By Dr. Lise Johnson (CSNE Education Manager)
If your intuition tells you that the adjective “neural” can be roughly translated as “having something to do with the brain,” then your intuition has brought you very close to the truth (thanks, intuition). In fact, “neural” means “having something to do with neurons.” Neurons are a type of cell, and the majority of those cells live in the semi-solid, three-pound lump of tissue between your ears and behind your eyes - your brain. Your brain houses about 100 billion neurons, which is quite a few. But while most of your neurons are in your brain, “most” is not the same as “all.” The neurons outside of your brain are in the minority, but they perform some critical functions, especially in the context of Sensorimotor Neural Engineering. They are still a part of the nervous system, so, when we say “neural”, those guys are invited to the party too.
Since “neural” is a big part of what the CSNE is about, we should probably talk a little bit about neurons. As I said, neurons are cells, and they have all of the regular cell parts – a nucleus, organelles, a membrane, etc. Like most of the cells in the body, these cells have a specific job (they are what we call differentiated cells) and they have some specialized parts for doing that job.So, let’s talk about that special job, because it turns out that neurons do something really important: they talk to each other. Okay, they don’t literally “talk” to each other, they don’t have lips (or lungs for that matter). What I mean is that they communicate. They send and receive little packets of information using a sort of code, like Morse code but vastly more complicated. They do this all the time; they’re chatty. That may seem like little more than interesting trivia until you consider that the information they’re busily swapping back and forth is the currency of behavior. When I say behavior, I mean pretty much all behavior – internal and external, conscious and subconscious, voluntary and involuntary. Walking and reading and eating and remembering and being thirsty and feeling happy and seeing and waiting and sleeping and peeing and hoping and even intuition are all based on the persistent chatter of your 100 billion odd neurons. So, they’re important. That’s why injuries and diseases that destroy neurons are so serious. A spinal cord injury can cause paralysis, a stroke can make it impossible to speak, a traumatic brain injury can change your personality, and Alzheimer’s disease can take away your memories.
So, we know that neurons don’t have lips and lungs for talking, so how do they do all this communicating? That’s why they need those special parts. Now, there are many different kinds of neurons, and they’re all a little bit different so that they can do their particular jobs. What they all have in common though, is that they are electrically excitable and they form electrochemical synapses. If I lost you just there, stay with me, I’ll explain. First, let’s talk about what they look like. All neurons have a cell body and dendrites, and most of them have an axon as well. You can see to the right a drawing of what a “typical” neuron would look like (of course, you should keep in mind that there is no such thing as a “typical” neuron!). The dendrites make up the bushy part on top that looks like the top of a tree. In fact, “dendrite” is Greek for tree, so there you go. The cell body is the round part in the middle (in real neurons it is always more blobby than round) and the axon is the long skinny projection that comes out the other side of the cell body. Okay, this is how a neuron works: information comes in through the dendrites, gets processed in the cell body, and, having been changed by the journey, it leaves through the axon. The axon makes a connection with the dendrite of another neuron; that neuron receives the information, processes it, and sends it out through its own axon. This is how the message gets passed around: axon to dendrite, axon to dendrite, etc. A couple of important points here: 1) there are lots of dendrites, but just one axon.This means that each cell gets a lot of inputs from other cells, and it has to turn all of those inputs into just one output. 2) There is only one axon, but it can split off into branches. Each branch carries exactly the same message as all of the others, but they spread out and take that message to lots of different neurons, near and far.To recap: each neuron receives messages from lots of other neurons, and in return, it sends its own message to many other neurons. When neurons are connected this way, they are said to be part of a network. These networks are where the action is; individual cells don’t control behaviors, networks control behaviors.
When an electrical pulse comes into a neuron, it responds by making its own electrical pulse to send out to other neurons. That is what we mean when we say neurons are electrically excitable cells; they respond to electrical inputs. As you read this your neurons are busily sending electrical signals all over you body, like the wires in the walls sending electricity all over the house. Except that neurons are not like wires. If you want to connect two wires, you just touch them together, and the electricity flows between them. Neurons, on the other hand, are connected with synapses. Synapses are complicated, but here are the basics. First of all, synapses are not a physical connection; the neurons don’t actually touch each other. They get very, very close, but they don’t touch, and if they don’t touch, the electricity can’t flow between them. Instead, when the electrical message reaches a synapse at the end of an axon on the sending cell, it sends chemicals across the gap between the cells. When the chemicals reach the cell on the other side of the synapse, that cell recognizes that a signal has been sent, and generates an electrical signal in response. It’s sort of like if the cell body were to make a phone call to the synapse; when the synapse gets the call it puts the message into a box and mails it the cell on the other side. When that cell gets the package in the mail, it calls the message in to its own cell body. Electrical, chemical, electrical. That’s what we mean by electrochemical synapse.
So these are the two things that allow neurons to do their exceedingly important jobs: they can represent information with electrical signals, and they can pass those signals back and forth at synapses.Of course, there is a lot more to know about neurons, but this is a good place to start, and for now, it’s a good place to stop. Next up, we’ll be talking about the nervous system and where we can find all those neurons that aren’t in your brain.