Vision

1
Vision

• eye can sense 10 million gradations of light
  intensity
• 7 million different shades of color
• eyes are responsible for 80 of all knowledge
  assimilated
• eye contains 70 of all sensory receptors
• optic nerve carries 1/3 of all afferent neurons
• see only 1/70 of the electromagnetic spectrum
• between 400 and 750 nm

2
Three Tunics of the Eye

• 1. Tunica fibrosa - fibrous or avascular tunic
• 2. Tunica vasculosa- vascular tunic or uvea
• 3. Tunica interna or nervous tunic or retina

3
Fibrous or Avascular Tunic

• a. sclera
• posterior 5/6
• dense connective tissue with collagen and elastic
  fibers
• maintains the shape of the eye
• called _______________ anteriorly
• contains insertion of 6 extrinsic eye muscles

4
Fibrous or Avascular Tunic

• b. cornea
• clear, dense connective tissue
• outer layer of stratified squamous epithelium
• continuous with __________________
• 95 transplantation success because has no blood
  vessels

5
Vascular Tunic or Uvea

• a. choroid
• lines most (2/3) of internal region of sclera
• darkly pigmented with concentrations of
  ______________
• contains many blood vessels
• delivers oxygen and nutrients to retina

6
Vascular Tunic or Uvea

• b. ciliary body
• thickened region around edge of cornea
• contains two regions
• 1. ciliary body or processes
• folds on internal surface where cells secrete
  _______________ humor
• suspensory ligaments attach to these processes
• 2. contains smooth muscles called ciliary
  muscles

7
Vascular Tunic or Uvea

• c. iris
• continuation of choroid, highly vascular
• thin, muscular diaphragm
• pupillary constrictor muscles
• pupillary dilator muscles
• contains the pigment _______________
• causes eye color
• regulates the size of pupils

8
Nervous Tunic or Retina

• a. outer pigmented layer
• simple cuboidal epithelium
• abuts choroid, provides nutrition for nervous
  layer
• acts as phagocytes, absorbs vitamin A
• b. inner nervous tissue layer
• photoreceptors
• rods
• cones
• two layers are close together but not fused
• can separate -- _____________ retina

9
Nervous Tunic or Retina

• neurons
• horizontal cells amacrine cells
• bipolar cells ganglion cells
• optic disc
• where optic nerve leaves rear of retina
• no photoreceptors, called ______________
• macula lutea
• no rods
• where visual image arrives
• _____________ -highest concentration of cones at
  center of macula lutea

10
LENS

• avascular like the cornea
• dense fibrous capsule covers entire lens
• filled with transparent proteins called
  crystallins
• stable proteins that remain intact and functional
  for a lifetime without need for replacement
• held by a fibrous suspensory ligament
• attaches to ciliary body
• normally pulls lens flat at rest

11
LENS CATARACTS

• lens hardens and thickens
• crystallin unfolds and becomes cloudy
• causes
• drug reactions, injuries
• radiation, aging
• lens turns yellow, may go completely opaque
• treatment
• removal of lens or piece of lens

12
ANTERIOR CAVITY

• anterior to lens and iris
• divided into two chambers
• 1. anterior chamber before the iris
• 2. posterior chamber in between iris and lens
• aqueous humor
• clear fluid similar to cerebrospinal fluid
• secreted by epithelium of ciliary processes into
  posterior chamber
• 5 to 6 ml per day
• maintains a pressure between 12 - 21 mm
• drains into the Canal of ___________ (venous
  sinus)

13
GLAUCOMA

• drainage of aqueous humor into Canal of Schlemm
  impaired
• increasing pressure on weak point- optic nerve
• 2 of population over 25
• treatments
• drugs - constrict pupil and tenses edge of iris
• laser surgery to perforate wall of anterior
  chamber

14
POSTERIOR CAVITY

• 80 of the cavities
• vitreous humor
• jellylike substance
• network of fine collagenous fibers embedded in a
  viscous connective matrix (proteoglycans)
• not replaced

15
ACCESSORY STRUCTURES

• 1. eyelids
• continuation of skin
• contain skeletal muscles
• blink 20-30 times per minute
• 2. eyelashes
• 200
• prevent foreign matter from reaching surface of
  eye

16
ACCESSORY STRUCTURES

• 3. sebaceous and sweat glands
• meibomian or tarsal glands
• embedded in tarsal plate
• modified sebaceous glands
• oily secretions keep eyelids from adhering to
  each other

17
ACCESSORY STRUCTURES

• 4. lacrimal caruncle
• at the medial canthus
• small papillary projection
• includes sebaceous and sweat glands
• produces the white material of the eye
• 5. conjunctiva
• mucous membrane
• lines inside of eyelids and anterior surface of
  the eye
• thin layer of stratified squamous epithelium
• inflammation called _______________ or pink eye
• viral or bacterial

