Retinal Wiring
There are five types of retinal cells:
- photoreceptors, the neurons which transduce photons into electrical potentials and release neurotransmitter onto the processing cells;
- horizontal cells, neurons that share information between neighboring photoreceptors;
- bipolar cells, which convey information between photoreceptors and ganglion cells;
- amacrine cells, neurons that share information between neighboring bipolar cells;
- ganglion cells, the output cells of the retina.
The retina processes information “sideways”, sharing information with neighboring cells. This is called the lateral pathway and is mediated mostly by the horizontal and amacrine cells.
The retina also passes information straight through. This is a pathway formed by photoreceptors, bipolar cells, and ganglion cells.
The nuclei of photoreceptors are found in the outer nuclear layer (ONL). Their synaptic terminals pass information to horizontal cells and bipolar cell dendrites in the outer plexiform layer (OPL, also called the outer synaptic layer).
As the name suggests, horizontal cells pass information horizontally in the retina, telling neighboring photoreceptors what is going on nearby. They have few discernable synapses, and both receptors and neurotransmitter release mechanisms are not very visible but must exist in the OPL.
Bipolar cells gather together information coming from photoreceptors and horizontal cells. While all photoreceptors release glutamate, the receptors on bipolar cells fall into two categories.
Sign-conserving synapses would be called “excitatory” in a different context. The more glutamate that is released from photoreceptors, the more the synapses on these bipolar cells are depolarized. But remember that light decreases glutamate release, so these postsynaptic contacts hyperpolarize in the light. Because these postsynaptic elements hyperpolarize when photoreceptors hyperpolarize, the “sign” of the synapse is preserved and they are called “sign-conserving”.
When bipolar cells have sign-conserving synapses on their central dendrites, and sign-inverting synapses in the faraway parts of the dendritic tree, they are called “OFF-center bipolars” because they turn off when more photons are present.
Sign-inverting synapses would be called “inhibitory” in a different context. The less glutamate that is released from photoreceptors, the more these bipolar cells are depolarized. This situation happens when photons interact with the rod and cone outer segments. If a collection of sign-inverting synapses are found in the center (think near the “trunk”) of the bipolar cell’s dendritic tree, then this bipolar cell is called an “ON-center” bipolar because it depolarizes when light is present.
The receptive field is the complete set of points in visual space that cause activation of a particular bipolar cell. A bipolar cell which is close to the fovea has no knowledge of what is going on in the peripheral retina near the ciliary body, and vice versa. Lateral interactions are hyperlocal and used only to set up the individual bipolar’s receptive field. The receptive field of a bipolar cell is an exact overlay, then, of the bipolar cell’s dendritic tree.
Bipolar cells have “center-surround” receptive fields. This means that in the center of the tree, nearest the “trunk”, one response predominates while in the faraway branches of the tree the opposite type of synapse is found. That means that ON-center bipolar cells have an OFF surround and OFF-center bipolar cells have an ON surround.
This type of receptive field organization, which predominates in the retina, is called a center-surround receptive field.