Neuro thinkery 2.0

I've now completed the aforementioned med school class and started thinking about research again. While the med school course focused on problems like "A patient walks in with X symptoms, what part of their brain is acting up?" my research is more focused on a specific portion of the brain and what it does normally. My hope is that studying this portion of the brain will bring out some general principles that apply more generally, but we'll see about that.

Rather than dive into explaining what I'm studying just yet, I thought I'd give a very brief introduction to the overall idea of how brains work we (people who study this stuff) think brains work. And before I get into that, I need to point out that I am going to be glossing over how we know most of this stuff. That is actually something that I want to go back and cover at some point because it's really cool, but for now I will stick to what we know rather than how we know it. Because of this I will also stick to subjects where there is a very high level of certainty that we're right because often the details of how we know something also helps clarify the limitations of that knowledge. Lastly, I'd just like to mention that I'm going to be sticking to the low level of how neurons work and interact rather than big scale anatomy like I talked in the previous post.

Ok, enough of the preamble. So as most of you probably know, the main interesting cells in the brain are called neurons. They stand out compared to the rest of the cells in the human body because of their very long projections (scientists like to call them "processes") that connect one part of them to some distant part of the body. For example, if you can feel the tip of your big toe, that is because there is a cell (actually many cells) that extend from the tip of your toe, up to your spine, into your spinal cord, and all the way up to your brainstem. So if you're six feet tall there is cell that is at least five foot six running almost the entire length of your body telling your brain if there is pressure, heat, vibration, or whatever on your big toe. But how does it do that?

Ooooh, pretty neurons! Fun fact: the sensory neurons I'm talking about all look like the one on the bottom right here

First, one of the sensors on your big toe has to be activated by a stimulus. As an example, let's say you bump your foot into something lightly enough that you aren't in pain. After all, we don't want you to be in pain during this example. Then there are tips of certain neurons that are designed to detect the pressure from the bump, and others that will detect the vibration in your skin. The tips of these neurons are excited by the stimulus. Different types of sensors are excited in different ways, and to be perfectly honest it isn't too important how those different sensors work other than to say that the toe-bumping excites the tips of some neurons.

What does it mean for the tip of the neuron to be excited? It means that some super-tiny specialized holes in the cell open up. First, some of these holes allow sodium (or calcium, but I'll stick to sodium for simplicity) into the cell. Sodium, when it is floating around in water/cells/your body, has an electric charge. So after some sodium enters the cell, there is a change in the electric field across the edge of the cell. Now if there is a small amount of sodium, then it might leave the cell, or it could spread out enough that there isn't much of an effect. If there's a bunch coming into the cell all at once, then the electric field will actually open up more sodium-holes and let more into the cell, creating an even bigger electric field, which leads to a loop of more sodium, more electric field, and more holes opening up. When the electric field gets high enough, a second part of the sodium holes closes up and stays shut until the cell has reset, and to help reset the high electric field opens up some potassium holes, that do the opposite of what the sodium did. Just to throw in a few vocab words, the edge of the cell is referred to as the cell membrane; the holes in the cell membrane are called sodium/potassium/calcium channels depending on what they let through; the electric field across the membrane is called a membrane potential or membrane voltage; and the spike in membrane potential from the sodium is called an action potential. One final note just for accuracy: though calcium can help start raising the electric field, it is always sodium channels that open up when the membrane potential gets high enough, causing an action potential.

Now the process I just referred to begins at the tip of a neuron and spreads down it. In the case of your big toe I guess it's actually spreading up the neuron until it gets to your brainstem. When it gets there, the neuron releases a chemical called a neurotransmitter into a bunch of tiny gaps connecting to different neurons in the brain. I'll try to touch on how those neurons react in the next post. At some point, though, it's important to remember that we scientists really lose track of the effects of individual neurons firing in the sea of thought action and memory that is the human brain. For now, it just helps to know that the sensory neurons act a bit like electric cables, sending little pulses of information when they are stimulated.