Visual system
Eye
o
image formation & phototransduction
Visual pathway
o
tranmission of nerve impulses
Visual cortex
o
occipital lobe of cerebral cortex
primary visual receiving area: sides of the calcarine fissure
o
visual processing & perception occurs here
Stimulus
Light
o
visible portion of the electromagnetic spectrum
Sensory organ
Eye
o
optical instrument for focusing of images on retina by refraction of
light rays
o
Refractive power
Cornea: 40 dioptres
fixed
Lens: 20 dioptres
adjustable
o
Photoreceptors on retina
rods
cones
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Refraction of light
when light beam passes through an angulated interface, light rays bend
measured in diopters
Biconvex spherical lens
o
convergence
Concave spherical lens
o
divergens
Accommodation
Parasympathetic response
When a person looks at a near object, 3 changes occur:
o
accommodation reflex
o
convergence of visual axis
o
pupil constrict
When accommodation relaxed:
o
Near object (<6 m)
diverging rays
image falls behind retina
o
Far object (>6 m)
parallel rays
image falls on retina
Physiology
o
ciliary muscles contracts
this relaxes the lens ligaments
lens spring into a more convex shape
near point of vision recedes throughout life
o
slowly at 1st
advancing rapidly with old age
o
due to increasing hardness of lens
impaired accommodation
receding of near point
o
Presbyopia
reading and close vision difficult
corrected by wearing convex lens
diverging rays
Errors of refraction
o
Emmetropia
normal vision
accommodation relaxed
far object, parallel rays – image falls on retina
o
Hyperopia
farsightedness
short eyes ball/weak lens
image from distant objects formed behind retina
accommodation all the time
ciliary muscle overworked
eye strain
headache
convergence of visual axes
squint/strabismus
corrected by
convex lens
o
Myopia
nearsightedness
long eye ball
image from distant objects is formend infront of retina
corrected by
concave lens
Astigmatism
Uneven corneal surface
o
curvatures at various meridians not equal
Different focal points
o
distorted image
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Photoreceptors
Receptor cells containing excitatory/inhibitory synaptic transmitters
Rods
Photopigments
o
protein: rhodopsin
o
Vitamin A aldehyde
retinal
retinene
Abundant in
o
peripheral retina
Low threshold receptors for
o
dim light
o
night vision (scotopic vision)
Most sensitive to
o
505 nm
o
Blue-Green
Cones
Photopigments
o
protein: photopsins
o
Vitamin A aldehyde
retinal
retinene
Abundant in
o
central retina
particularly in fovea centralis (in macula lutea)
Higher threshold receptors for
o
daylight
o
detailed vision (photopic vision)
o
colour vision
Visual acuity greatest at
o
fovea centralis
Phototransduction
In the photoreceptors,
In the dark
o
Na+ channel open
Na+ entry
Depolarization
Inhibitory neurotransmitter release
o
Bipolar cells inhibited
o
Ganglion cells (axons of optic nerves)
decrease discharge
In the light
o
Activates rhodopsin
decomposition of rhodopsin
cis-retinal bleached –> trans-retinal (opsin & retinal)
retinal detaches from opsin
activated opsin
o
Na+ channels close
Na+ entry decrease
Hyperpolarization
Decrease inhibitory neurotransmitter release from the rod
removal of inhibition
o
Bipolar cells excited
o
Ganglion cells
action potential initiated
transmit to the brain –> vision occurs
Dark adaptation
If one moves from a brightly lighted room to a dimly lighted room
o
retina slowly becomes more sensitive to light
pupil dilate – to capture more light into the retina
o
this decline in visual threshold: dark adaptation
Dark adaptation depends on
o
rate of regeneration of rhodopsin
which depends on vitamina A (retinol)
Vitamin A deficiency:
Signs & Symptoms
impaired dark adaptation
Nactalopia
night blindness
Bitot spots
Xerophthalmia
eye drying
Keratomalacia
corneal softening
Ulceration
Scars
Pathophysiology
degeneration of rods & cones
degeneration of neural layers of retina
blindness
