Hehehe this was one of my favourite bits of neuro...
In the absence of light stimulation, Sodium channels in photoreceptors are kept open by the action of cyclic guanosine monophosphate (cGMP), because cGMP is produced by the enzyme guanylate cyclase that is constitutively active in the photoreceptor cell.
Rhodopsin of a rod cell exists in its normal, inactive state bound to 11-cis retinene (carrots are a good source of this compound, hence the old wives' tale). When light energy stimulates the rhodopsin, this causes a change in its conformation that isomerises 11-cis retinene to trans-retinene. By chemistry bla bla, trans-retinene activates a g-coupled protein called Transducin, which in turn causes activation of cGMP phosphodiesterase enzymes, that cleave cGMP to GMP. The reduced [cGMP] resulting means that sodium channels close as they are not held open by cGMP, therefore causing hyperpolarisation and thus reduced neurotransmitter secretion.
The neurotransmitter involved is usually glutamate, which is excitatory in most parts of the nervous system, but is inhibitory in the retina.
Rod cell is highly permeable to sodium when not activated
This is because an enzyme called guanylate cyclase is constantly producing cGMP
cGMP causes sodium channels to open
Light > Rhodopsin
This causes rhodopsin to isomerise 11-cis retinene to trans-retinene.
Trans-retinene activates a protein called transducin.
Transducin activates phosphodiesterases that cleave cGMP, therefore reducing its concentration in the rod cell.
Less cGMP means less sodium channels open.
Last edited by Friar Chris; 02-06-2012 at 20:49.