detached retina

Separation of most layers of the retina of the eye from the choroid, the pigmented middle layer of the eyeball. With age, small tears can develop in the retina, and the vitreous humour inside the eyeball leaks through, separating the retina from the choroid. The disease retrolental fibroplasia or accidents can also cause retinal detachment. It usually develops slowly, without pain. Floating black spots and flashes of light appear in the affected eye, and vision becomes increasingly blurred. Without prompt treatment, it causes permanent blindness. Draining the fluid behind the retina and applying heat, a laser beam, or extreme cold causes scarring that seals the tears and prevents the retina from detaching again.

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Retinal, technically called retinene₁ or retinaldehyde, is a light-sensitive retinene molecule found in the photoreceptor cells of the retina. Retinal is the fundamental chromophore involved in the transduction of light into visual signals, i.e. nerve impulses, in the visual system of the central nervous system.

Overview

The molecule that takes part in the initial step in the vision process, rhodopsin, has two components called 11-cis retinal and opsin. Retinal is a light-sensitive derivative of vitamin A, and opsin is a protein molecule. Rhodopsin is found in the rod cells of the eye. 11-cis retinal is a powerful absorber of light because it is a polyene; its 6 alternating single and double bonds make up a long conjugated electron network. When no light is present, the 11-cis retinal molecule is found in a "bent (cis) configuration" (fig A), and as such it is attached to the opsin molecule in a stable arrangement:

When light strikes the retina, a retinal molecule may absorb a photon, promoting it into an excited electronic state. The nature of the excited state is not well understood, but it is known that within 200 femtoseconds it returns to the ground electronic state. One third of these events cause no net change, while the remaining two thirds induce a rotation in the pi bond found between the eleventh and twelfth carbon atoms. In other words, the 11-cis retinal is transformed into the all-trans retinal (fig B) in a straightened configuration.

The all-trans retinal configuration, subsequently, does not fit into the binding site of the opsin molecule; as a result, upon isomerization, the trans isomer separates from the protein, which triggers a G protein signaling pathway' including transducin, that results in the generation of an electrical impulse, which is transmitted through the optic nerve to the brain for processing.

It takes a minimum of five photons to trigger a nerve impulse. In the absence of light, enzymes mediate the isomerization of all-trans back to the 11-cis configuration, and rhodopsin is regenerated by a new formation of a Schiff base linkage, which actuates the binding of the cis isomer to opsin. This is the basic mechanism of the vision cycle.

All-trans-retinal is also an essential component of type I, or microbial, opsins such as bacteriorhodopsin, channelrhodopsin, and halorhodopsin. In these molecules, light causes the all-trans-retinal to become 13-cis retinal, which then cycles back to all-trans-retinal in the dark state.

11-cis-retinal	all-trans-retinal

History

This photon induced retinal-bending mechanism was discovered in 1958 by the American biochemist George Wald and his co-workers. For his work, Wald won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit.

See also

• Sensory system
• Vision
• The Visual Cycle

References

External links

• First Steps of Vision - National Health Museum
• Vision and Light-Induced Molecular Changes
• Retinal Anatomy and Visual Capacities
• Retinal