Chapter 17 The Special Senses Lecture Outline Chapter 17 The

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Chapter 17

• The Special Senses
• Lecture Outline

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Chapter 17The 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

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Chemical Senses

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

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Anatomy of olfactory receptors

• The receptors for olfaction, which are bipolar
  neurons, are in the nasal epithelium in the
  superior portion of the nasal cavity (Figure
  17.1).
• They are first-order neurons of the olfactory
  pathway.
• Supporting cells are epithelial cells of the
  mucous membrane lining the nose.
• Basal stem cells produce new olfactory receptors.

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Olfactory Epithelium

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

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Cells of the Olfactory Membrane

• Olfactory receptors
• bipolar neurons with cilia or olfactory hairs
• Supporting cells
• columnar epithelium
• Basal cells stem cells
• replace receptors monthly
• Olfactory glands
• produce mucus
• Both epithelium glands innervated cranial nerve
  VII.

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Physiology of Olfaction - Overview

• Genetic evidence suggests there are hundreds of
  primary scents.
• In olfactory reception, a generator potential
  develops and triggers one or more nerve impulses.
• Adaptation to odors occurs quickly, and the
  threshold of smell is low only a few molecules
  of certain substances need be present in air to
  be smelled.
• Olfactory receptors convey nerve impulses to
  olfactory nerves, olfactory bulbs, olfactory
  tracts, and the cerebral cortex and limbic
  system.
• Hyposmia, a reduced ability to smell, affects
  half of those over age 65 and 75 of those over
  80. It can be caused by neurological changes,
  drugs, or the effects of smoking .

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Olfaction Sense of Smell

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

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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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Olfactory Pathway

• Axons from olfactory receptors form the olfactory
  nerves (Cranial nerve I) that synapse in the
  olfactory bulb
• pass through 40 foramina in cribriform plate
• Second-order neurons within the olfactory bulb
  form the olfactory tract that synapses on
  primary olfactory area of temporal lobe
• conscious awareness of smell begins
• Other pathways lead to the frontal lobe (Brodmann
  area 11) where identification of the odor occurs

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GUSTATORY SENSE OF SMELL

• Taste is a chemical sense.
• To be detected, molecules must be dissolved.
• Taste stimuli classes include sour, sweet,
  bitter, and salty.

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Gustatory Sensation Taste

• Taste requires dissolving of substances
• Four classes of stimuli--sour, bitter, sweet, and
  salty
• Other tastes are a combination of the four
  taste sensations plus olfaction.
• 10,000 taste buds found on tongue, soft palate
  larynx
• Found on sides of circumvallate fungiform
  papillae
• 3 cell types supporting, receptor basal cells

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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.

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Physiology of Taste

• Receptor potentials developed in gustatory hairs
  cause the release of neurotransmitter that gives
  rise to nerve impulses.
• 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
• Mechanism
• dissolved substance contacts gustatory hairs
• receptor potential results in neurotransmitter
  release
• nerve impulse formed in 1st-order neuron

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Gustatory Pathway

• First-order gustatory fibers found in cranial
  nerves
• V
• VII (facial) serves anterior 2/3 of tongue
• IX (glossopharyngeal) serves posterior 1/3 of
  tongue
• X (vagus) serves palate epiglottis
• Signals travel to thalamus or limbic system
  hypothalamus
• Taste fibers extend from the thalamus to the
  primary gustatory area on parietal lobe of the
  cerebral cortex
• provides conscious perception of taste

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VISION

• More than half the sensory receptors in the human
  body are located in the eyes.
• A large part of the cerebral cortex is devoted to
  processing visual information.

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Accessory Structures of Eye - Overview

• Eyelids or palpebrae
• protect lubricate
• epidermis, dermis, CT, orbicularis oculi m.,
  tarsal plate, tarsal glands conjunctiva
• Tarsal glands
• oily secretions
• Conjunctiva
• palpebral bulbar
• stops at corneal edge

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Eyelids

• The eyelids shade the eyes during sleep, protect
  the eyes From superficial to deep, each eyelid
  consists of epidermis, dermis, subcutaneous
  tissue, fibers of the orbicularis oculi muscle, a
  tarsal plate, tarsal glands, and conjunctiva
  (Figure 17.4a).
• The tarsal plate gives form and support to the
  eyelids.
• The tarsal glands secrete a fluid to keep the eye
  lids from adhering to each other.
• The conjunctiva is a thin mucous membrane that
  lines the inner aspect of the eyelids and is
  reflected onto the anterior surface of the
  eyeball.
• Eyelashes and eyebrows help protect the eyeballs
  from foreign objects, perspiration, and the
  direct rays of the sun.

