Evolution of inhibitory synapses in rods

[robmcm]

In eyes, the impulse that travels down the optic nerve comes about due to the removal of the inhibition exerted on that neurone from the rod cells. When light hits a rod cell it causes repolarisation of that cell, which in turn stops the production of an inhibitory neurotransmitter and so the optic neurone can depolarise. It's a double-negative system.

My question is this - why have this 'double-negative' system in the first place? What's the evolutionary reason for it that makes it better to do this than it is to have a simple 'light --> depolarisation' system that is the intuitive way of doing things?

Any ideas??

[safra]

This is done so that the impulse is unidirectional. If it wasn't uni-directional, the impulse picked up by the neuron can travel back up to where it was picked up.