By Dr. Lise Johnson (CSNE Education Manager)
In the last post I talked a little bit about what neurons are, in this post I want to talk about where neurons live. They are not uniformly distributed throughout your body, they are highly organized into a system - the nervous system. On the highest level of organization the nervous system is split into two parts, central and peripheral. The central nervous system, or CNS, includes your brain and your spinal cord. Sometimes the retinas are also included, but for now we’ll leave them out. We call it the “central” nervous system not because it is in the center of your body (your brain is in your head, after all) but because it functions like a central command station.
Your CNS coordinates your behavior; it integrates all of the available information about the world and it decides which, if any, actions you will take. If you consider how much information your brain has to deal with, and how many different actions you are capable of generating, you can start to appreciate what a complex task this is. That’s why your brain needs so many neurons; it’s doing a lot of heavy lifting. What the CNS does not do, however, is interact with the outside world. Your brain cannot know anything about the world or change anything about the world without the help of the peripheral nervous system (PNS). If you think about the CNS like a military command center, then the components of the PNS are like the troops at the front line. The PNS includes the sensors that allow you to feel and hear and smell and taste and see as well as to know where your limbs are with respect to your trunk and whether you’re standing upright or lying on your back or hanging upside down. It also includes the motor neurons that tell your muscles when and how to contract to achieve smooth, purposeful and effective movements.
By numbers, the PNS is much smaller than the central nervous system, but you can see why it is so important. It is critical that the central and peripheral nervous systems maintain communication. The PNS is clueless without the CNS, and the CNS is powerless without the PNS. If you were to sever the nerves that run from your spinal cord to your arm, for example, your arm would become insensate (meaning you would no longer feel anything with it) and paralyzed (meaning you wouldn’t be able to move it). Although it would still be there, alive and healthy, it would be for decoration only, completely non-functional. This is essentially what happens in a spinal cord injury -- the CNS and the PNS lose their connection. The nervous system on both sides of the injury might be completely healthy, but no messages get through.
The CNS is further organized into many different subsystems with each subsystem performing specialized functions related to specific aspects of behavior. These subsystems are anatomically as well as functionally distinct. You can tell them apart based on the organization of cells, the type of cells and the connections that the cells make with different parts of the nervous system. The set of component subsystems falls into a sort of hierarchy, or ladder. As you get closer to the top of the hierarchy the associated behaviors become more and more sophisticated. The spinal cord is at the base of the ladder; it is primarily a conduit for the rest of the brain to access the body, but it is also responsible for very primitive behaviors such as reflexes. At the top of the ladder is the frontal cortex, it does things like moral evaluations and contemplating the blueness of blue. The same ladder can be used to evaluate the age of brain areas. As you go up the hierarchy, the evolutionary age of the brain decreases so that the parts at the top are “younger” than the parts at the bottom. By this I mean that the brain stem (which allows you to do some very important things, like breathing) evolved before the hippocampus (a brain area involved in memory, among other things) which evolved before the visual cortex (which is, not surprisingly, involved in visual perception). The neocortex is the newest part of the brain, which is why we call it the “neo” cortex. It is the wrinkly part that covers the outside of the brain, and it does a lot of very complicated things. But here’s the problem, while the brain is sort of a hierarchy, it’s also sort of a funnel. All of the really deep thoughts and sophisticated behaviors that happen in the neocortex have to go through the brainstem and the spinal cord in order to be expressed. As a result, if you have a problem with your brainstem or your spinal cord, you have a problem with everything. In fact, people who have damage to their brainstem may completely lose the ability to move, not even to speak or to breathe. These people are what we call “locked-in,” they are prisoners in their own bodies. In general, the brain is much more resilient to damage higher on the hierarchy than it is to damage lower on the hierarchy.
Obviously there is a lot more to know about the nervous system, but I’m only going to hit one more point here: glia. So far, the only cells I have mentioned are neurons. That’s because neurons are really important. However, there is another class of cell in the nervous system, and that is (you guessed it), the glia, or neuroglia. “Glia” is Greek for glue, and it’s an apt description of their function. Glia are the support staff for the neurons, they protect them, feed them, provide them with electrical insulation and keep them in the right places. Generally, they just keep everything from falling apart. For a long time we thought that was pretty much all they did. The neurons are like important political figures, the glia are the junior staffers that do the grunt work.We need them, but we don’t have a lot of respect for them. Recently, however, the glia have emerged as the dark horses of neural computation. It turns out that, some of them at least, have a lot of power to determine when neurons are activated and when they are not. Instead of junior staffers, they are more like back-room political bosses. The neurons make the speeches, but the glia tell them what to say.
Hopefully this gives us a place to start when we talk about the “neural” part of sensorimotor neural engineering. Up next: is sensorimotor a real word, and if so, what does it mean?
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