phototransduction, visible light

Definition

Target type: biologicalprocess

The sequence of reactions within a cell required to convert absorbed photons from visible light into a molecular signal. A visible light stimulus is electromagnetic radiation that can be perceived visually by an organism; for organisms lacking a visual system, this can be defined as light with a wavelength within the range 380 to 780 nm. [GOC:go_curators, ISBN:0198506732]

Phototransduction is the process by which light is converted into an electrical signal in photoreceptor cells of the eye. This complex process begins with the absorption of light by photopigments, specialized molecules that undergo conformational changes upon light absorption. In humans, the primary photopigments are rhodopsin in rod cells and cone opsins in cone cells.

Rhodopsin, a transmembrane protein embedded in the outer segment of rod cells, consists of a protein called opsin and a light-absorbing molecule called retinal. Retinal exists in two forms: cis-retinal and trans-retinal. In the dark, retinal is in its cis form, and rhodopsin is inactive. When light strikes the retina, it isomerizes cis-retinal to trans-retinal, causing a conformational change in the rhodopsin molecule. This change activates rhodopsin, triggering a cascade of biochemical reactions.

The activated rhodopsin initiates a signal transduction pathway involving a protein called transducin. Transducin is a heterotrimeric G protein composed of alpha, beta, and gamma subunits. When activated rhodopsin binds to transducin, it causes the alpha subunit of transducin to dissociate from the beta-gamma subunits. The alpha subunit, now carrying a bound GTP molecule, activates another protein called phosphodiesterase.

Phosphodiesterase is an enzyme that breaks down cyclic GMP (cGMP), a second messenger molecule involved in regulating the permeability of ion channels in the plasma membrane of photoreceptor cells. In the dark, cGMP levels are high, keeping these ion channels open. This allows sodium ions to flow into the cell, maintaining a depolarized state.

Upon light activation, phosphodiesterase hydrolyzes cGMP, causing a decrease in its concentration. This drop in cGMP levels leads to the closure of ion channels, reducing the flow of sodium ions into the cell. This results in hyperpolarization of the photoreceptor cell, which is the electrical signal that is transmitted to other neurons in the visual pathway.

The hyperpolarization signal travels along the optic nerve to the brain, where it is processed and interpreted as visual information.

The entire process of phototransduction is remarkably efficient, allowing us to see in a wide range of light conditions. In addition, the system is rapidly reset, allowing for continuous visual perception. This is achieved by a series of feedback mechanisms that deactivate rhodopsin, replenish cGMP, and return the photoreceptor cells to their resting state.'
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Proteins (5)

Compounds (11)