*Read foundation 1 notes

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

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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

image

  • 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

image

  • 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)

image

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)

image

  • 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

image

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Visual fields & Visual pathways

image

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  • 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)

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  • 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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Terri is obsessed with making medical school as painless as possible. She studies and compiles medical school notes in a concise, easy-to-understand format. She also enjoys reading contributions by others. She is an investor in sustainability projects. Her ideal weekend is wine tasting and experimenting on bread-making. She has yet to master the art of Sourdough baking.

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