Eye

• Eye diagram

  A) lens

  B) iris

  C) anterior chamber

  D) conjuctivia

  E) cornea

  F) pupil

  G) suspendatory ligament

  H) ciliary body

  I) ciliary muscles

  J) sclera

  K) blind spot

  L) optic nerve

  M) fovea

  N) retina

  O) choroid

  P) vitreous humour

  
• The iris:

  • light hitting the retina causes a response in the iris / the iris controls the amount of light entering the eye
  • in bright light circular muscles in the iris contract (and radial muscles relax) to reduce the diameter of the pupil / amount of light entering the eye
  • this protects the retina from damage
  • in dim light the radial muscles contract (and circular muscles relax) to dilate the pupil / increase the amount of light entering the eye
  • to ensure sufficient light can reach the retina to stimulate photoreceptors
• The lens:

  • the lens is for fine adjustment of the convergence of light onto the retina (at the front of the eye the transparent cornea is responsible for much of the refraction of light)
  • for a distant object, the ciliary muscles relaxing causes the suspensory ligaments to become taut and make the lens thinner
  • which means it is less refracting
  • with a near object, the ciliary muscles contract, reducing the tension in the suspensory ligaments making the lens wider
  • which causes greater refraction
• The rods:

  • rods contain the photosensitive pigment rhodopsin which is readily broken down (bleached) in low light intensity into opsin and retinine
  • if the threshold level is reached an action potential is initiated in the receptor neurone
  • in bright light rods remain bleached
  • the opsin and retinine only recombine in darker conditions
  • several rods synapse with one bipolar neurone (retinal convergence)
  • the resultant convergence allows summation of generator potentials
  • convergence results in a lack of visual acuity (poor resolution)
• The cones:

  • cones contain the pigment iodopsin which is broken down in high light intensity
  • each cone synapses individually with bipolar neurones
  • which gives cones greater visual acuity
  • there are three types of cones
  • each type of cone has a different type of iodopsin sensitive to red, green or blue light
  • the variety of colours we preceive depends on the relative amount of stimulation of each of these three types