Photoreceptor Cell - Phototransduction

The process of phototransduction occurs in the retina. The retina is thick with cells. The photoreceptor cells (rods and cones) form the innermost layer. The middle layer contains bipolar cells, which collect neural signals from the rods and the cones and then transmit them to the outermost layer of the retina where the neurons called retinal ganglion cells (RGCs) organize the signals and send them to the brain. The bundled RGC axons form the optic nerve, which leaves the eye through a hole in the retina creating the blind spot.

Activation of rods and cones is actually hyperpolarization; when they are not being stimulated, they depolarize and release glutamate continuously. In the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens ion channels (largely sodium channels, though calcium can enter through these channels as well). The positive charges of the ions that enter the cell down its electrochemical gradient change the cell's membrane potential, cause depolarization, and lead to the release of the neurotransmitter glutamate. Glutamate can depolarize some neurons and hyperpolarize others.

When light hits a photoreceptive pigment within the photoreceptor cell, the pigment changes shape. The pigment, called iodopsin or rhodopsin, consists of large proteins called opsin (situated in the plasma membrane), attached to a covalently bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes it to activate a regulatory protein called transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters. The entire process by which light initiates a sensory response is called visual phototransduction.