18
ACCESSORY STRUCTURES

• 6. lacrimal apparatus or gland
• superior and lateral region of the orbit
• 6 to 12 ducts
• components
• lacrimal gland
• lacrimal punctum
• lacrimal canaliculi
• lacrimal sac
• nasolacrimal duct

19
ACCESSORY STRUCTURES

• 6. lacrimal apparatus or gland
• produces tears which it releases when you blink
• components of tears - water, mucin, salt and
  lysozymes
• functions
• cleans, lubricates and moistens eye
• production
• an average of 1 ml produced per day
• drains into inferior meatus

20
FOCUSING IMAGES

• light is bent ( refraction) 3 times
• 1. cornea
• 2. entering lens
• 3. leaving lens
• three basic processes to focusing
• 1. refraction of light
• 2. accommodation of lens
• 3. constriction of pupil

21
FOCUSING IMAGES - LENS

• when ciliary muscles contract
• when they contract (parasympathetic stimulation)
  ciliary body moves toward lens
• relaxes the tension on the suspensory ligament
• allows the lens to assume a more spherical shape
• when ciliary muscle relax
• tension in suspensory ligament tightens the
  capsule
• causes lens to form a more flattened shape
• accommodation
• ability of lens to change shape

22
FOCUSING IMAGES - IRIS

• 2 sets of smooth muscles
• 1. circular muscle (pupillary constrictor
  muscle)
• constricts the pupils
• stimulated by the _____________ nervous system
• 2. radial muscle (pupillary dilator muscle)
• dilates pupils
• stimulated by the _______________ nervous system

23
ACCOMMODATION PROBLEMS

• a. myopia or nearsightedness
• distant objects are focused in front of the
  retina
• lens is too thick or eyeball is too long
• correction - concave or biconcave lens
• b. hyperopia or farsightedness
• image is focused behind the retina
• lazy lens (poor refractory period) or eyeball
  that is too short
• correction - biconvex lens

24
ACCOMMODATION PROBLEMS

• c. astigmatism
• unequal curvature of the lens or cornea
• d. presbyopia
• lens tends to lose its elasticity and ability to
  accommodate

25
ANATOMY OF RODS AND CONES

• a. synaptic body or ending
• connects to bipolar neurons
• releases the neurotransmitter
• b. soma or nerve cell body
• contains the _____________
• c. inner segment
• contains mitochondria
• makes ________________

26
ANATOMY OF RODS AND CONES

• d. outer segment
• composed of shelves of membranes or discs
• made up of about 1000 discs
• contains the visual pigments
• rhodopsin - rods
• iodopsin - cones
• embedded in the pigmented layer of the retina

27
RODS

• more numerous than cones, 130 million per retina
• detects presence or absence of photons without
  regard to wavelength
• night vision and peripheral vision
• perceive as gray
• rhodopsin ------gt scotopsin retinene or
  retinal
• retinal is derived from vitamin A
• takes 5 minutes to regenerate 50 of bleached
  rhodopsin in rods
• rod stimulation is believed to occur at the very
  instant that rhodopsin becomes excited by light

28
PHOTORECEPTION - RODS

• unexcited - dark current
• inner segment continually pumps Na to outside
  creating a negative potential inside the cell (-
  40 mV)
• outer segment is in an unexcited state and is
  very leaky to Na
• Na continually leaks back inside neutralizing
  much of the negativity on the inside of the cell
• rods in the dark releases the neurotransmitter
  ___________ at a constant rate

29
PHOTORECEPTION - RODS

• excited - exposed to light
• when rhodopsin is exposed to light in the outer
  segment it begins to break down
• decreases the leakage of Na into the rods
• Na continues to be pumped out
• see a net loss of Na
• called ___________________ (-40 mV to -70 mV)
• hyperpolarization causes the rods to
  _____________release of neurotransmitter
  glutamate
• inhibition of rod causes depolarization of
  bipolar neuron
• excites ganglion cells

30
LIGHT AND DARK ADAPTATION

• after 30 minutes in dark, almost all visual
  pigments will be fully receptive to stimulation -
  dark- adapted state
• a single rod will hyperpolarize in response to a
  single photon of light
• when lights come on, tremendous amount of
  bleaching occurs
• sensitivity changes 25,000 times
• night blindness - lack of vitamin A

31
PHOTORECEPTION - CONES

• color and clear vision
• 6.5 million cones per one retina
• photochemical in cones have almost exactly the
  same chemical composition as that of rhodopsin in
  rods
• protein portion is photopsin instead of scotopsin
  in rods
• iodopsin ------gt photopsin retinal
• takes 90 seconds to regenerate 50 of iodopsin in
  cones
• 3 different types of photochemicals in cones
• red (erythrolobe)- 74 - 558 nm
• green (chlorolobe)- 10 - 531 nm
• blue (cyanolobe)- 16 - 420 nm

