Neuroscience Basics III: Action Potential
I said in Part II that whether a neuron fires depends on how the summed positive and negative ion flux adds up during periods of stimulus. So, say some positive sodium ions start flowing in and depolarizing the cell, and that they outnumber the negative chloride ions; what happens next is pretty clever. Remember that I said in Part I that some (but not all) of the ion channels in the cell were voltage-senstive (or "voltage-gated"); now two of these become important.
First, there are sensitively voltage-gated sodium channels that start to open up when the cell depolarizes, allowing more sodium ions in and depolarizing it further in a wave of positive feedback that starts in the dendrites and then spreads along the cell body toward the axon. (When chloride ions are predominant, they act to stop this process at its starting point by overpowering the positive charge and keeping the downstream sodium gates from opening.) Once the charge in a region of the cell dips past a certain point, these gates close again and the sodium stops flowing in.
As the influx of sodium spreads through the cell, it triggers the opening of a special set of potassium channels that are normally closed, but open up when the cell depolarizes past a certain threshold, allowing positive potassium ions to flow out of the neuron and causing the cell's voltage to start going negative again. As I mentioned in Part II, the action of these potassium gates is slower those for sodium, so you end up with a second wave of net negative charge lagging behind the initial positive one.
Once the positive wave hits the axon, the real action begins. Most axons in mammals are coated with a sheath of myelin (basically a special extra-thick layer of phospholipids), which acts as an insulator preventing any ion transfer across the cell membrane and makes the axon the most highly conductive region of the neuron. If it isn't obvious why, you can think of an unmeylinated axon as very leaky pipe: when a pipe is full of holes, it takes a lot of force to push water all the way through it because the water keeps diffusing outward. Myelin effectively "plugs the holes", which means it requires less force to push "water" (charge) through the "pipe" (axon) at a faster rate. So myelin speeds up transmission along the length of the axon by easing propagation of electrical charge.
However, there are still only so many ions, so in order to keep the charge from dimnishing as it travels, fresh influxes of sodium are necessary. For this purpose, there are periodic breaks in the myelin sheath about a micron wide, known as nodes of Ranvier (how these guys got a piece of neuroanatamy named after them, I'll never know). These nodes are each loaded with a whole bunch of hair-trigger sodium channels that will open up as soon as the voltage goes slightly more positive, and their combined effect is to make the current move along the axon in a series of fast hops, basically acting as voltage repeaters.
The potassium channels continue to play catch-up, bringing the charge back to normal negative polarity in the wake of the positive wave. Eventually the ion pumps in the cell will clean everything up and bring all the ion levels back to baseline, but right now the cell is too busy just trying to normalize its voltage. This entire process is called an "action potential", or simply a "spike" because of the spike that appears on a voltmeter monitoring the neuron during an action potential. One feature of an action ptential that will become important later is that "a spike is a spike is a spike"-the amplitude of the wave is always the same for every single action potential. This is the only part of the neuron that can be considered "digital": either it fires or it doesn't, with no grey area. (What can change is the number and frequency of spikes, but that's for later posts.)
Once the wave of positive charge reaches the endpoints of the axons ("axon terminals"), sodium's job is done and voltage-gated calcium channels are waiting to pick up the ball. When the voltage at the axon terminal goes positive, there's a sudden flood of calcium into the axon terminal, which triggers . . . well, we'll get into that later.