Week 6 - Lecture 2a - Alterations in vision Flashcards
What percentage of the body’s sensory receptors are in the eye
70%
3 step of vision
- light/energy strikes the retina
- converts energy into action potentials
- relayed to brain for processing
Where is vision processed in the cerebral cortex
primary visual cortex and visual association areas
pathway of light entering eye
- lights enters the cornea
- light enters the aqueous humor of the anterior chamber
- light passes through the pupil of the iris
- light enters the lens which sits on top of the vitreous humor
- light is refracted to the retina which is composed of an entire layer of photoreceptors
Describe how the pupils control light
Think of pupils as a window, the iris is a sphincter, the iris contracts or relaxes (constricts or dilates the pupil) to allow more or less light in.
- close vision and bright light- sphincter pupillae (circular muscles) contract; pupils constrict
- distant vision and dim light - dilator papillae (radial muscles) contract; pupils dilate - sympathetic fibres - pupil dilation also occurs in response to change in emotional state
Rods
more numerous (20 rods for 1 cone)
- Mostly in peripheral retina
- vision in dim light
- very sensitive to light
- best suited for night vision and peripheral vision
- contain single pigment : rhodopsin (opsin + retinae)
- contract with light breaks down rhodopsin
- perceived input in gray tones only
cones
less numerous
mostly in central retina
visual acuity (clarity), bright light and colour
- need bright light for activation, breaking down photopigments
-react more quickly
- have 1 of 3 photopigments for coloured view
vision photopigments
erythrolabe : red cone absorbs light at 625-nm wavelength
chlorolabe : green cone absorbs light at 530nm wavelength
cyanolabe : blue cone absorbs light at 455nm wavelength
colour blindness
lack of one or more cone pigments
Refraction
bending of light rays
- due to change in speed when light passes
- occurs when light meets surface of different medium at an oblique angle
light refracted 3 times along pathway
- entering cornea
- entering lens
- leaving lens
majority of refractory power in cornea
change in lens curvature allows for fine focusing
light passing through convex lens (as in eye) is bent so that rays converge at focal point
- image formed at focal point is upside down and reversed right to left
Control of eye movements
6 muscles - originate from bony orbit ; insert on eyeball
- enable eye to follow moving objects
- maintain shape of eyeball
- hold in orbit
4 rectus muscles - originate from common tendinous ring : name indicates movement
superior, inferior, lateral, medial rectus muscles
2 oblique muscles move eye in vertical plane and rotate eyeball
- superior and inferior oblique muscles
innervation
CN III, IV, VI
5 movement types
saccades : looking from object A to object B
Pursuit : smoothly following a moving object
Convergence/divergence : both eyes turning inward/outward simultaneously
Vestibular : eyes sensing and adjusting to head movement via connections with nerves in the inner ear
Fixation maintenance: minute eye movements that position and accommodate both eyes
Eyebrows function
protection/shade eye from sunlight
prevent perspiration from reaching eye
eye lids function
protect eye anteriorly
blink reflexively every 3-7 seconds
spread secretions to moisten eye
eye lashes function
nerve endings of follicles initiate reflex blinking
lubricating glands associated with eyelids
conjuntiva function
transparent mucous membrane
produces a lubricating mucous secretion
Lacrimal glands
primary producers of tear
lacrimal secretion (tears) - 1ml/day
dilute saline solution containing mucous, antibodies and lysosome
Protection:
enzymes in tears neutralise harmful substances, protect against bacterial infection
provide nutrients and moisture
remove debris and waste from the eye
-closure of eyelid (blinking) forces tear downward, toward nose
-tears enter paired lacrimal canaliculi via lacrimal
- - taste of eye drops
Aqueous humor role
maintain eye pressure
provide nutrient to the cornea
production: ciliary body
reabsorption (main) : trabecular network (meshwork)
Reabsorption (minor) : uveal- scleral outflow pathway
3 steps for aqueous humor
aqueous humor forms by filtration from the capillaries in the ciliary processes