32
PHOTORECEPTION - CONES

• color vision
• orange light
• red cones 99
• blue cones 0
• green cones 42
• blue light
• red cones 0
• blue cones 97
• green cones 0
• white light
• equal stimulation of all three cones

33
PHOTORECEPTION - CONES

• colorblindness
• 8 of all men
• 0.5 of all women
• most common is red-green
• genes found on X chromosome
• deficiency of either red or green cones
• reds and greens are seen as the same color

34
PHOTORECEPTION - CONES

• visual acuity
• fovea centralis
• 4000 cones converge on 4000 bipolar cells
• peripheral to fovea
• 600 rods converge on one bipolar cell
• blindspot
• where the optic nerve leaves the retina
• has no rods or cones

35
ANATOMY OF THE EAR

• 1. external ear
• pinna or auricle
• elastic cartilage
• external auditory canal or ear canal
• curved passageway 3 cm long
• lined with skin and near entrance are fine hairs
  and sebaceous glands
• ceruminous glands
• produce cerumen
• slows growth of microorganisms
• ends at the ______________ membrane

36
ANATOMY OF THE EAR

• 2. middle ear- small air-filled cavity
• tympanic membrane at junction of middle and outer
  ear
• semitransparent fibrous connective tissue
• hole in tympanic membrane is a perforated
  eardrum
• oval and round windows
• mastoid sinus
• separated from brain by only thin bony partition

37
ANATOMY OF THE EAR

• 2. middle ear
• eustachian or auditory tube
• 4 cm long and opens into nasopharynx
• can open passageway by yawning or swallowing
• equalizes air pressure
• infections may travel along this passageway

38
ANATOMY OF THE EAR

• 2. middle ear
• middle ear bones
• a. malleus or hammer
• connects to tympanic membrane
• b. incus or anvil
• c. stapes or stirrup
• connects to oval window

39
ANATOMY OF THE EAR

• 3. inner ear
• labyrinth, two divisions
• a. bony labyrinth
• divided into three areas
• a. semicircular canals
• b. vestibule
• c. cochlea
• lined with periosteum and filled with perilymph
• similar to cerebrospinal fluid

40
ANATOMY OF THE EAR

• 3. inner ear
• b. membranous labyrinth
• lined with epithelium and contains a fluid called
  endolymph
• different from those of typical body fluids
• 3 parts to bony labyrinth
• 1. cochlea
• spiral canal that resembles a snail's shell
• makes almost 2.5 turns

41
ANATOMY OF THE EAR

• 3. inner ear
• bony labyrinth
• 2. vestibule
• oval central portion of bony labyrinth
• contains a membranous labyrinth consisting of two
  sacs
• a. utricle
• b. saccule
• 3. semicircular canals
• three - one in each plane
• one end of each canal enlarges into a swelling
  called the ampulla

42
VESTIBULE

• two sacs
• a. utricle
• b. saccule
• equilibrium sensations whether body is moving or
  not
• macula - small thickened area
• contains hair and supporting cells
• each cell has 40 - 70 stereocilia (microvilli)
  and one kinocilium
• imbedded in a gelatinous substance are particles
  of calcium called the otolith

43
VESTIBULE

• concerned with static and dynamic equilibrium
• movement of stereocilia initiates depolarizing or
  hyperpolarizing receptor potentials
• static equilibrium - position of head
• dynamic equilibrium - linear acceleration

44
SEMICIRCULAR CANALS

• respond to rotational movements of the head
• anterior, posterior and lateral semicircular
  ducts are continuous with the utricle
• each duct has an expanded regions called the
  ampulla
• inside ampulla are many hair cells with their
  kinocilium and sterocilia embedded in a
  gelatinous mass called the cupula
• crista
• contains group of hair cells and supporting cells

45
MOTION SICKNESS

• temporary disturbance in functioning of
  semicircular canals
• results in headache, sweating, nausea and
  vomiting
• results from repetitive changes in rate and
  direction of movement and from conflicting
  vestibular and visual signals
• treatment
• dramamine - depresses activity at the vestibular
  nuclei

46
MENIERES DISEASE

• caused by irritation or damage to inner ear
• leads to accumulation of excess endolymph
• if semicircular canals are affected
• dizziness and loss of equilibrium
• if cochlea is affected
• tinnitus

47
COCHLEA - STRUCTURE

• modiolus
• bony core of cochlea
• three passageways
• 1. scala vestibuli
• 2. scala tympani
• 3. scala media or cochlear duct
• contains endolymph
• scala vestibuli and tympani are continuous
• both contain perilymph