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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
• Innervated by CN III, IV or VI.
• 4 rectus muscles -- superior, inferior, lateral
  and medial
• 2 oblique muscles -- inferior and superior

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Tunics (Layers) of Eyeball

• The eye is constructed of three layers (Figure
  17.5).
• 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
• 3 layers
• nonkeratinized stratified squamous
• collagen fibers fibroblasts
• simple squamous epithelium
• Transplants
• common successful
• no blood vessels so no antibodies to cause
  rejection
• Nourished by tears aqueous humor

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Fibrous Tunic -- Description of Sclera

• White of the eye
• Dense irregular connective tissue layer --
  collagen fibroblasts
• Provides shape support
• At the junction of the sclera and cornea is an
  opening (scleral venous sinus)
• Posteriorly pierced by Optic Nerve (CNII)

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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
• Autonomic reflexes
• circular muscle fibers contract in bright light
  to shrink pupil
• radial muscle fibers contract in dim light to
  enlarge pupil

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Vascular Tunic -- Muscles of the Iris

• Constrictor pupillae (circular) are innervated by
  parasympathetic fibers while Dilator pupillae
  (radial) are innervated by sympathetic fibers.
• Response varies with different levels of light

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Vascular Tunic -- Description of lens

• Avascular
• Crystallin proteins arranged like layers in onion
• Clear capsule perfectly transparent
• Lens held in place by suspensory ligaments
• Focuses light on fovea

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Vascular Tunic -- Suspensory ligament

• Suspensory ligaments attach lens to ciliary
  process
• Ciliary muscle controls tension on ligaments
  lens

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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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Photoreceptors

• shapes of their outer segments differ
• Rods
• specialized for black-and-white vision in dim
  light
• allow us to discriminate between different shades
  of dark and light
• permit us to see shapes and movement.
• Cones
• specialized for color vision and sharpness of
  vision (high visual acuity) in bright light
• most densely concentrated in the central fovea, a
  small depression in the center of the macula
  lutea.

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Photoreceptors

• The macula lutea is in the exact center of the
  posterior portion of the retina, corresponding to
  the visual axis of the eye.
• The fovea is the area of sharpest vision because
  of the high concentration of cones.
• Rods are absent from the fovea and macula and
  increase in density toward the periphery of the
  retina.

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Layers of Retina

• Pigmented epithelium
• nonvisual portion
• absorbs stray light helps keep image clear
• 3 layers of neurons (outgrowth of brain)
• photoreceptor layer
• bipolar neuron layer
• ganglion neuron layer
• 2 other cell types (modify the signal)
• horizontal cells
• amacrine cells

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Rods Cones--Photoreceptors

• Rods----rod shaped
• shades of gray in dim light
• 120 million rod cells
• 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 (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

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Lens

• The eyeball contains the nonvascular lens, just
  behind the pupil and iris.
• The lens fine tunes the focusing of light rays
  for clear vision.
• With aging the lens loses elasticity and its
  ability to accommodate resulting in a condition
  known as presbyopia.

