How do different retinal ganglion cells send distinct signals to the brain?

[Wrichik Basu]

I was thinking how nerve impulses may differ from each other, if at all they do differ. I searched on Google, but the answers didn't provide a satisfactory answer to my question.

Take, for example, the human eye. The cone cells are responsible for vision on bright light, and they have the photosensitive iodopsin pigment.

Now, cone cells are mainly of three types. As Wikipedia puts it: "Humans normally have three types of cones. The first responds the most to light of long wavelengths, peaking at about 560 nm ; this type is sometimes designated L for long. The second type responds the most to light of medium-wavelength, peaking at 530 nm, and is abbreviated M for medium. The third type responds the most to short-wavelength light, peaking at 420 nm, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively, depending on the individual."

It is but certain that the nerve impulses that these three types of cone cells send to the brain, differ in some form or the other, otherwise the brain would not be able to distinguish between which impulse comes from which type of cell, and corresponds to which wavelength of light.

However, each type of cell itself must be able to send different types of impulses, because we can (more or less) distinguish subtle difference in colours within our range of vision. In order to make the brain distinguish between different wavelengths, the impulses should vary from each other.

If a single nerve wants to carry different informations, then there must be some difference in the impulses they carry.

Can you explain how nerve impulses can differ?

 

[TeethWhitener]

If a single nerve wants to carry different informations, then there must be some difference in the impulses they carry.

It's significantly more complicated than that. I'll try to explain it at a simple imprecise level and a more precise, less simple level. Action potentials (what you're referring to as "nerve impulses") are all-or-nothing and are generally triggered by a sufficiently strong electrical stimulus. More precisely, the interiors of nerve cells are polarized to -70mV versus the extracellular matrix through active pumping of ions against a concentration gradient. When a threshold depolarizing stimulus is reached, the nerve "fires:" a series of channels in the cell opens which allows ions to freely flow into and out of the cell along the gradient. This signal propagates along the length of the neuron. (Brief but important aside: neurons are unidirectional. They receive signals from the dendrite side and propagate them toward the axon side.)

This is where it gets more complicated. When the signal gets to the end of the neuron (the synapse at the end of an axon), it usually causes the release of a neurotransmitter--a chemical compound that binds to receptors on (the dendrites of) an adjacent neuron. There are many kinds of neurotransmitters. Some of them cause the next neuron to depolarize, leading to a new action potential and propagating the signal onward. These are called excitatory neurotransmitters. Others cause the next neuron to resist depolarization (or to hyperpolarize), thus decreasing the chance that the next neuron will fire. These are called inhibitory neurotransmitters.

Further complicating matters is the fact that one neuron will be stimulated by many neurons (sometimes in the thousands), all carrying their own signal. The firing behavior of this neuron is a complicated function of all of the signals it receives from the neurons that terminate at it, both spatially and temporally. This process is called summation:
https://en.wikipedia.org/wiki/Summation_(neurophysiology)

This process of summation is far more important to the brain's signal processing than individual action potential. Individual neurons are very simple: they either fire or they don't. The summation process, however, allows them to process information in a vast number of ways as a function of the behavior of their neighbors.

 

[Wrichik Basu]

@TeethWhitener though the first two paragraphs was not new to me, the summation principle is quite interesting. Thanks for the help.

 

[BillTre]

Of all the neurons in the retina, only the retinal ganglion cells project their axons to other areas of the brain.
The retina's many other neurons do preliminary information processing before any information leaves the eye for the brain.
These cells provide particular retinal ganglion cells with specific kinds of information about the contents of the visual field, like color, or contrast, or lines.

The different kinds of retinal ganglion cells than send their signals (about particular kinds of information) from the eye to the brain.
By this time, distinct kinds of retinal ganglion cells comprise different (but parallel) pathways for different kinds of information to get to various regions of the brain.

The signaling used by the different kinds of retinal ganglion cells does not have to be differ based on the information it is carrying because they are already anatomically distinct (at a microscopic level) and make distinct connections at both the incoming end (dendrites, for acquiring the information) and outgoing end (axon/nerve terminal-where they deliver the signal).
The anatomical specificity of the connections frees the cell's physiology of having to convey that information.