Saturday, April 07, 2007

Neuroscience Basics I: Electricity and Equilibrium   posted by Matt McIntosh @ 4/07/2007 11:40:00 AM

One of the awkward things about GNXP is that we've got an audience that's very mixed in terms of knowledge, so it's hard to know just how much background information you should assume when writing posts. In the archives we've got "basic concepts" posts on population genetics and psychometrics that hopefully help lay-readers follow the more technical posts on these subjects, but so far we don't have too much "neuroscience 101" stuff. With that in mind, this is going to be the first in a series of posts that try to give the uninitated reader an adequate background to follow the more advanced posts by Amnestic.

Most people are familiar with terms like "neuron", "synapse", and "neurotransmitter" and have a vague notion that the brain operates with electrical impulses and chemical messenger molecules like serotonin and dopamine, but don't have a clear idea of how these things all fit together—much the same way that I know that there are things called an "alternator" and "transmission" under the hood of a car but don't really know how these things work together to make the car go. But the basic idea of how neurons work is pretty easy to understand.

Neurons are fundamentally devices for transmitting and storing small amounts of information in an analog format. Most of you have probably seen diagrams of what neurons look like: a fat cell body called the "soma" with a lot of branches radiating from it called "dendrites", and a big fat tube extending from it called the "axon" which also has a bunch of braches sprouting from it. The dendrites are the input sites, the axon is the output, and synapses are the sites where the endpoints of dendrites and axons meet. A synapse is where the actual communication between two neurons occurs, but before we get to that it'd be better to understand what decides whether they even communicate at all.

Neurons, like all animal cells, have a membrane "skin" composed of two layers of phospholipids (fatty acids attached to a phosphate group). This is what keeps the cell's insides inside and everything else outside, and it works in a rather ingenious way: the phospholipids are polar molecules which at one end are attracted to fats and at the other end attracted to water. The reason why oil and water don't mix is the same reason your cells stay cohesive: the lipid molecules cling together and form a collective barrier that most molecules won't pass through. In the case of a cell membrane, instead of just forming round globules they form a uniform wall.

Of course this membrane isn't totally impermeable, since a cell needs to allow certain chemicals to enter and exit, just as complex living organisms do. There are a variety of different doors embedded within the wall—specialized channels that permit this or that kind of atom or molecule to bypass the membrane based on their electrical charges and/or shape, like bouncers at a nightclub. This selective permeability is central to cell function in general, and in neurons the most important purpose it serves is to control the difference in electric charge between the inside and outside of the cell via different concentrations of ions.

If you recall your chemistry classes, ions are atoms that have had one or more electrons either removed or added to their outermost electron shell, giving them either a positive or negative charge. Ions are the basis of bioelectricity—the bridge between biochemistry and electricity. When people talk about electrical impulses in your nervous system, they're talking about changes in the proportions of various ions.

In accordance with the second law of thermodynamics, ions (like everything else) tend to spread out evenly unless impeded. That's where the cell membrane comes in: by selectively channeling ions in and out of the cell while maintaining a barrier, it can control the difference in concentrations of ions inside and outside the cell. Because of the electric charge of ions, this creates a difference in charge, AKA "potential [electrical] difference"—in a word, voltage. (Voltage is defined as the difference in electrical charge between two points with a resistive barrier between them.) So cells in general and neurons in particular are always operating out of equilibrium with their environments, chemically and electrically.

The neuron has a negative feedback system that works to maintain a voltage "set point", just as your thermostat works to keep your house's atmosphere at a temperature set point. Just what that set point is will vary from neuron to neuron for reasons I'll explain in later posts; for now all you need to know is what role that system plays in neural activity—arguably, this feedback system is the foundation of all neural function.

Many of the ion channels embedded within the cell membrane are voltage-senstive and will alter their shape if the voltage gets "too high" or "too low", thereby closing or opening those channels to the free flow of particular kinds of ions (there are different channels for potassium ions, calcium ions, etc). You can probably see how this works now: when the voltage drops or rises beyond a certain point, some of the gates will open and permit ions to go the way they want to—toward equilibrium, which could be into or out of the cell depending on the charge of the ions and the voltage across the membrane. This will tend to bring the neuron back to its voltage set point.

Ohm's law governs the relationship between current, voltage and resistance in a conductive medium: voltage equals current multiplied by resistance. In the cell, the impermeability of the membrane corresponds to resistance and ion flow corresponds to current. Holding voltage constant (the set point), changing the resistance (permeability) necessarily means a corresponding change in the current (ion flow). I highlight this because this simple relationship is a helpful way to summarise the whole process I've just described, and is easier to remember than the details about the biochemical specifics.

So now that we've got the basic mechanism down, next up I'll discuss the exogenous causes of changes in voltage, which is where dendrites and axons come in.