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Cavities of the Interior of Eyeball

• Anterior cavity (anterior to lens)
• filled with aqueous humor
• produced by ciliary body
• continually drained
• replaced every 90 minutes
• 2 chambers
• anterior chamber between cornea and iris
• posterior chamber between iris and lens
• Posterior cavity (posterior to lens)
• filled with vitreous body (jellylike)
• formed once during embryonic life
• floaters are debris in vitreous of older
  individuals

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Eye Anatomy

• The pressure in the eye, called intraocular
  pressure, is produced mainly by the aqueous
  humor.
• The intraocular pressure, along with the vitreous
  body, maintains the shape of the eyeball and
  keeps the retina smoothly applied to the choroid
  so the retina will form clear images.
• Glaucoma
• increased intraocular pressure
• problem with drainage of aqueous humor
• may produce degeneration of the retina and
  blindness

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Aqueous Humor

• Continuously produced by ciliary body
• Flows from posterior chamberinto anterior
  through the pupil
• Scleral venous sinus
• canal of Schlemm
• opening in white of eyeat junction of cornea
  sclera
• drainage of aqueous humor from eye to bloodstream

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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

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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
• Light rays from gt 20 are nearly parallel and
  only need to be bent enough to focus on retina
• Light rays from lt 6 are more divergent need
  more refraction
• extra process needed to get additional bending of
  light is called accommodation

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Accommodation the Lens

• Accommodation is an increase in the curvature of
  the lens, initiated by ciliary muscle
  contraction, which allows the lens to focus on
  near objects (figure 17.10c).
• Convex lens refract light rays towards each other
• Lens of eye is convex on both surfaces
• Viewing a distant object
• lens is nearly flat by pulling of suspensory
  ligaments
• View a close object
• ciliary muscle is contracted decreases the pull
  of the suspensory ligaments on the lens
• elastic lens thickens as the tension is removed
  from it
• increase in curvature of lens is called
  accommodation
• The near point of vision is the minimum distance
  from the eye that an object can be clearly
  focused with maximum effort.

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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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Refraction Abnormalities

• Myopia is nearsightedness (Figure 17.11).
• Hyperopia is farsightedness (Figure 17.11).
• Astigmatism is a refraction abnormality due to an
  irregular curvature of either the cornea or 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
• Narrows beam of light that enters the eye
• 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.
• In convergence, the eyeballs move medially so
  they are both directed toward an object being
  viewed.
• required so that light rays from the object will
  strike both retinas at the same relative point
• extrinsic eye muscles must coordinate this action

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Physiology of Vision

• The first step in vision transduction is the
  absorption of light by photopigments (visual
  pigments) in rods and cones (photoreceptors)
  (Figure 17.12).

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Photoreceptors

• Named for shape of outer segment
• Receptors transduce light energy into a receptor
  potential in outer segment
• Photopigment is integral membrane protein of
  outer segment membrane
• photopigment membrane is folded into discs
  replaced at a very rapid rate
• Photopigments
• opsin (protein) retinal (derivative of vitamin
  A)

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Physiology of Vision

• Photopigments are undergo structural changes upon
  light absorption.
• Retinal is the light absorbing part of all visual
  photopigments.
• All photopigments involved in vision contain a
  glycoprotein called opsin and a derivative of
  vitamin A called retinal.
• There are four different opsins
• A cone contains one of three different kinds of
  photopigments so there are three types of cones.
• permit the absorption of 3 different wavelengths
  (colors) of light
• Rods contain a single type of photopigment
  (rhodopsin)

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Physiology of Vision

• Figure 16.14 shows how photopigments are
  activated and restored.
• Bleaching and regeneration of the photopigments
  accounts for much but not all of the sensitivity
  change during light and dark adaptation.

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Photopigments

• Isomerization
• light cause cis-retinal to straighten become
  trans-retinal shape
• Bleaching
• enzymes separate the trans-retinal from the opsin
• colorless final products
• Regeneration
• in darkness, an enzyme converts trans-retinal
  back to cis-retinal (resynthesis of a
  photopigment)

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Application Color Blindness Night Blindness

• Most forms of colorblindness (inability to
  distinguish certain colors) result from an
  inherited absence of or deficiency in one of the
  three cone photopigments and are more common in
  males. A deficiency in rhodopsin may cause night
  blindness (nyctalopia)
• 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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Regeneration of Bleached Photopigments

• Pigment epithelium near the photoreceptors
  contains large amounts of vitamin A and helps the
  regeneration process.
• After complete bleaching, it takes 5 minutes to
  regenerate 1/2 of the rhodopsin
• Full regeneration of bleached rhodopsin takes 30
  to 40 minutes
• Rods contribute little to daylight vision, since
  they are bleached as fast as they regenerate.
• Only 90 seconds are required to regenerate the
  cone photopigments

