Written by Travis M. Moore
Last edited 5-Jun-2020
Just like a circuit board, your brain controls your body through electrical signals traveling through "wires." Below we'll discuss how this happens, known commonly as basic neurophysiology. If we break that word down, neuro- refers to the nerves or nervous system, and physiology is the study of how biological systems work. Not as intimidating as you might have first thought, right? Instead of electricity flowing through wires, we'll discuss electrical potentials traveling along axons. Figure 1 shows electrical impulses traveling down an axon just like a wire.
When talking about biological systems, like the human body, we're no longer focusing on neutral atoms, like in the previous page about electricity for electronics applications. Instead, we'll be looking at atoms with a negative or positive charge. An atom with a negative charge would have more electrons than protons. On the other hand, an atom with a positive charge would have more protons than electrons. Charged atoms are referred to as ions. The two ions we'll focus on here are sodium (Na+) and potassium (K+). The important thing to note for biological signals is that electricity is no longer movement of electrons from atom to atom, but the flow of ions moving from one location to another. However, the principle is the same: just like in electronics, charges are flowing whether electrons are moving or charged ions are moving. Take a look at Figure 2 to see what it looks like when ions flow across the membrane of a neuron.
There's a lot to talk about in Figure 2. First, look at the left side labeled "Depolarizaton." We see Na+ separated from K+ by a membrane. This situation is the biological equivalent of a battery. That is, one side of the membrane is positive, and the other side is relatively negative. What you're seeing is called a gradient. For our purposes, you can think of a gradient as a state where ions with different charges are separated from each other. A gradient implies a difference - more of one charge on one side than the other. You need to understand that nature hates gradients. Physics always wants things to be equal; after all, keeping charges apart requires energy. If there were permanent holes in the membrane in Figure 2, what do you think would happen? The Na+ and the K+ would move so that they were spread evenly along both sides of the membrane. Figure 3 demonstrates the principle of diffusion, or the spreading out of particles until they are evenly distributed.
Let's look at an actual battery for comparison. Figure 4 shows how a household battery works by using a gradient. The negative end (bottom) of the battery is filled with electrons, which are separated from protons at the positive end (top). That's a gradient. It's a scenario similar to Figure 2. When you connect the negative and positive terminals as shown in Figure 4, the electrons move to balance out the positive charge at the top of the battery. Connecting the two battery terminals is like opening channels in the membrane in a neuron - open channels allow ions to move in a way that balances the charge inside the neuron with the charge outside the neuron. Believe it or not, this is essentially what happens when a neuron "fires." The gradient of ions moves to balance itself when membrane channels open, and that movement is electrical current. This "firing" activity is called depolarization, and occurs when sodium channels open in the neuron's membrane allowing Na+ ions to flow into the cell.
A major difference between a battery and a neuron is how long the flow of electricity lasts. A battery releases electrons in a slow, measured fashion. A neuron, on the other hand, opens many channels and lets ions rush into the cell all at once. Then the channels close and energy is required to "reset" the ions back to their original positions. Remember it takes energy to separate charges (like the energy you have to exert to pull two magnets apart), and it takes energy to make a gradient (i.e., to put ions out of balance, where they do not want to go). The resetting phase in the neuron is like the "replacing" phase in Figure 4. The battery is being recharged, which requires energy to move the electrons back to the negative terminal. We see this same process happening in the right side of Figure 2, labelled repolarization. This process uses energy to power ion pumps in the membrane that move Na+ back outside of the cell, thus "recharging" the neuron so it can fire (depolarize) again.