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rod cells/ cone cells

I'm currently doing AQA A Level Biology and am a bit confused about the pigments associated with rod cells and cone cells.
My understanding is that these photoreceptors produce transmitters when NOT stimulated (when pigment hasn't broken down) and don't produce transmitters when stimulated (when pigment has broken down)? However I don't know how this works
Any help/guidance would be greatly appreciated

Reply 1

Wannabevetnurse

In cone cells, the pigment is iodopsin which breaks down due to high light intensity and therefore you can see in bright environments.

In rod cells, the pigment is rhodopsin which breaks down due to low light intensity and therefore you can see in dark environments.

When each one is broken down, an action potential occurs.

Reply 2

TheVirtualPhoton

These videos may help if you're super interested:
https://www.youtube.com/watch?v=dhd2fja0LZ4
https://www.youtube.com/watch?v=CqN-XIPhMpo (a fair bit of detail)
https://www.youtube.com/watch?v=GKaFjw8N8zQ&t=2168s (lots of detail)

In summary:
-The structure of the retina is: rods & cones -> bipolar cells -> ganglion cells (that form the optic nerve), with some connections between (horizontal cells and amacrine cells)
-Dark: rod cells are depolarised due to Na+ entry and release glutamate, a neurotransmitter.
-Light: retinal of rhodopsin pigment undergoes a conformational change when it absorbs light, a cascade results in the closure of Na+ channels, the cell becomes hyperpolarised and stops releasing glutamate.

-Glutamate is usually excitatory but it inhibits 'on' bipolar cells
-Removal of inhibitory glutamate allows 'on' bipolar cell excitation
-Depolarisation of these bipolar cells results in the release of glutamate, which are excitatory on ganglion cells
-Action potentials are triggered in ganglion cells and travel down the optic nerve

There are even more pathways involved with vision, but hopefully this answers your question. I believe that it is beyond what is required for A level (especially the last two linked videos - this is degree-standard stuff) but I understand the need for some extra info to avoid confusion!

Reply 3

Jpw1097

Original post by helloworld121

The phototransduction pathway is quite complex, as explained by @TheVirtualPhoton. The ‘on’ photoreceptors themselves release neurotransmitter when not being stimulated, however, the neurotransmitter is inhibitory. The photoreceptors synapse with bipolar cells which in turn synapse with ganglion cells.