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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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Details Formation of Receptor Potentials

• In darkness
• Na channels are held open and photoreceptor is
  always partially depolarized (-30mV)
• continuous release of inhibitory neurotransmitter
  onto bipolar cells suppresses their activity
• In light
• enzymes cause the closing of Na channels
  producing a hyperpolarized receptor potential
  (-70mV)
• release of inhibitory neurotransmitter is stopped
• bipolar cells become excited and a nerve impulse
  will travel towards the brain

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Release of Neurotransmitters
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Visual Pathway

• Horizontal cells transmit inhibitory signals to
  bipolar cells
• bipolar or amacrine cells transmit excitatory
  signals to ganglion cells
• ganglion cells which depolarize and initiate
  nerve impulses (Figure 17.8).
• Impulses are conveyed through the retina to the
  optic nerve, the optic chiasma, the optic tract,
  the thalamus, and the occipital lobes of the
  cortex (Figure 17.15).

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Retinal Processing of Visual Information

• Convergence
• one cone cell synapses onto one bipolar cell
  produces best visual acuity
• 600 rod cells synapse on single bipolar cell
  increasing light sensitivity although slightly
  blurry image results
• 126 million photoreceptors converge on 1 million
  ganglion cells
• Horizontal and amacrine cells
• horizontal cells enhance contrasts in visual
  scene because laterally inhibit bipolar cells in
  the area
• amacrine cells excite bipolar cells if levels of
  illumination change

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

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Visual fields

• Fibers from nasal 1/2 of each retina cross in
  optic chiasm
• 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
  and Right occipital lobe sees left 1/2 of the
  world.

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Anatomy of the Ear Region
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HEARING AND EQUILIBRIUM - Overview

• The external (outer) ear collects sound waves.
• The middle ear (tympanic cavity) is a small,
  air-filled cavity in the temporal bone that
  contains auditory ossicles (middle ear bones, the
  malleus, incus, and stapes), the oval window, and
  the round window (Figure 17.17).
• The internal (inner) ear is also called the
  labyrinth because of its complicated series of
  canals (Figure 17.18).

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Anatomy of the Ear Region
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External Ear

• The external (outer) ear collects
• sound waves and passes them
• inwards (Figure 17.16)
• 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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Muscles of the Ear

• Stapedius m. inserts onto stapes
• prevents very large vibrations of stapes from
  loud noises
• Tensor tympani attaches to malleus
• limits movements of malleus stiffens eardrum to
  prevent damage

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Bony Labyrinth

• The bony labyrinth is a series of cavities in the
  petrous portion of the temporal bone.
• It can be divided into three areas named on the
  basis of shape the semicircular canals and
  vestibule, both of which contain receptors for
  equilibrium, and the cochlea, which contains
  receptors for hearing.
• The bony labyrinth is lined with periosteum and
  contains a fluid called perilymph. This fluid,
  chemically similar to cerebrospinal fluid,
  surrounds the membranous labyrinth.

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Inner Ear---Bony Labyrinth

• 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
• utricle, saccule, ampulla, 3 semicircular ducts
  cochlea

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Membranous Labyrinth

• The membranous labyrinth is a series of sacs and
  tubes lying inside and having the same general
  form as the bony labyrinth.
• lined with epithelium.
• contains a fluid called endolymph, chemically
  similar to intracellular fluid.
• The vestibule constitutes the oval central
  portion of the bony labyrinth. The membranous
  labyrinth in the vestibule consists of two sacs
  called the utricle and saccule.
• Anterior to the vestibule is the cochlea, which
  consists of a bony spiral canal that makes almost
  three turns around a central bony core called the
  modiolus (Figure 17.19a).

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Semicircular Canals

• Projecting upward and posteriorly from the
  vestibule are the three bony semicircular canals.
  
• arranged at approximately right angles (X-Y-Z
  axis)
• The anterior and posterior semicircular canals
  are oriented vertically the lateral semicircular
  canal is oriented horizontally.
• Two parts
• One end of each canal enlarges into a swelling
  called the ampulla.
• The portions of the membranous labyrinth that lie
  inside the semicircular canals are called the
  semicircular ducts (membranous semicircular
  canals).

