Physiology of vision

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Visual system

• Eye
• image formation & phototransduction
• Visual pathway
• tranmission of nerve impulses
• Visual cortex
• occipital lobe of cerebral cortex
• primary visual receiving area: sides of the calcarine fissure
• visual processing & perception occurs here

Stimulus

• Light
• visible portion of the electromagnetic spectrum

Sensory organ

• Eye
• optical instrument for focusing of images on retina by refraction of light rays
• Refractive power
• Cornea: 40 dioptres
• fixed
• Lens: 20 dioptres
• adjustable
• 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
• convergence
• Concave spherical lens
• divergens

Accommodation

• Parasympathetic response
• When a person looks at a near object, 3 changes occur:
• accommodation reflex
• convergence of visual axis
• pupil constrict
• When accommodation relaxed:
• Near object (<6 m)
• diverging rays
• image falls behind retina
• Far object (>6 m)
• parallel rays
• image falls on retina
• Physiology
• ciliary muscles contracts
• this relaxes the lens ligaments
• lens spring into a more convex shape
• near point of vision recedes throughout life
• slowly at 1st
• advancing rapidly with old age
• due to increasing hardness of lens
• impaired accommodation
• receding of near point
• Presbyopia
• reading and close vision difficult
• corrected by wearing convex lens
• diverging rays
• Errors of refraction
• Emmetropia
• normal vision
• accommodation relaxed
• far object, parallel rays – image falls on retina
• 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
• Myopia
• nearsightedness
• long eye ball
• image from distant objects is formend infront of retina
• corrected by
• concave lens

Astigmatism

• Uneven corneal surface
• curvatures at various meridians not equal
• Different focal points
• distorted image

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Photoreceptors

• Receptor cells containing excitatory/inhibitory synaptic transmitters

Rods

• Photopigments
• protein: rhodopsin
• Vitamin  A aldehyde
• retinal
• retinene
• Abundant in
• peripheral retina
• Low threshold receptors for
• dim light
• night vision (scotopic vision)
• Most sensitive to
• 505 nm
• Blue-Green

Cones

• Photopigments
• protein: photopsins
• Vitamin A aldehyde
• retinal
• retinene
• Abundant in
• central retina
• particularly in fovea centralis (in macula lutea)
• Higher threshold receptors for
• daylight
• detailed vision (photopic vision)
• colour vision
• Visual acuity greatest at
• fovea centralis

Phototransduction

In the photoreceptors,

• In the dark
• Na+ channel open
• Na+ entry
• Depolarization
• Inhibitory neurotransmitter release
• Bipolar cells inhibited
• Ganglion cells (axons of optic nerves)
• decrease discharge
• In the light
• Activates rhodopsin
• decomposition of rhodopsin
• cis-retinal bleached –> trans-retinal (opsin & retinal)
• retinal detaches from opsin
• activated opsin
• Na+ channels close
• Na+ entry decrease
• Hyperpolarization
• Decrease inhibitory neurotransmitter release from the rod
• removal of inhibition
• Bipolar cells excited
• 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
• retina slowly becomes more sensitive to light
• pupil dilate – to capture more light into the retina
• this decline in visual threshold: dark adaptation
• Dark adaptation depends on
• 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
• mixture of various proportions of
• Red wavelength
• Green wavelength
• Blue wavelengh
• 3 types of cones
• Red cones
• absorb long wavelength, L cones
• Green cones
• absorb medium wavelength, M cones
• Blue cones
• absorb short wavelength, S cones
• Cones most sensitive to
• 405 nm (yellow-green)

Color blindness

• Trichromats (have 3 cone systems -RGB)
• Normal trichromats
• individuals with normal colour vision
• 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)
• Protanopia
• red blindness
• no red-sensitive cones
• Deuteranopia
• green blindness
• no green-sensitive cones
• Tritanopia
• blue blindness
• no blue-sensitive cones
• Monochromats (1 cone system)

• Causes of colour blindness
• Inherited
• X-linked recessive
• defective opsins
• most frequently: red-green weakness
• Lesions of visual cortex concerned with colour vision
• achromatopsia
• 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
• if not, double vision (diplopia)
• Diplopia
• 2 different images sent to the brain
• brain will suppress one of the images
• to prevent seeing double
• 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
• Perimetry

Lesions in visual pathways (Visual field defect)

• Anterior pituitary tumours
• pressure on optic chiasma from below
• eg. visual field defects in gigantism

Visual acuity

• Defined in terms of minimum separable
• shortest distance by which 2 lines can be separated & still be perceived as 2 lines
• corresponds to a visual angle of about 1 minutes
• Snellen letter charts
• 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
• test for near vision (reading)
• When visual acuity is markedly reduced
• 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
• may occur on sudden exposure to darkness/bright light
• blinding light
• light adaptation= 5 minutes
• Transient monocular blindness
• associated with an increased risk of subsequent stroke
• Night blindenss
• Colour blindness
• Total blindness
• Visual field contraction/blindness
• Legal blindness
• 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
• Lens opacity increases
• due to aging
• leading cause of blindness
• Glaucoma
• 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)
• trachoma (infection)
• other corneal opacities
• diabetic retinopathy
• retinal detachment
• Eye conditions in children
• cataract
• retinopathy of prematurity
• vitamin A deficiency
• amblyopia
• associated with refractive error/strabismus/squint

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