most common cause of preventable blindness
treatment before receptors are destroyed
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Colour vision
The sensation of white/any spectral colour can be produced by
o
mixture of various proportions of
Red wavelength
Green wavelength
Blue wavelengh
3 types of cones
o
Red cones
absorb long wavelength, L cones
o
Green cones
absorb medium wavelength, M cones
o
Blue cones
absorb short wavelength, S cones
Cones most sensitive to
o
405 nm (yellow-green)
Color blindness
Trichromats (have 3 cone systems -RGB)
o
Normal trichromats
individuals with normal colour vision
o
Anomalous trichromats (1 weak cone system)
Protanomaly
red weakness
defective red-sensitive cones
Deuteranomaly
green weakness
defective green-sensitive cones
Tritanomaly
blue weakness
defective blue-sensitive cones
Dichromats (2 cone systems)
o
Protanopia
red blindness
no red-sensitive cones
o
Deuteranopia
green blindness
no green-sensitive cones
o
Tritanopia
blue blindness
no blue-sensitive cones
Monochromats (1 cone system)
Causes of colour blindness
o
Inherited
X-linked recessive
defective opsins
most frequently: red-green weakness
o
Lesions of visual cortex concerned with colour vision
achromatopsia
o
Sildenafil (viagra)
inhibits retinal as well as penile form of phosphodiesterase
transient blue-green colour weakness
Test for colour vision
Colour matching
Ishihara Pseudoisochromatic plates
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Visual fields & Visual pathways
Fusion of 2 images occurs in the visual cortex
Visual axes of 2 eyes must be properly aligned
o
if not, double vision (diplopia)
Diplopia
o
2 different images sent to the brain
o
brain will suppress one of the images
to prevent seeing double
o
in children (<6 y/o)
suppression of one eye’s input to the brain leads to reduced
vision in the suppressed eye
amblyopia
Examination of visual fields
o
Perimetry
Lesions in visual pathways (Visual field defect)
Anterior pituitary tumours
o
pressure on optic chiasma from below
o
eg. visual field defects in gigantism
Visual acuity
Defined in terms of minimum separable
o
shortest distance by which 2 lines can be separated & still be perceived
as 2 lines
o
corresponds to a visual angle of about 1 minutes
Snellen letter charts
o
designed so that the height of the letters in the smallest line a normal
individual can read at 20 ft (6m) subtends a visual angle of 5 minutes
Jaeger’s cards
o
test for near vision (reading)
When visual acuity is markedly reduced
o
can be quantified in terms of
Count fingers (CF)
distance at which the patient can count fingers
Hand movement (HM)
discern hand movement
Perceive light
No light perception (NLP)
if an eye is totally blind, examination will reveal NLP
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Blindness (Visual impairment)
Transient blindness
o
may occur on sudden exposure to darkness/bright light
o
blinding light
o
light adaptation= 5 minutes
Transient monocular blindness
o
associated with an increased risk of subsequent stroke
Night blindenss
Colour blindness
Total blindness
Visual field contraction/blindness
Legal blindness
o
a level of visual impairment that has been defined by law to determine
eligibility for benefits
central visual acuity of 20/200 or less
in the better eye
with best possible correction
visual field of 20 degrees or less
Common causes of blindness
Cataract
o
Lens opacity increases
o
due to aging
o
leading cause of blindness
Glaucoma
o
increased accumulation of aqueous humor
increased pressure in anterior & posterior chambers
increased pressure on vitreous humor
increased pressure on retinal layers & optic nerve
Age-related macular degeration (AMD)
o
trachoma (infection)
o
other corneal opacities
o
diabetic retinopathy
o
retinal detachment
o
Eye conditions in children
cataract
retinopathy of prematurity
vitamin A deficiency
amblyopia
associated with refractive error/strabismus/squint
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