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Cranial nerves of the Ear Region
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Nerve

• Vestibulocochlear nerve CN VIII
• The vestibular branch of the vestibulocochlear
  nerve consists of 3 parts
• ampullary, utricular, and saccular nerves
• cochlear branch has spiral ganglion in bony
  modiolus

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Overview of Physiology of Hearing
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Physiology of Hearing - Overview

• 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 is attached to the
  eardrum
• Stapes pushes on oval window producing fluid
  pressure waves in scala vestibuli tympani
• oval window vibration is 20X more vigorous than
  eardrum (but the frequency of vibration is
  unchanged)
• Pressure fluctuations inside cochlear duct move
  the hair cells against the tectorial membrane
• Microvilli are bent producing receptor potentials

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Tubular Structures of the Cochlea

• Stapes pushes on fluid of scala vestibuli at oval
  window
• At helicotrema, vibration moves into scala
  tympani
• Fluid vibration dissipated at round window which
  bulges
• The central structure is vibrated (cochlear duct)

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Cochlea

• Cross sections through the cochlea show that it
  is divided into three channels by partitions that
  together have the shape of the letter Y (Figure
  17.19 a-c).
• The channel above the bony partition is the scala
  vestibuli, which ends at the oval window.
• The channel below is the scala tympani, which
  ends at the round window.
• The scala vestibuli and scala tympani both
  contain perilymph and are completely separated
  except at an opening at the apex of the cochlea
  called the helicotrema.
• The third channel (between the wings of the Y) is
  the cochlear duct (scala media).
• The vestibular membrane separates the cochlear
  duct from the scala vestibuli, and the basilar
  membrane separates the cochlear duct from the
  scala tympani.

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Cochlear Anatomy Zoom In

• Section thru one turn of Cochlea
• Partitions that separate the channels are Y
  shaped
• bony shelf of central modiolus
• vestibular membrane above basilar membrane
  below form the central fluid filled chamber
  (cochlear duct)
• Fluid vibrations affect hair cells in cochlear
  duct

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Cochlear Anatomy Zoom Out

• 3 fluid filled channels found within the cochlea
• scala vestibuli, scala tympani and cochlear duct
• Vibration of the stapes upon the oval window
  sends vibrations into the fluid of the scala
  vestibuli

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Anatomy

• Resting on the basilar membrane is the spiral
  organ (organ of Corti), the organ of hearing
  (Figure 17.19, c,d).
• Projecting over and in contact with the hair
  cells of the spiral organ is the tectorial
  membrane, a delicate and flexible gelatinous
  membrane.

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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

• Sound waves result from the alternate compression
  and decompression of air molecules.
• The sounds heard best by human ears are at
  frequencies between 1000 and 4000 Hertz (Hz
  cycles per minute), but many people perceive a
  range of 20 to 20,000 Hz
• speech is 100 to 3000 Hz
• Frequency of a sound vibration is percieved as
  pitch
• higher frequency is higher pitch
• The volume of a sound is its intensity (the
  greater the size of the 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
• possibly nerve damage (CN VIII), but usually
  damage to hair cells from antibiotics, high
  pitched sounds, anticancer drugs, etc.
• the louder the sound the quicker the loss of
  hearing
• person may fail to notice loss until they have
  difficulty hearing frequencies of speech
• Conduction deafness
• perforated eardrum
• otosclerosis
• vibrations are not conducted to hair cells

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Physiology of Hearing

• The events involved in hearing are seen in Figure
  17.20.
• The auricle directs sound waves into the external
  auditory canal.
• Sound waves strike the tympanic membrane, causing
  it to vibrate back and forth.
• The vibration conducts from the tympanic membrane
  through the ossicles (through the malleus to the
  incus and then to the stapes).
• The stapes moves back and forth, pushing the
  membrane of the oval window in and out.

