Visual phototransduction

Visual phototransduction is a process by which light is converted into electrical signals in the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye.

Photoreceptors

The process of phototransduction is a complicated one, and in order to understand it, one must have an understanding of the structure of the photoreceptors in the eye: the rods and cones. These cells contain opsin (a protein) and retinal (the light-sensitive compound). There are three different kinds of opsin, which respond best to certain wavelengths, or colours, of light: red, green, or blue. This only applies for cones, which can perceive colour; rods deal with low light level, black and white images.

Process

The process starts when a light photon interacts with the retinal in a photoreceptor, causing the retinal to undergo isomerization which changes the potential, or charge, of the membrane of the cell. This electrical change activates a G-protein (a special kind of protein) known as transducin. This protein opens sodium-potassium pumps in the cell membrane, causing depolarization. This in turn causes an influx of Ca2+ ions into the cell. These ions cause small bubbles, or vesicles, which are filled with special chemicals known as neurotransmitters to merge with the membrane of the cell, releasing the neurotransmitters into the synaptic cleft, the area between the end of one cell and beginning of a neuron. These neurotransmitters then open sodium gates in the end of the next neuron, allowing sodium ions to flood into that cell, therefore continuing the action potential. The message travels from neuron to neuron in this fashion until it reaches the brain, where the information in decoded.

External links

• Visual pigments and visual transduction
• A General Overview on Visual Perception