What Exactly Are All Those Electrical Impulses In Your Brain?

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This is the second in a series of articles explaining – using plain English – what exactly are the electrical impulses in your brain responsible for how it learns, represents, and processes information. In the first article we introduced the anatomy of a neuron brain cell, some important high level concepts about how neurons talk to each other, and set the stage for this second piece – understanding the biophysical basis of the membrane potential and why it is important to the brain.

First thing first. Before putting in the work to understand what the membrane potential is, we need to ask why is it so important. In the most straight forward sense, it is critical because the electrical impulses that travel down the length of the neuron responsible for encoding information are physically represented, in other words, are encoded, as fluctuations in the membrane potential. (Make sure you read that first article if this does not make sense.) So it would not be possible to understand what the electrical impulses in your brain are, or how they encode information, without a solid understanding of what the membrane potential is first.

The neurons in your brain have a resting membrane electric potential of roughly -70 millivolts (mV). This means that the membrane of these cells is ‘electrically charged’. There is a differential distribution of electric charges on the outside of the membrane compared to the inside of the membrane. Specifically, there is differential distribution of sodium (Na+) and potassium (K+) ions across the cell membrane. These ions are the physical carriers of electric charge. Sodium and potassium each have an electric charge of +1, meaning they each carry one positive charge. (In technical terms, they are monovalent cations. ‘Mono’ meaning ‘one’ and ‘cation’ implying that their charge is positive.)

Here are two straightforward examples to illustrate the above concept. If one side (the outside) of the cell membrane had 10 Na+ ions and the other side (the inside) of the membrane had 10 K+ ions, the number of ‘like’ positive electric charges on both sides of the membrane would be the same, 10 on either side. So the next difference between the charges on either side of the membrane would be zero, which means that there would be no electric charge difference – or electric potential – across the membrane.

But if the outside of the membrane had 10 Na+ ions and the inside of the membrane had 3 K+ ions, well, now there are 7 extra positive charges on the outside, which is the same as saying there are an extra 7 negative charges on the inside. Note how the negative charges in this case really just reflect the absence or missing positive charges, given there are fewer K+ ions compared to the number of Na+ ions! Now there exists an electrical imbalance (as a count of the electric charges themselves) between the inside and outside of the membrane. It is precisely this imbalance that is what the membrane electric potential is. Pretty simple actually.

That a typical neuron has a resting membrane potential just means that when the neuron is sitting quietly, i.e. not receiving signals from other neurons and not generating any electrical impulses, its membrane potential during this resting state is not zero, but rather about -70 millivolts. In other words, at rest when the neuron is doing nothing there is an inherent imbalance in the electric charges across the membrane that produces an electric potential. In fact, Na+ is at a much higher concentration outside the membrane than inside. (Meaning there are more Na+ ions outside than inside the membrane.) While for K+ it is reversed: The concentration of K+ ions is greater inside the membrane than outside. But in total, the number of ions – and the charges they carry – are not equal across the cell membrane. There is an imbalance of charges that is responsible for producing the membrane potential. It is precisely that imbalance that we call the membrane potential.

There is one last subtle but crucial consideration we need to take into account. The imbalance of charges, represented by a differential unequal physical distribution of Na+ and K+ ions, only applies to a very very thin ‘shell’ right on either side of the membrane. In other words, the imbalance that produces the membrane potential is restricted to the distribution of ions very very close to the membrane itself (just nanometers actually). Once you move even a tiny distance away from the cell membrane, on either side of it, that differential distribution of ions disappears and on average there is no difference in the number of Na+ and K+ ions between the inside of the neuron and the bulk outside ‘fluid’ the neuron lives in. In other words, there is no electric potential once you move away from the cell membrane itself. This fact will be important later in a subsequent article.

With the stage now set, the next article in this series will explore what happens when the flood gates open: What happens when the ions channels in the cell membrane open and Na+ and K+ ions are allowed to freely cross the membrane. Because it is the movement of these ions across the membrane that are the electrical impulses we are attempting to understand.

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