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Physiology of Hearing - Review

• The movement of the oval window sets up fluid
  pressure waves in the perilymph of the cochlea
  (scala vestibuli).
• Pressure waves in the scala vestibuli are
  transmitted to the scala tympani and eventually
  to the round window, causing it to bulge outward
  into the middle ear.
• As the pressure waves deform the walls of the
  scala vestibuli and scala tympani, they push the
  vestibular membrane back and forth and increase
  and decrease the pressure of the endolymph inside
  the cochlear duct.

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Physiology of Hearing - Review

• The pressure fluctuations of the endolymph move
  the basilar membrane slightly, moving the hair
  cells of the spiral organ against the tectorial
  membrane the bending of the hairs produces
  receptor potentials that lead to the generation
  of nerve impulses in cochlear nerve fibers.
• Pressure changes in the scala tympani cause the
  round window to bulge outward into the middle ear.

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Hair Cell Physiology - Review

• Hair cells convert mechanical deformation into
  electrical signals
• As microvilli are bent, mechanically-gated
  channels in the membrane let in K ions
• This depolarization spreads causes
  voltage-gated Ca2 channels at the base of the
  cell to open
• Triggering the release of neurotransmitter onto
  the first order neuron
• more neurotransmitter means more nerve impulses

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More on Pitch and Volume

• Differences in pitch are related to differences
  in the width and stiffness of the basilar
  membrane and sound waves of various frequencies
  that cause a standing wave.
• High-frequency (high-pitch) tone causes the
  basilar membrane to vibrate near the base of the
  cochlea (where it is stiff and narrow.)
• Low-frequency (low-pitch) tone causes the basilar
  membrane to vibrate near the apex of the cochlea
  (where it is flexible and wide.)
• Hair cells beneath the vibrating region of the
  basilar membrane convert the mechanical force
  (stimulus) into an electrical signal (receptor
  potential)
• Sounds of the same pitch vibrate the same region
  of the membrane, and thus stimulate the same
  cells, but a louder sound causes a greater
  vibration amplitude -- which our brain interprets
  as louder.

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Auditory Pathway

• Cochlear branch of CN VIII sends signals to
  cochlear and superior olivary nuclei (of both
  sides) within medulla oblongata
• differences in the arrival of impulses from the
  ears, allows us to locate the source of a sound
  along the horizon (right vs. left)
• Fibers ascend to the
• medulla, most impulses then cross to the opposite
  side and then travel to the
• midbrain (inferior colliculus)
• to the thalamus
• to the auditory area of the temporal lobe
• primary auditory cortex (areas 41 42)

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Otoacoustic Emissions

• The cochlea can produce sounds called otoacoustic
  emissions.
• caused by vibrations of the outer hair cells that
  occur in response to sound waves and to signals
  from motor neurons.
• vibration travels backwards toward the eardrum
• can be recorded by sensitive microphone next to
  the eardrum
• Purpose
• as outer hair cells shorten, they stiffen the
  tectorial membrane
• amplifies the responses of the inner hair cells
• increasing our auditory sensitivity

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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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Applications

• Otosclerosis
• a condition is which there is an overgrowth of
  spongy bone over the oval window that immobilizes
  the stapes.
• prevents the transmission of sound waves to the
  inner ear and leads to conductive hearing loss

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Vestibular Apparatus

• Notice semicircular ducts with ampulla, utricle
  saccule

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Physiology of Equilibrium (Balance)

• Static equilibrium
• maintain the position of the body (head) relative
  to the force of gravity
• macula receptors within saccule utricle
• Dynamic equilibrium
• maintain body position (head) during sudden
  movement of any type--rotation, deceleration or
  acceleration
• crista receptors within ampulla of semicircular
  ducts

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Otolithic Organs Saccule and Utricle

• The maculae of the utricle and saccule are the
  sense organs of static equilibrium.
• They also contribute to some aspects of dynamic
  equilibrium (Figure 17.21).

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Otolithic Organs Saccule Utricle

• Cell types in the macula region
• hair cells with stereocilia (microvilli) one
  cilia (kinocilium)
• supporting cells that secrete gelatinous layer
• Gelatinous otolithic membrane contains calcium
  carbonate crystals called otoliths that move when
  you tip your head

101
Detection of Position of Head

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

102
Membranous Semicircular Ducts

• The three semicircular ducts, along with the
  saccule and utricle maintain dynamic equilibrium
  (Figure 17.22).
• anterior, posterior horizontal ducts detect
  different movements (combined 3-D sensitivity)
• The cristae in the semicircular ducts are the
  primary sense organs of dynamic equilibrium.

