Axo-Axo-Somatic Inhibition

Neurons communicate to eachother in the brain by chemical signals. They send out long ramifications called axons which synapse (connect) with other neurons (generally) on parts called dendritic trees. These connections are not physical in the sense that the cells do not share any inner-membrane space, but they do allow the communication of intercellular chemical signals with incredible speed. There is a small space called the synaptic cleft into which the signalling cell releases a chemical signal and at which the cell receiving the signal has receptors specific to that signal. Dendritic trees are places where signals from many other neurons are summed up, when a neuron recieves enough signals from those cells which synapse onto it, it fires an action potential at it's soma or body. An action potential is a large transient increase in the voltage of the cell (don't forget your brain is electric). That transient (called a spike) propagates down the axons of that cell much like an electrical signal does in a telephone wire or a television cable, to the synapses it forms with other cells and triggers the release of the chemical signals used to communicate. This is the story whether the signalling cell is telling it's target to turn on (excitation) or turn off (inhibition). One difference is that excitatory signals generally go to the dendritic tree of another neuron while inhibitory connections can go to the dendrites or the soma (body). This is useful because the inhibition acts like opening a voltage drain and preventing the action potential from building up which (as I mentioned) it generally does at the soma. The specifics of the signalling molecules and receptors determines whether inhibition or excitation is being transmitted, and in general cells send either one or the other kind of signal. So if an excitatory cell wants to inhibit another excitatory cell, it must excite an inhibitory cell which in turn inhibits the target excitatory cell. When might the brain want to do that? Well lateral inhibition is a ubiquitous concept in brain circuitry. This is the idea that if I have a bunch of neurons all designed to report something appearing in different parts the visual field, it makes sense for them all to mutually inhibit eachother. Because the areas of the visual field to which single neurons respond overlap a bit, a line might excite a sort of fuzzy set of neurons in the cortex, if the ones that are responding most strongly inhibit the ones that are only responding weakly I can detect cleaner edges and have better spatial precision in general.

A new paper in Science presents evidence that inhibitory synapses can have another form¹. In the image above you can see an illustration of each othese kinds of inhibitory synapses. Both the type I've already described (above) and the new kind (below). The difference is that in the new type, instead of having to send a signal to the inhibitory interneuron which in turn inhibits the target excitatory cell, the 1st excitatory can hijack the inhibitory synapse of the interneuron to rapidly and directly inhibit the target cell! Why is this so exciting? Well any time we can figure out something new about how that intricate mass of electrified flesh in our heads might accomplish some of the seemingly miraculous feats it does, I get excited. I take particular pleasure in understanding the mechanistic underpinnings of consciousness, and being a materialist (in the philosophical sense), I think that neuroscience is the way to do that. Beyond this, the specificity of this new mechanism is potentially greater than the earlier described variant. An inhibitory cell projects to many other excitatory cells so if an inhibitory cell gets turned on, it will turn off many other cells which may not be "what the 1st excitatory cell wants." The other reason I'm excited by this is that I see it as a way to modify the receptive field properties of a cell while during processing rather than through some longer term "learning" or modification in general. Below is an illustration of the receptive field structure of a simple cell in the primary visual cortex.

These cells are the first place that visual information is represented in the visual cortex, and all visual percepts are built up from them. Consequently, it doesn't make sense to change them over time too much. It's like a computer monitor, pixels are a good universal way to represent many different types of images. You wouldn't change the shape of pixels depending on the type of images you were going to see because we don't have technology that could do it quickly and efficiently enough. Similarly, one keeps the receptive fields of cells in the primary visual cortex constant and then changes how that information is used at later stages. However, if there was some way to easily change the shape of pixels back and forth depending on the stimulus being displayed, it would be very useful. That's one consequence of this, online modification of receptive field properties at a low level. We've got a lot to learn from the brain.

References

1. Ren, M., Yoshimura, Y., Takada, N., Horibe, S. & Komatsu, Y. (2007) Specialized Inhibitory Synaptic Actions Between Nearby Neocortical Pyramidal Neurons. Science 316, 758–761 (2007).