The brain is made up of two types of cells. Glial cells act as support cells for neurons, the cells responsible for sending signals in the brain. Action potentials — electrical signals that travel across neurons — are the means for receiving, analyzing, and conveying information in the brain. Action potentials have an amplitude of about 100 millivolt (mV) and last for around 1 millisecond (ms).

A neuron is made up of four distinct parts. The cell body contains the nucleus and other cell structures. Dendrites branch out from the cell body like the branches on a tree, and receive information from other neurons. The axon is a long extension on one side of the cell body, similar to the trunk of the tree, and it ends in the presynaptic terminals.

A neuron is polarized, meaning the electrical charge inside the neuron is different from the charge outside the cell. Dendrites receive signals from other neurons which can change the charge inside the cell. A neuron at rest is more negatively charged than the surrounding area. Excitatory postsynaptic potentials bring the charge closer to zero, and inhibitory postsynaptic potentials make the charge even more negative.

At the axon hillock, all of these potentials are averaged. There are two ways these charges are averaged: across time and across space. The further away from the axon hillock a potential is, the less effect it has. The longer a potential lasts, the more effect it has on the axon hillock.

If the averaged charge from the postsynaptic potentials reaches a certain threshold, an action potential is generated. Postsynaptic potentials can be different sizes depending on the signals received by the dendrites, but the action potential operates on an all-or-none principle, meaning there is no gradient. There either is an action potential or there is no action potential.

The action potential is an electrical signal that travels down the neuron’s axon. The axon is coated in a myelin sheath, which, similar to the insulation on an electrical wire, allows the action potential to travel faster. The action potential carries the electrical signal to the presynaptic terminals, which then communicate to another neuron.

Between two neurons there is a gap called a synapse. When the presynaptic terminal on a neuron receives a signal from an action potential, it sends chemicals called neurotransmitters into the synapse. These chemicals are absorbed by another neuron. Neurotransmitters are the mechanism for sending signals between neurons.