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Crista Ampulla of Semicircular Ducts

• Small elevation within each of three semicircular
  ducts
• Hair cells are covered with cupula (gelatinous
  material)
• When you move, fluid in canal tends to stay in
  place, thus bending the cupula and bending the
  hair cells - and altering the release of
  neurotransmitter

104
Detection of Rotational Movement

• Nerve signals to the brain are generated
  indicating which direction the head has been
  rotated

105
Equilibrium Pathways in the CNS

• Most vestibular branch fibers of the
  vestibulocochlear nerve (CN VIII) enter the brain
  stem and terminate in the medulla the remaining
  fibers enter the cerebellum.
• Fibers from these areas connect to
• cranial nerves that control eye and head and neck
  movements (III,IV,VI XI)
• vestibulospinal tract that adjusts postural
  skeletal muscle contractions in response to head
  movements
• The cerebellum receives constant updated sensory
  information which it sends to the motor areas of
  the cerebral cortex
• motor cortex can then adjust its signals to
  maintain balance

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DEVELOPMENT OF THE EYES AND EARS
107
Eyes

• Eyes begin to develop when the ectoderm of the
  lateral walls of the prosencephalon bulges to
  form a pair of optic grooves (Figure 17.23a)
• As the neural tube closes the optic grooves
  enlarge and move toward the surface of the
  ectoderm and are known as optic vesicles (Figure
  17.23b)
• When the optic vesicles reach the surface, the
  surface ectoderm thickens to form the lens
  placodes and the distal portions of the optic
  vesicles invaginate to form the optic cups
  (Figure 17.23c).
• The optic cups remain attached to the
  prosencephalon by the optic stalks (Figure
  17.23d).

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Ears

• Inner ear develops from a thickening of surface
  ectoderm called the otic placode (Figure 17.24a).
• Otic placodes invaginate to form otic pits
  (Figure 17.24 a and b)
• Optic pits pinch off from the surface ectoderm to
  form otic vesicles (Figure 17.24d)
• Otic vesicles will form structures associated
  with the membranous labyrinth of the inner ear.
• Middle ear develops from the first pharyngeal
  (branchial) pouch.
• The external ear develops from the first
  pharyngeal cleft (Figure 17.24).

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AGING AND THE SPECIAL SENSES

• Age related changes in the eyes
• Presbyopia
• Cataracts
• Weakening of the muscles that regulate the size
  of the pupil
• Diseases such as age related macular disease,
  detached retina, and glaucoma
• Decrease in tear production
• Sharpness of vision as well as depth and color
  perception are reduced.

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AGING AND THE SPECIAL SENSES

• After age 50 some individuals experience loss of
  olfactory and gustatory receptors.
• Age related changes in the ears
• Presbycusis hearing loss due to damaged or loss
  of hair cells in the organ of Corti
• Tinnitus (ringing in the ears) becomes more common

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DISORDERS HOMEOSTATIC IMBALANCES

• A cataract is a loss of transparency of the lens
  that can lead to blindness.
• Glaucoma is abnormally high intraocular pressure,
  due to a buildup of aqueous humor inside the
  eyeball, which destroys neurons of the retina. It
  is the second most common cause of blindness
  (after cataracts), especially in the elderly.

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DISORDERS HOMEOSTATIC IMBALANCES

• Deafness is significant or total hearing loss. It
  is classified as sensorineural (caused by
  impairment of the cochlear or cochlear branch of
  the vestibulocochlear nerve) or conduction
  (caused by impairment of the external and middle
  ear mechanisms for transmitting sounds to the
  cochlea).
• Menieres syndrome is a malfunction of the inner
  ear that may cause deafness and loss of
  equilibrium.
• Otitis media is an acute infection of the middle
  ear, primarily by bacteria. It is characterized
  by pain, malaise, fever, and reddening and
  outward bulging of the eardrum, which may rupture
  unless prompt treatment is given. Children are
  more susceptible than adults.

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