Question: How does the brain generate electrical signals?

Nina Rzechorzek answered on 12 May 2020:

Hi thelocalnerd26, excellent question –
Neurons (the electrically-active cells in the central nervous system = brain and spinal cord) communicate via both electrical and chemical signals. The electrical signals are action potentials; the chemical signals are neurotransmitters. This is an overview of the electrical side of things:

The Resting Potential
The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane and regulate the relative concentrations of different ions inside and outside the cell. Cells can use energy to preferentially move certain ions either inside or outside of the membrane, setting up a difference in ion charge across the membrane, where one side is relatively more negative and the other side is relatively more positive. The difference in total charge between the inside and outside of the cell is called the membrane potential.

The membrane potential of a neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (-70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. The resting potential is established and maintained by two main processes: an ATP-powered ion channel called the sodium-potassium pump, and a passive ion channel called the potassium leak channel.

The sodium-potassium pump, which is also called Na+/K+ ATPase, transports sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in neurons, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of neurons, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used. This process is so important for neurons that it accounts for the majority of their ATP usage.

In addition to the sodium potassium pump, neurons possess potassium leak channels and sodium leak channels that allow the two cations to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. Thus the combined effects of the sodium-potassium pump and the potassium leak channels is that the interior of the cell is more negative than the outside of the cell. It should also be noted that chloride ions (Cl–) tend to accumulate outside of the cell because they are repelled by negatively-charged proteins within the cytoplasm.

The Action Potential
When we talk about neurons “firing” or being “active,” we’re talking about the action potential: a brief, positive change in the membrane potential along a neuron’s axon. When an action potential occurs, the neuron sends the signal to the next neuron in the communication chain, and, if an action potential also occurs in the next neuron, then the signal will continue being transmitted. What causes an action potential? When a neuron receives a signal from another neuron (in the form of neurotransmitters, for most neurons), the signal causes a change in the membrane potential on the receiving neuron. The signal causes opening or closing of voltage-gated ion channels, channels that open or close in response to changes in the membrane voltage. The opening of voltage-gated ion channels causes the membrane to undergo either a hyperpolarization, where the membrane potential increases in magnitude (becomes more negative) or a depolarization, where the membrane potential decreases in magnitude (becomes more positive). Whether the membrane undergoes a hyperpolarization or a depolarization depends on the type of voltage-gated ion channel that opened.

Not all depolarizations result in an action potential. The signal must cause a depolarization that is large enough in magnitude to overcome the threshold potential, or the specific voltage that the membrane must reach for an an action potential to occur. The threshold potential is usually about -55 mV, compared to the resting potential of about -70 mV. If the threshold potential is reached, then an action potential is initiated at the axon hillock in the following stages:

Depolarization: voltage-gated sodium channels open quickly after depolarization past the threshold potential. As sodium enters the axon (influx), the inside becomes relatively electrically positive (approximately +30 mV, compared to the initial resting potential of approximately -70 mV).
Repolarization: shortly after the initial depolarization, the voltage-gated sodium channels close and remain closed (and cannot be opened) for about 1-2 milliseconds. Voltage-gated potassium channels then open, allowing potassium to exit the axon (efflux), causing the membrane to repolarize (become more negative).
Hyperpolarizaton: potassium continues leaving the axon to the point that the membrane potential dips below the normal resting potential. Sodium channels return to their resting state, meaning they are ready to open again if the membrane potential again exceeds the threshold potential.
Reset resting potential: The sodium-potassium pump and potassium leak channels reset the locations of sodium and potassium ions, reestablishing the membrane potential to allow another action potential to fire

There are a few important universal features of action potentials:

The action potential travels down the axon, proceeding as a wave of depolarization.
Action potentials always proceed in one direction only, from the cell body (soma) to the synapse(s) at the end of the axon. Action potentials never go backward, due to the refractory period of the voltage-gated ion channels, where the channels cannot re-open for a period of 1-2 milliseconds after they have closed. The refractory period forces the action potential to travel only in one direction.
Action potentials do not vary in magnitude or speed; they are “all-or-nothing.” When a given neuron fires, the action potential always depolarizes to the same magnitude and always travels at the same speed along the axon. There is no such thing as a bigger or faster action potential. The parameter that can vary is the frequency of action potentials, or how many action potentials occur in a given amount of time.

Much of this was summarized from the following website which has some great schematics and links to very digestible videos to go through how this works (be warned that there are some spelling and grammatical errors in places:

Neurons