• Light enters the eye
  • Light hits the photoreceptor and is absorbed by light sensitive pigments
  • Light bleaches the pigments resulting in a chemical change and altering the membrane permeability to Na+
  • Generator potential is created and once it reaches a threshold a nerve impulse is sent along a bipolar neurone
  • Biplolar neurones connect photoreceptors to the optic nerve



Mainly located in peripheral parts of retina

Mainly located in fovea part of retina

Black and white


Many rods linked to same bipolar cell

One cone linked to one bipolar cell

High sensitivity to light

Low sensitivity to light

Low visual acuity

High visual acuity



  • Rods are sensitive to light as there are many rods to one bipolar neurone
    • Therefore, it is easier for 3 rod cells to reach the threshold value relative to 1
  • Cones are less sensitive than rods as they have one bipolar neurone each
    • Therefore, requires more light to reach threshold value

Visual Acuity

  • Visual acuity is the ability to tell the different points that are close together
  • Rods give a low visual acuity as many rods join to the same bipolar neurone
    • Therefore, light from two points close together cannot be differentiated
  • Cones give a high visual acuity as each cone is close together and one cone joins to one bipolar neuron
    • When light from two points hits two cones, two action potentials are given