aqueous humor flows from the posterior chamber through the pupil into the anterior chamber
aqueous humor is reabsorbed into the venous sinus through the trabecular meshwork and scheme canal (some reabsorbed into larger blood vessels through anterior ciliary body)
REVIEW visual lesions on page 14
3
What are eyes best adapted for
distant vision
far point of vision - distance beyond which no change in lens shape needed for focusing - 6m for emmetropic (normal) eye- cornea and lens focus light precisely on retina
ciliary muscles relax - sympathetic activation
Close focus
light from close objects (<6m) diverges as approaches eye
requires eye to make active adjustments using 3 simultaneous processes
- accommodation
- constriction
- convergence
what is accommodation
changing lens shape to increase refraction
presbyopia - loss of accommodation over age 50
what is constriction
accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye
what is convergence
medial rotation of eyeballs toward object being viewed
what is myopia
nearsightedness
eye focuses an image in front of retina
correction: biconcave lens to promote divergence of light
what is hyperopia
farsightedness
focusing image behind the retina
correction: biconvex lens to promote the convergence of light
What is astigmatism
irregular curvature of the lens
prevents the focusing of an image
blurring vision
cause: corneal scar, scattering in the crystalline lens
correction : glasses, contact lenses, surgery
What is presbyopia
farsightedness associated with ageing
in ability of ciliary muscle and lens to accommodate for near vision
Correction: bifocals
What is strabismus
“crossed eyes”
misalignment of visual axes results in inability to focus on a single object (there are many types)
lack of coordination between extra ocular muscle function
prevents the eyes in lining up in the same direction
what is amblyopia
“lazy eye”
loss of visual detail from uncoordinated eye movement and focus
- visual axes are misaligned in children
- may result form strabismus to avoid diplopia
The brain suppresses on of the images
- visual impairment of the ignored eye
- from dimness of vision to permanent loss of one eye
- the impairment may not be fully correctable
what is Diplopia
double vision
perception of 2 images of a single object
images fall on non-corresponding areas of the retina
may be the result of strabismus
lack of coordination of extraocular muscle
- mechanical problems
- disorders of neuromuscular junction
- CN 3,4,6 palsy
(lens problems can cause diplopia as well : cataract)
(temporary diplopia : alcohol intoxication)
What is nystagmus
involuntary oscillation of the eye
caused by abnormal functioning of the brain, labyrinth of the inner ear or vestibular pathways responsible for controlling eye movement
congenital : oculomotor abnormalities during neural development
post-natal - acquired : any visual input disruptions
Head injury
space occupying lesion that interferes with vestibular pathways/cerebellar function
stroke
meunière disease
MS
drug, alcohol toxicity
Conjunctiva function
transparent mucous membrane - produces a lubricating mucous secretion
conjuntivitis "pink eye" inflammation of the mucous membrane lining of the eye due to viral bacterial or allergy causes
what is cataract
clouding of lens
-altering vision focus by scattering incoming light
protein component of lens (crystallin) clumps or aggregate
increasing opaque area, increasing impairment
cataract causes
nuclear
- most common
- form in the centre, or nucleus of the lens
- caused by ageing
cortical
- forms in the lens cortex
- extends from outer lens towards the centre
posterior sub scapular
- begins at the back of the lens
- may result from diabetes and severe hyperopia
other risk factors
- heavy smoking
- frequent exposure to intense sunlight
- some congenital
- large doses of vitamin C increases cataract formation
manifestations and evaluation of alterations in vision
decreasing acuity an claret occurs over time
visual acuity : far and near vision
far vision : snellen or E chart
near vision
- handheld card
- jaegar chart
treatment of alterations in vision n
specific to identified impairment muscle imbalances -glasses -patching -surgery
errors in refraction
- glasses, contact lenses
- surgery
myopia, hyperopia, astigmatism
-laser assisted in situ keratomileusis (LASIK)
cataract
-lens and cataract removal, lens implantation
