vision

• Iris – muscle – changing size of pupil - coloured part
• Pupil – light goes in
• Lens – changes shape to refract light more or less
• Cornea – focuses on retina – sensitive to pain
• THE CORNEA

  has sensory nerve supply:

  mechanoreceptors linked to thin myelinated Aδ type fibres.

  • They convey acute sharp pain in response to mechanical contact with the corneal surface.

  polymodal nociceptors

  • with slow-conducting unmyelinated C-fibres
  • They convey sharp and sustained pain in response to chemical stimuli, heat and mechanical irritants.
  • Aδ and C fibre cold receptors.
• Corneal nerves induce tear production and stimulate the blinking reflex
• diabetes causes loss of fibres in cornea
• • raised blood glucose
  • reduced epithelial cell proliferation
  • increases apoptosis
  • inhibits epithelial wound healing
  • Prolonged hyperglycemia results in the accumulation of glycation end products which promotes inflammation and oxidative stress and damages nerves.
• • photosensitive cells behind nerve fibres
  • creates optic disc (blind spot)
• fovea - greatest visual acuity

  • densely packed with cones
  • centre of yellow spot
  • pitted structure
• Muller Glia:

  • optical fibres for colour vision
  • funnel shaped
  • guide light to photoreceptors in deep layers
• light detection

  • Retinal pigment epithelium absorbs light unabsorbed by photoreceptors
  • Photoexcited pigment initiates a signal that amplifies light signals
  • closure of cation channels in Rods & Cones
  • hyperpolarization.
  • less NTM released
• Rods = low-light vision

  • more sensitive to light - dim light detected
• cones - daylight and colour vison

  • red, green and blue
  • cones in centre and rods on outside
• Phototransduction in rods – the dark current

  • rods full of cGMP and sodium channels
  • in no light - channels always open, always depolarised and glutamate always released

  LIGHT

  • enzyme that degrades cGMP activated
  • closure of ion channels
  • hyperpolarises
• rhodopsin absorbs photon = conformal change

  • activates transduction
  • activates enzyme PDE
  • breaks down cGMP
• structure of rod

  ATP provided by the inner segment powers the sodium-potassium pump.

  
• bipolar cells - filter on and off signals