeball
• .
• 4 rectus muscles -- superior, inferior, lateral
  and medial
• 2 oblique muscles -- inferior and superior

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Tunics (Layers) of Eyeball

• Fibrous Tunic(outer layer)
• Vascular Tunic (middle layer)
• Nervous Tunic(inner layer)

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Fibrous Tunic -- Description of Cornea

• Transparent
• Helps focus light(refraction)
• astigmatism
• Transplants
• common successful
• no blood vessels so no antibodies to cause
  rejection

15
Fibrous Tunic -- Description of Sclera

• White of the eye
• Dense irregular connective tissue layer --
  collagen fibroblasts
• Provides shape support

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Vascular Tunic -- Choroid Ciliary Body

• Choroid
• pigmented epithilial cells (melanocytes) blood
  vessels
• provides nutrients to retina
• black pigment in melanocytes absorb scattered
  light
• Ciliary body
• ciliary processes
• folds on ciliary body
• secrete aqueous humor
• ciliary muscle
• smooth muscle that alters shape of lens

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Vascular Tunic -- Iris Pupil

• Colored portion of eye
• Shape of flat donut suspended between cornea
  lens
• Hole in center is pupil
• Function is to regulate amount of light entering
  eye

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Vascular Tunic -- Description of lens

• Avascular
• Crystallin proteins arranged like layers in onion
• Clear capsule perfectly transparent

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Nervous Tunic -- Retina

• Posterior 3/4 of eyeball
• Optic disc
• optic nerve exiting back of eyeball
• Central retina BV
• fan out to supply nourishment to retina
• visible for inspection
• hypertension diabetes
• Detached retina
• trauma (boxing)
• fluid between layers
• distortion or blindness

View with Ophthalmoscope
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Rods Cones--Photoreceptors

• Rods----rod shaped
• shades of gray in dim light
• 120 million rod cells
• discriminates shapes movements
• distributed along periphery
• Cones----cone shaped
• sharp, color vision
• 6 million
• fovea of macula lutea
• densely packed region
• at exact visual axis of eye
• 2nd cells do not cover cones
• sharpest resolution or acuity

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Pathway of Nerve Signal in Retina

• Light penetrates retina
• Rods cones transduce light into action
  potentials
• Rods cones excite bipolar cells
• Bipolars excite ganglion cells
• Axons of ganglion cells form optic nerve leaving
  the eyeball (blind spot)
• To thalamus then the primary visual cortex

22
Aqueous Humor

• Continuously produced by ciliary body
• Flows from posterior chamberinto anterior
  through the pupil
• Glaucoma
• increased intraocular pressure that could produce
  blindness
• problem with drainage of aqueous humor

23
Major Processes of Image Formation

• Refraction of light
• by cornea lens
• light rays must fall upon the retina
• Accommodation of the lens
• changing shape of lens so that light is focused
• Constriction of the pupil
• less light enters the eye

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Definition of Refraction

• Bending of light as it passes from one substance
  (air) into a 2nd substance with a different
  density(cornea)
• In the eye, light is refracted by the anterior
  posterior surfaces of the cornea and the lens

25
Refraction by the Cornea Lens

• Image focused on retina is inverted reversed
  from left to right
• Brain learns to work with that information
• 75 of Refraction is done by cornea -- rest is
  done by the lens

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Near Point of Vision and Presbyopia

• Near point is the closest distance from the eye
  an object can be still be in clear focus
• 4 inches in a young adult
• 8 inches in a 40 year old
• lens has become less elastic
• 31 inches in a 60 to 80 year old
• Reading glasses may be needed by age 40
• presbyopia
• glasses replace refraction previously provided by
  increased curvature of the relaxed, youthful lens

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Correction for Refraction Problems

• Emmetropic eye (normal)
• can refract light from 20 ft away
• Myopia (nearsighted)
• eyeball is too long from front to back
• glasses concave
• Hypermetropic (farsighted)
• eyeball is too short
• glasses convex (coke-bottle)
• Astigmatism
• corneal surface wavy
• parts of image out of focus

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Constriction of the Pupil

• Constrictor pupillae muscle contracts
• Prevents light rays from entering the eye through
  the edge of the lens
• Sharpens vision by preventing blurry edges
• Protects retina very excessively bright light

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Convergence of the Eyes

• Binocular vision in humans has both eyes looking
  at the same object
• As you look at an object close to your face, both
  eyeballs must turn inward.
• convergence

30
Photoreceptors

• Photopigment is integral membrane protein of
  outer segment membrane
• photopigment membrane folded into discs
  replaced at a very rapid rate
• Photopigments opsin (protein) retinal
  (derivative of vitamin A)
• rods contain rhodopsin
• cone photopigments contain 3 different opsin
  proteins permitting the absorption of 3 different
  wavelengths (colors) of light

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Color Blindness Night Blindness

• Color blindness
• inability to distinguish between certain colors
• absence of certain cone photopigments
• red-green color blind person can not tell red
  from green
• Night blindness (nyctalopia)
• difficulty seeing in low light
• inability to make normal amount of rhodopsin
• possibly due to deficiency of vitamin A

