Question

1. Describe the main reaction that occurs in the rod cells in the retina, which generates a nerve impulse that allows us to see. Which role does the geometry of 11-cis-retinal play in this process? What reactions convert all-trans-retinal back to 11-cis-retinal so that the process of vision can continue?

Answer 1

The light is mapped as an image along the surface of the retina by activating a series of light-sensitive cells known as rods and cones. These photoreceptor cells convert the light into electrical impulses which are transmitted to the brain via nerve fibers.

The retina is lined with many millions of photoreceptor cells that consist of two types: 7 million cones provide color information and sharpness of images, and 120 million rods are extremely sensitive detectors of white light to provide night vision. The outer segments of the rods and cones contain a region filled with membrane bound discs, which contain proteins bound to the chromophore 11-cis-retinal. ( A chromophore is a molecule that can absorb light at a specific wavelength, and thus typically displays a characteristic color).

When visible light hits the chromophore, the chromophore undergoes an isomerization, or change in molecular arrangement, to all-trans-retinal. The new form of retinal does not fit as well into the protein, and so a series of conformational changes in the protein begins. As the protein changes its conformation, it initiates a cascade of biochemical reactions that result in the closing of Na+ channels in the cell membrane. Na+ ions flow freely into the cell to compensate for the lower potential which exists inside the cell. When the Na+ channels are closed, however, a large potential difference builds up across the plasma membrane. This potential difference is passed along to an adjoining nerve cell as an electrical impulse at the synaptic terminal, the place where these two cells meet. The nerve cell carries this impulse to the brain, where the visual information is interpreted.

To generate enough 11-cis retinal for the normal function and survival of photoreceptors, all-trans retinal is converted back into 11-cis retinal through a series of enzymatic steps known as the visual cycle.

For rods, such re-isomerization depends on the adjacent retinal pigment epiment epithelium (RPE), while cones rely on Muller glia as well as the RPE. By contrast, photoactivated melanopsin retains all-trans-retinal and re-isomerizes it to 11-cis-retinal upon subsequent light exposure, in a process known as photoreversal.