48
COCHLEA - STRUCTURE

• other structures
• helicotrema
• opening which connects scala vestibuli and
  tympani
• vestibular membrane
• separates cochlear duct from scala vestibuli

49
COCHLEA - STRUCTURE

• basilar membrane
• separates cochlear duct from the scala tympani
• contains 20,000 to 30,000 basilar fibers
• elastic and reedlike and can vibrate like reeds
  of an harmonica
• lengths of basilar fibers increase from 0.04 mm
  near the windows to 0.5 mm at the helicotrema
• 12 fold increase in length
• diameters of fibers decrease from the windows
  to the helicotrema

50
COCHLEA - STRUCTURE

• basilar membrane
• short, stiff fibers near windows
• vibrate at higher frequencies
• long, limber fibers near helicotrema
• vibrate at lower frequencies
• organ of corti or spiral organ
• sits on the basilar membrane
• contains a coiled sheet of epithelial cells
• supporting cells and about 20,000 hair cells in 4
  rows
• lack kinocilia but stereocilia are in contact
  with overlying tectorial membrane
• not neurons

51
COCHLEA - STRUCTURE

• organ of corti
• hair cells are receptors for auditory sensations
• two groups of hair cells with supporting cells
• a. inner- single row
• b. outer- several rows
• when portion of basilar membane vibrates, the
  sterocilia of the hair cells are pressed against
  the tectorial membrane and become distorted

52
PATHWAY OF SOUND

• 1. auricle directs sound waves into external
  auditory canal
• 2. sound waves cause the tympanic membrane to
  vibrate
• tympanic membrane has 18X the surface area of the
  oval window
• central areas of tympanic membrane connects to
  the malleus which also starts to vibrate

53
PATHWAY OF SOUND

• 3. vibration continues to the incus and stapes
• as stapes moves back and forth, it pushes the
  membrane of the oval window in and out
• energy is magnified 18X to the oval window
• movement of oval window sets up fluid pressure
  waves in the perilymph of the cochlea

54
PATHWAY OF SOUND

• 4. as oval window bulges in, it pushes on the
  perilymph of the scala vestibuli
• pressure waves are transmitted from scala
  vestibuli to scala tympani
• pressure waves eventually hit the round window
  which bulges outwards

55
PATHWAY OF SOUND

• 5. as pressure waves deform the walls of the
  scala vestibuli and scala tympani they also push
  the vestibular membrane back and forth
• pressure of endolymph inside the cochlear duct
  increases and decreases
• 6. pressure fluctuations of the endolymph move
  the basilar membrane slightly

56
PATHWAY OF SOUND

• 7. when the basilar membrane vibrates, the hair
  cells of the spiral organ move against the
  tectorial membrane
• bending of the hairs opens ion channels and
  produces receptor potentials that lead to the
  generation of nerve impulses
• 8. pressure changes in the scala tympani cause
  the round window to bulge outward into the middle
  ear

57
SOUND

• a. pitch
• determined by frequency of vibrations
• higher frequency (short wavelength) stimulates
  window end of basilar membrane
• lower frequency (long wavelength) stimulates
  helicotrema end of basilar membrane
• human ear is sensitive to a range from 20 -
  20,000 hertz (Hz) or cycles per second (cps)

58
SOUND

• b. intensity
• three ways
• 1. amplitude of vibration increases
• 2. number of hair cells vibrating increases
• 3. some hair cells have different thresholds
• can discriminate a 100 trillion fold difference
  in sound from a whisper to the loudest noise
• sound intensity is expressed in decibels
• 1 decibel actual increase in intensity of 1.26
  times

59
SOUND

• b. intensity
• 20 decibels - ticking watch
• 45 decibels -normal conversation
• 60 decibels - crowd noise
• 80 decibels - alarm clock at 2 feet, subway
• damage at 8 hours
• 100 decibels - chain saw, pneumatic drill
• damage at 2 hours
• 120 decibels - sandblasting, rock concert
• damage immediate
• 140 decibels - gunshot, jet plane
• 160 decibels - rocket launching pad

60
ATTENUATION REFLEX

• muscles in middle ear pull malleus and stapes to
  make ossicles rigid
• can reduce sound by 30-40 decibels but is a
  relatively slow reflex - 40 msec

61
DEAFNESS

• 1. conduction deafness
• a. ear wax buildup
• b. otosclerosis
• c. destruction of tympanic membrane

62
DEAFNESS

• 2. nerve deafness
• damage to receptors or nerve fibers - cochlea,
  auditory nerve
• causes
• a. low frequency sounds
• excessive and prolonged exposure to very loud
  sounds like rock band, jet engine
• b. all frequency sounds
• drug sensitivity of organ of corti to some
  antibiotics like neomycin and gentamicin