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Light and Dark Adaptation

• Light adaptation
• adjustments when emerge from the dark into the
  light
• Dark adaptation
• adjustments when enter the dark from a bright
  situation
• light sensitivity increases as photopigments
  regenerate
• during first 8 minutes of dark adaptation, only
  cone pigments are regenerated, so threshold burst
  of light is seen as color
• after sufficient time, sensitivity will increase
  so that a flash of a single photon of light will
  be seen as gray-white

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Brain Pathways of Vision
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Processing of Image Data in the Brain

• Visual information in optic nerve travels to
• occipital lobe for vision
• midbrain for controlling pupil size
  coordination of head and eye movements
• hypothalamus to establish sleep patterns based
  upon circadian rhythms of light and darkness

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Visual fields

• Left occipital lobe receives visual images from
  right side of an object through impulses from
  nasal 1/2 of the right eye and temporal 1/2 of
  the left eye
• Left occipital lobe sees right 1/2 of the world
• Fibers from nasal 1/2 of each retina cross in
  optic chiasm

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Anatomy of the Ear Region
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External Ear

• Function collect sounds
• Structures
• auricle or pinna
• elastic cartilage covered with skin
• external auditory canal
• curved 1 tube of cartilage bone leading into
  temporal bone
• ceruminous glands produce cerumen ear wax
• tympanic membrane or eardrum
• epidermis, collagen elastic fibers, simple
  cuboidal epith.
• Perforated eardrum (hole is present)
• at time of injury (pain, ringing, hearing loss,
  dizziness)
• caused by explosion, scuba diving, or ear
  infection

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Middle Ear Cavity
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Middle Ear Cavity

• Air filled cavity in the temporal bone
• Separated from external ear by eardrum and from
  internal ear by oval round window
• 3 ear ossicles connected by synovial joints
• malleus attached to eardrum, incus stapes
  attached by foot plate to membrane of oval window
• stapedius and tensor tympani muscles attach to
  ossicles
• Auditory tube leads to nasopharynx
• helps to equalize pressure on both sides of
  eardrum
• Connection to mastoid bone mastoiditis

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Inner Ear---Bony Labyrinth
Vestibule
canals
ampulla

• Bony labyrinth set of tubelike cavities in
  temporal bone
• semicircular canals, vestibule cochlea lined
  with periosteum filled with perilymph
• surrounds protects Membranous Labyrinth

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Inner Ear---Membranous Labyrinth

• Membranous labyrinth set of membranous tubes
  containing sensory receptors for hearing
  balance and filled with endolymph
• utricle, saccule, ampulla, 3 semicircular ducts
  cochlea

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Anatomy of the Organ of Corti

• 16,000 hair cells have 30-100 stereocilia(microvil
  li )
• Microvilli make contact with tectorial membrane
  (gelatinous membrane that overlaps the spiral
  organ of Corti)
• Basal sides of inner hair cells synapse with 1st
  order sensory neurons whose cell body is in
  spiral ganglion

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Sound Waves

• Vibrating object causes compression of air around
  it sound waves
• audible range is 20 to 20,000 Hz
• hear best within 500 to 5000 cycles/sec or Hz
• speech is 100 to 3000 Hz
• Frequency of a sound vibration is pitch
• higher frequency is higher pitch
• Greater intensity (size) of vibration, the louder
  the sound measured in decibels (dB)
• Conversation is 60 dB pain above 140dB
• OSA requires ear protection above 90dB

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Deafness

• Nerve deafness
• damage to hair cells from antibiotics, high
  pitched sounds, anticancer drugs
• the louder the sound the quicker the hearing loss
• fail to notice until difficulty with speech
• Conduction deafness
• perforated eardrum
• otosclerosis

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Physiology of Hearing

• Auricle collects sound waves
• Eardrum vibrates
• slow vibration in response to low-pitched sounds
• rapid vibration in response to high-pitched
  sounds
• Ossicles vibrate since malleus attached to
  eardrum
• Stapes pushes on oval window producing fluid
  pressure waves in scala vestibuli tympani
• oval window vibration 20X more vigorous than
  eardrum
• Pressure fluctuations inside cochlear duct move
  the hair cells against the tectorial membrane
• Microvilli are bent producing receptor potentials

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Overview of Physiology of Hearing
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Cochlear Implants

• If deafness is due to destruction of hair cells
• Microphone, microprocessor electrodes translate
  sounds into electric stimulation of the
  vestibulocochlear nerve
• artificially induced nerve signals follow normal
  pathways to brain
• Provides only a crude representation of sounds

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Physiology of Equilibrium (Balance)

• Static equilibrium
• maintain the position of the body (head) relative
  to the force of gravity
• Dynamic equilibrium
• maintain body position (head) during sudden
  movement of any type--rotation, deceleration or
  acceleration

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Detection of Position of Head

• Movement of stereocilia or kinocilium results in
  the release of neurotransmitter onto the
  vestibular branches of the vestibulocochler nerve

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Detection of Rotational Movement

• When head moves, the attached semicircular ducts
  and hair cells move with it
• Nerve signals to the brain are generated
  indicating which direction the head has been
  rotated