What absorbes any light passing through the retina?

Answer 1

Four kinds of light-sensitive receptors are found in the retina:

• rods
• three kinds of cones, each "tuned" to absorb light from a portion of the spectrum of visible light
  • cones that absorb long-wavelength light (red)
  • cones that absorb middle-wavelength light (green)
  • cones that absorb short-wavelength light (blue)

  Each type of receptor has its own special pigment for absorbing light. Each consists of:

• a transmembrane protein called opsin coupled to
• the prosthetic group retinal. Retinal is a derivative of vitamin A (which explains why night blindness is one sign of vitamin A deficiency) and is used by all four types of receptors.

The amino acid sequence of each of the four types of opsin are similar, but the differences account for their differences in absorption spectrum. The retina also contains a complex array of interneurons:

• bipolar cells and ganglion cells that together form a path from the rods and cones to the brain
• a complex array of other interneurons that form synapses with the bipolar and ganglion cells and modify their activity.

Ganglion cells are always active. Even in the dark they generate trains of action potentials and conduct them back to the brain along the optic nerve. Vision is based on the modulation of these nerve impulses. There is not the direct relationship between visual stimulus and an action potential that is found in the senses of hearing, taste, and smell. In fact, action potentials are not even generated in the rods and cones.