Chapter 22 The Special Senses

1
Chapter 22The 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
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.

5
Olfaction Sense of Smell

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

6
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

7
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
• Found on sides of circumvallate fungiform
  papillae
• 3 cell types supporting, receptor basal cells

8
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.

9
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
• Mechanism
• dissolved substance contacts gustatory hairs
• receptor potential results in neurotransmitter
  release
• nerve impulse formed in 1st-order neuron

10
Gustatory Pathway

• First-order gustatory fibers found in cranial
  nerves
• 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
• providing conscious perception of taste

11
Accessory Structures of Eye

• Eyelids or palpebrae
• protect lubricate
• epidermis, dermis, CT, orbicularis oculi m.,
  tarsal plate, tarsal glands conjunctiva
• Tarsal glands
• oily secretions keep lids from sticking together
• Conjunctiva
• palpebral bulbar
• stops at corneal edge
• dilated BV--bloodshot

12
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

13
Lacrimal Apparatus

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

14
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

15
Tunics (Layers) of Eyeball

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

16
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

17
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)

18
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

19
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

20
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

21
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

22
Vascular Tunic -- Suspensory ligament

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

23
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
24
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

25
Rods Cones--Photoreceptors

• Rods----rod shaped
• 120 million rod cells
• discriminates shapes movements
• shades of gray in dim light
• 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

26
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

27
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

28
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
• Glaucoma
• increased intraocular pressure that could produce
  blindness
• problem with drainage of aqueous humor

29
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

30
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

31
Accommodation the Lens

• Convex lens refract light rays towards each other
• Lens of eye is convex on both surfaces
• View 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

32
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

33
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

34
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

35
Photoreceptors

• Named for shape of outer segment
• Transduction of light energy into a receptor
  potential in outer segment
• 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

36
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

37
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)

38
Regeneration of 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 but only 90
  seconds to regenerate the cone photopigments
• 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.

39
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

40
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
• 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

41
Release of Neurotransmitters
42
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 excited bipolar cells if levels of
  illumination change

43
Brain Pathways of Vision
44
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

45
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

46
Anatomy of the Ear Region
47
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

48
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

49
Middle Ear Cavity
50
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

51
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

52
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

53
Cranial nerves of the Ear Region

• Vestibulocochlear nerve CN VIII
• ampullary, utricular saccular brs. form
  vestibular branch
• cochlear branch has spiral ganglion in bony
  modiolus

54
Cochlear Anatomy

• 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

55
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)

56
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

57
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

58
(No Transcript)
59
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

60
Overview of Physiology of Hearing
61
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 both
  ears, allows us to locate the source of a sound
• Fibers ascend to the
• inferior colliculus
• thalamus
• primary auditory cortex in the temporal lobe
  (areas 41 42)

62
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

63
Vestibular Apparatus

• Notice semicircular ducts with ampulla, utricle
  saccule

64
Otolithic Organs Saccule Utricle

• Thickened regions called macula within the
  saccule utricle of the vestibular apparatus
• 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

65
Detection of Position of Head

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

66
Crista Ampulla of Semicircular Ducts

• Small elevation within each of three semicircular
  ducts
• anterior, posterior horizontal ducts detect
  different movements
• Hair cells covered with cupula of gelatinous
  material
• When you move, fluid in canal bends cupula
  stimulating hair cells that release
  neurotransmitter

67
Detection of Rotational Movement

• When head moves, the attached semicircular ducts
  and hair cells move with it
• endolymph fluid does not and bends the cupula and
  enclosed hair cells
• Nerve signals to the brain are generated
  indicating which direction the head has been
  rotated

68
Equilibrium Pathways in the CNS

• Fibers from vestibulocochlear nerve (VIII) end in
  vestibular nuclei and 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
• Cerebellum receives constant updated sensory
  information which it sends to the motor areas of
  the cerebral cortex
• motor cortex can adjust its signals to maintain
  balance