Week 6 - Lecture 2a - Alterations in vision

Question 1

What percentage of the body’s sensory receptors are in the eye

Answer 1

70%

Question 2

3 step of vision

Answer 2

1. light/energy strikes the retina
2. converts energy into action potentials
3. relayed to brain for processing

Question 3

Where is vision processed in the cerebral cortex

Answer 3

primary visual cortex and visual association areas

Question 4

pathway of light entering eye

Answer 4

1. lights enters the cornea
2. light enters the aqueous humor of the anterior chamber
3. light passes through the pupil of the iris
4. light enters the lens which sits on top of the vitreous humor
5. light is refracted to the retina which is composed of an entire layer of photoreceptors

Question 5

Describe how the pupils control light

Answer 5

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.

1. close vision and bright light- sphincter pupillae (circular muscles) contract; pupils constrict
2. 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

Question 6

Rods

Answer 6

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

Question 7

cones

Answer 7

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

Question 8

vision photopigments

Answer 8

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

Question 9

colour blindness

Answer 9

lack of one or more cone pigments

Question 10

Refraction

Answer 10

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

Question 11

Control of eye movements

Answer 11

6 muscles - originate from bony orbit ; insert on eyeball

1. enable eye to follow moving objects
2. maintain shape of eyeball
3. 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

Question 12

5 movement types

Answer 12

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

Question 13

Eyebrows function

Answer 13

protection/shade eye from sunlight

prevent perspiration from reaching eye

Question 14

eye lids function

Answer 14

protect eye anteriorly
blink reflexively every 3-7 seconds
spread secretions to moisten eye

Question 15

eye lashes function

Answer 15

nerve endings of follicles initiate reflex blinking

lubricating glands associated with eyelids

Question 16

conjuntiva function

Answer 16

transparent mucous membrane

produces a lubricating mucous secretion

Question 17

Lacrimal glands

Answer 17

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

Question 18

Aqueous humor role

Answer 18

maintain eye pressure
provide nutrient to the cornea

production: ciliary body
reabsorption (main) : trabecular network (meshwork)
Reabsorption (minor) : uveal- scleral outflow pathway

Question 19

3 steps for aqueous humor

Answer 19

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)

Question 20

REVIEW visual lesions on page 14

Answer 20

3

Question 21

What are eyes best adapted for

Answer 21

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

Question 22

Close focus

Answer 22

light from close objects (<6m) diverges as approaches eye

requires eye to make active adjustments using 3 simultaneous processes

1. accommodation
2. constriction
3. convergence

Question 23

what is accommodation

Answer 23

changing lens shape to increase refraction

presbyopia - loss of accommodation over age 50

Question 24

what is constriction

Answer 24

accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye

Question 25

what is convergence

Answer 25

medial rotation of eyeballs toward object being viewed

Question 26

what is myopia

Answer 26

nearsightedness eye focuses an image in front of retina correction: biconcave lens to promote divergence of light

Question 27

what is hyperopia

Answer 27

farsightedness focusing image behind the retina correction: biconvex lens to promote the convergence of light

Question 28

What is astigmatism

Answer 28

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

Question 29

What is presbyopia

Answer 29

farsightedness associated with ageing in ability of ciliary muscle and lens to accommodate for near vision Correction: bifocals

Question 30

What is strabismus

Answer 30

"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

Question 31

what is amblyopia

Answer 31

"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

Question 32

what is Diplopia

Answer 32

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)

Question 33

What is nystagmus

Answer 33

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

Question 34

Conjunctiva function

Answer 34

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 ```

Question 35

what is cataract

Answer 35

clouding of lens -altering vision focus by scattering incoming light protein component of lens (crystallin) clumps or aggregate increasing opaque area, increasing impairment

Question 36

cataract causes

Answer 36

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

Question 37

manifestations and evaluation of alterations in vision

Answer 37

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

Question 38

treatment of alterations in vision n

Answer 38

``` 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