Let's first start with a depiction of the neuron membrane (Figure 1). I recall a great Star Trek alien that refered to humans as "Great ugly bags of mostly water." It's true, and neurons are no different. Each neuron has a thin membrane of lipids (fats) and proteins that enclose a salt solution. The chemical composition of the salt solutions are different inside and outside the neuron as shown at right. The plus (+) and minus (-) signs indicate the "charge" of the ions, and it does not quite even out, allowing the inside of the neuron to have a slight negative charge. Note the detail in the left of the Figure 1 that says – these ions do *not* naturally cross the neuron membrane. They have to have channels.
The critical component of the solution for our discussion right now is sodium, Na+. Yes, too much sodium in the diet is a bad thing, but sodium is an essential chemical in the body. You will notice the concentrations are listed as "mM". That's "millimolar." One "molar" concentration is 35 grams of sodium per liter of water, or 4.72 ounces per gallon. "Milli" means 1/1000, therefore, 145 millimolar sodium is 0.145 x 4.72 = 0.56 ounces per gallon of water. You could *barely* taste that concentration of salt in water, so it really isn't much, and most people consume enough salt in the foods they eat and water they drink without the need to add salt for *nutritional* purposes. For flavor is another story.
So, sodium has a higher concentration outside the neuron than in, and it has an electrical charge. We can calculate the resulting electrical "potential" voltage simply by asking how much electrical energy would be required to prevent the positive charged sodium from entering the neuron and *balance* the concentrations at the same level as if no diffusion were possible (see Figure 2 at left).
However, the most important part is that (A) sodium has to flow through a channel to get into the neuron, (B) that channel is normally closed. Thus when we *do* open the channel, we can get 66 millivolts worth of electrical energy out of this neuron (ionic current) by allowing the sodium ions to diffuse until they reach equal concentrations on both sides of the neuron membrane.
That's the complicated part – chemically. Now for the electrical part.
The whole process takes about 2 to 5 milliseconds, and the neuron is ready to "fire" again. Action potentials travel at about 10 meters per second. That's maybe 25 miles per hour, although the *really* long neurons are specially insulated to speed that up 10 times or faster. Yes, signals from the brain to hands and feet travel at over 250 mph! And they do it over and over and over again – as much as 100 times per second, every minute, every hour, every day.
That's an amazing system, and it is all biologically based. The *information* content comes from changing the timing, the frequency, and the specific connections between neurons in a complex pattern not unlike the way a laser show makes complex patterns out of a single beam of light that simply turns off and on very fast.
Sound familiar? Kind of like a computer making information out of just ones and zeros? Well, perhaps, but that's a matter for the next blog.