THE EYE

• Rods have high light sensitivity: they can be activated by low light intensities.
• Cones have low light sensitivity: they can only be activated by high light intensities.
• • Rod and cone cells each contain a specific optical pigment that absorbs light.
• • Light breaks down (“bleaches”) this pigment to activate the photoreceptor cell so that impulses are then sent to the brain.
• • Rods contain a pigment called rhodopsin; rhodopsin can be broken down even by low-intensity light, so rod cells are activated in low-light conditions.
  • Cones contain a pigment called iodopsin; iodopsin is only broken down in high light intensities, since there is more light energy available. Therefore, cones only activate in bright light.
• Upon the break down of the pigment molecule, the photoreceptor only sends impulses for a short time before returning to its resting state. A fresh pigment molecule needs to be broken down for the photoreceptor to be activated again.
• Re-synthesis of optical pigments

  • For a photoreceptor cell to be become responsive to light again, the optical pigment needs to be re-made so it can be broken down again.
  • Rhodopsin in rod cells takes a long time to reform.
  • Bright light will rapidly bleach rhodopsin in almost all rod cells of the retina. This rhodopsin then takes a long time to reform, meaning that rod cells are not useful for “bright light” vision.
  • Iodopsin in cones reforms much faster, ensuring cones remain responsive during bright light vision.
• The connections between photoreceptors and their neurones can also explain differing light sensitivities.
• • More light being absorbed by a photoreceptor cell leads to more neurotransmitter being released.
  • Neurotransmitters are released across a synapse by photoreceptor cells to stimulate the nearby neurone.
  • Neurones will only be activated if the connected photoreceptors release enough neurotransmitter to reach the neurone’s threshold.
  • The “threshold” is a minimum value of stimulation that must be reached for impulses to be sent.
• Rods are very weakly stimulated under low light intensities.

  Each rod therefore releases relatively little neurotransmitter.
• rods are crucial for vision in low light intensities
• • Several rods are connected to a single neurone.
  • This makes it more likely that enough neurotransmitter will be released to reach threshold for a set of nerve impulses to be sent to the brain.
• Rods have high light sensitivity:

  • Contain rhodopsin, which can be broken down by low-intensity light (to activate the rod);
  • Several rods are connected to a single neurone;
  • This makes it more likely that enough neurotransmitter will be released to reach threshold for a set of nerve impulses to be sent to the brain.
• • Each cone connects to its own single, separate neurone.
  • This makes it less likely that enough neurotransmitter will be released to reach the threshold for a set of nerve impulses to be sent to the brain.
• Cones have low light sensitivity:

  Contain iodopsin, which can only be broken down by high-intensity light;

  Each cone connects to its own single, separate neurone;

  This makes it less likely that enough neurotransmitter will be released to reach the threshold for a set of nerve impulses to be sent to the brain;
• Human colour vision

  • There are three cone types that each maximally absorb light at a different wavelength (blue, green and red);
  • This gives the cones their names: blue-sensitive, green-sensitive and red-sensitive.
  • Many wavelengths are absorbed by more than one type of cone cell, so the relative extent to which each type of cone cell is stimulated allows the brain to determine the exact colour being seen
• Visual acuity: measuring the ability to see detail
• Visual acuity

  Several rods connect to a single neurone. This leads to a single set of impulses travelling to the brain despite several rods being stimulated. This leads to lower visual acuity from rods.

  Each cone connects to its own single, separate neurone. Therefore, cones each send separate sets of impulses to the brain. This leads to higher visual acuity.
• • Most light is focused on a part of the retina called the fovea when you are looking straight ahead.
  • The fovea contains only cones.
  • The rest of the retina (the “peripheral” retina) consists mostly of rods (with some cones).
  • The region where the optic nerve leaves the eye has no photoreceptor cells at all, creating a “blind spot”.
• • Cones (especially the red-sensitive) will still be activated by the red light, providing a better visual acuity than night vision alone.
  • Rhodopsin in rod cells does not significantly absorb red light.
  • This means red light will not bleach rhodopsin, meaning rhodopsin remains intact to be used for night vision (e.g. being able to see at night if the power fails in an emergency).
• Several rods


  Multiple rod cells in the eye that detect light and dark. Connect to a single neuron, resulting in lower visual acuity.
• Single neuron


  A nerve cell that transmits impulses from several rods to the brain. Connects to multiple rods, resulting in lower visual acuity.
• Cones


  Light-sensitive cells in the eye that detect color and fine detail. Connect to their own separate neuron, resulting in higher visual acuity.
• Separate neuron


  A nerve cell that transmits impulses from each cone to the brain. Connects to each cone, resulting in higher visual acuity.