Retinal neurons and relationships, part 2

Hey, just thought of a design name. Retinal Images, anyone? See Me. The Shapes Behind Your Eyes. A Vision In Aluminium. Sight Unseen.

Stop me, please.

Anyway, The bipolar cell is firing. Let's talk about its destinations: Amacrine cells and Ganglion cells.

Amacrine cells are another processing waystation, like Horizontal cells in the last post. Except that if Horizontal cells were analogous to pressing 'sharpen image' in photoshop, Amacrine cells are like tuning contrast, colour balance and intensity. They are also involved in the perception of moving objects. They are complex and not well understood, like tiny cellular James Joyces. There are 20+ subtypes that use several different neurotransmitters. The do not have axons (long thin unbranched processes), instead sending signals out along dendrites (shorter, highly branched).

(In case you're interested, by this analogy the brain and its 30+ visual cortices are annotating everything, getting out contrast, colour and intensity histograms and tuning the image reeeeallly carefully, putting in animations, spraying the clone brush everywhere, photoshopping faces onto everything and sending messages to the photographer about what to take pictures of next, except much more complicated and extensive than that. And doing it in microseconds.)

Ganglion cells take the bipolar and amacrine inputs, integrate the signals and fire messages to the brain. The axons from ganglion cells stretch down the optic nerve, traveling many centimetres into the brain. Given the size of the cell body, this is an axon 100,000 times longer than its cell. And that's pretty short as some neurons go, which is amazing.

When I say neurons 'integrate' signals... Well, it's more like the interference between waves than computer-style 'if x and y = input then z = output'. There are inhibitory and excitatory waves of electricity of different sizes travelling different distances along dendrites to the cell body, and once they get there they can combine to cancel or amplify each other, and the sum of the waves at a particular point in the cell at a particular time determines if the cell fires and the axon carries a signal too the synapse.

I LOVE neuroscience. It just gets more and more complicated the more you look. Yipeee!

Practicing scientists, I apologise deeply if my analogies have hurt you.

A further consideration: Thanks to the vagaries of evolution, this all happens BACKWARDS. Your photoreceptors are in the layer of the retina furthest from the light. All this circuitry passes information back towards the light, and the axons of the ganglion cells run over the surface of the retina till they reach the optic nerve head, where they bundle together and head for the brain.

This of course means that all these neurons are transparent, so that light can actually reach the photoreceptors. Acrylic on aluminium is sounding better and better :)

Phew. That was fun. More images in a bit.