6-1-10-the-human-retina

The human retina

• The eye is a sense organ containing receptors that are sensitive to:

  • light intensity

  • light wavelength

• Light receptors are located in the retina, a layer of cells at the back of the eye; the retina contains:

  • rod cells

    • sensitive to light intensity only

    • distributed across the entire retina

  • cone cells

    • sensitive to different wavelengths of visible light

    • concentrated in the fovea

• Rods and cones detect light as follows:

  1. light causes the chemical breakdown of optical pigments inside rods and cones

  2. the breakdown of these optical pigments results in a generator potential

  3. if the generator potential is large enough then a nerve impulse is sent along a bipolar neurone to the optic nerve

• Information is relayed to the brain via the optic nerve

Sensitivity to light and wavelength

• Rod and cone cells differ in their sensitivity to light and wavelength

• Rod cells are:

  • very sensitive to light

    • Multiple rod cells connect to a single bipolar cell, meaning that many weak generator potentials together can trigger a nerve impulse in the bipolar cell

      • This is an example of spatial summation

  • not sensitive to wavelength

• Cone cells are

  • less sensitive to light

    • Each cone cell connects to its own bipolar neurone, so the generator potential from an individual cone cell must be higher than in a rod cell to trigger a nerve impulse in the bipolar cell

  • sensitive to different wavelengths of light

    • When red-sensitive, blue-sensitive and green-sensitive cones cells are stimulated in different numbers and locations, colour images are generated

Visual acuity

• Visual acuity is:

the ability to distinguish between two separate points when viewing an image

• The higher the visual acuity, the more detailed an image will be

• Rod cells have low visual acuity

  • As described above, multiple rod cells synapse with a single bipolar cell

  • Information from many neighbouring rod cells is detected as a single point when the brain generates an image

    • The brain is not able to determine which impulses have been generated by which specific rod cell

• Cone cells have high visual acuity

  • A single cone cell synapses with a single bipolar cell, allowing each cell to send a separate impulse to the brain

  • Information from each individual cone cell will be interpreted as separate points when the brain generates an image