The Eye

• The retina contains 2 types of photoreceptor cells, rods and cones, which both then connect to bi-polar neurons and these cells connect to the optic nerve.
• Visual acuity: ability to distinguish between 2 points of light.
• Rods:

  • Low visual acuity
  • High sensitivity to light
  • Monochrome

  Cones:

  • High visual acuity
  • Low sensitivity to light
  • Colour vision
• Colour vision: ability to distinguish between different wavelengths of light.
• Rods:

  • Distributed throughout the retina with exceptions of the fovea and blind spot.
  • Contain a photosensitive chemical rhodopsin. Only 1 type of rhodopsin so rods are monochromatic.
  • Light causes rhodopsin to break down to retinene and opsin- this causes sodium ions to enter the cell and create a generator potential.
• Rods have a high sensitivity to light. Low levels of light will produce a low frequency of action potential. When an action potential reaches a synapse at the 'rod-bipolar neurone' junction, a chemical neurotransmitter is produced. Neurotransmitter from 1 action potential from the rod is NOT enough to reach the threshold in the bipolar neurone.
• Several rods connect to 1 bipolar neurone= convergence (of rod cells).
• Spatial Summation: Neurotransmitter from multiple rod cells arrive at the same time.
• Spatial summation reaches the threshold and causes an action potential ( even in low light when few action potentials are produced by each rod).
• Rods have low visual acuity. But only 1 action potential is sent to the brain from a region of the retina containing many rod cells. The brain cannot distinguish between 2 points of light that are close together.
• Cones:

  • Distributed mainly in the fovea (high density)
  • Contain the pigment iodopsin
  • Light causes iodopsin to break down- causes Na+ channels to open and create generator potential.
  • 3 types of cone cell due to 3 types of iodopsin sensitive to different wavelengths- red, blue, green.
• Sensitivity to colour: Red, blue and green cone cells are sensitive to different wavelengths of light.

  The frequency of impulses from each type of cell that reaches the brain's visual cortex is 'interpreted' as colour.
• Iodopsin breaks down to photopsin and retinine.
• 1 cone cell connects with 1 bipolar neurone.
• Cones give high visual acuity because each cone cell forms a synapse with its own bipolar neurone that is stimulated beyond the threshold a separate impulse is sent along the optic nerve to the brain (visual cortex).
• Cones are not sensitive to low light. In low light, a low frequency of action potentials results in not enough neurotransmitter arriving at the bipolar neurone from the 1 rod cell to reach the threshold. The stimulus has to be intense (brighter light) to produce more frequent action potentials to produce enough neurotransmitter to reach the threshold (temporal summation).