Everything the nervous system does, from pulling a hand away from a hot pan to reading this sentence, depends on cells passing signals to one another. Those cells are neurons. The overall picture is well understood at the textbook level, though many details remain active research.
The parts of a neuron
A typical neuron has three main regions:
- Dendrites are branching extensions that receive incoming messages from other cells.
- The cell body contains the nucleus and adds up the incoming input.
- The axon is a long, thin fiber that carries the outgoing signal away. At its far end it divides into small endings that meet other cells.
Axons vary enormously in length. Some are a fraction of a millimeter, while others, such as those running from the spinal cord down the leg, can be a meter or more. Many axons are wrapped in a fatty layer called myelin, produced by supporting glial cells. Myelin acts somewhat like insulation on a wire and makes signals travel faster.
A neuron at rest
Cells are surrounded by fluid containing charged particles called ions, such as sodium, potassium and chloride. The cell membrane is largely a barrier to these ions, but it has channels that allow particular ones through and pumps that move them against their natural flow. As a result, the inside of a resting neuron is electrically negative compared with the outside. The difference is typically tens of millivolts, a tiny voltage, and it is called the resting potential.
The action potential
When a neuron receives enough stimulation to push its voltage past a threshold, voltage-sensitive channels in the membrane open. Sodium ions rush in, making the inside briefly positive. Moments later, those channels close and potassium channels open, letting potassium flow out and restoring the negative state. This whole swing in voltage is the action potential, or nerve impulse. It lasts only about a millisecond.
Several features are worth knowing:
- All or nothing. An action potential either fires fully or does not fire. A stronger stimulus does not make a bigger impulse; it generally makes impulses fire more often.
- It regenerates as it travels. The change in one patch of membrane triggers the next patch, so the impulse moves along the axon without fading. In myelinated axons, the signal effectively jumps between small gaps in the insulation, which speeds it up.
- A brief recovery period. Right after firing, a neuron cannot fire again for a moment, which keeps impulses moving in one direction.
Speeds vary widely, from roughly a meter per second in thin, unmyelinated fibers to over a hundred meters per second in thick, myelinated ones.
Crossing the synapse
When the impulse reaches the end of an axon, it meets the next cell at a junction called a synapse. In most synapses in the human body there is a tiny gap, so the electrical signal does not simply jump across. Instead, it is converted into a chemical one:
- The arriving impulse opens channels that let calcium ions into the axon ending.
- This causes small packages, called vesicles, to release chemical messengers known as neurotransmitters into the gap.
- The neurotransmitters drift across and attach to receptor proteins on the next cell.
- This changes the receiving cell’s membrane, making it either more or less likely to fire its own impulse.
- The neurotransmitter is then cleared away, either taken back up or broken down, so that the signal ends.
Some synapses are excitatory, pushing the next cell toward firing, and others are inhibitory, pushing it away. A single neuron may receive thousands of such inputs and effectively adds them up. If the total reaches threshold, it fires.
Neurotransmitters and drugs
Many neurotransmitters have been identified, including glutamate (mainly excitatory), GABA (mainly inhibitory), dopamine, serotonin and acetylcholine. It is tempting to give each a single role, such as “dopamine is the pleasure chemical,” but real roles depend on which receptors and circuits are involved, and such slogans oversimplify.
Because synapses are chemical, many medicines and other substances act there, by changing how much transmitter is released, how long it stays, or how well it binds. How a particular substance affects a particular person is a question for a doctor or pharmacist, not something to infer from a general outline like this.
From signals to thinking
A single impulse is simple. What makes the brain remarkable is the scale and pattern of connections. Neurons form circuits, and synapses can strengthen or weaken with use, which is thought to underlie learning. Our article on memory formation touches on how this might work, though a complete account of how circuits produce memory, perception and thought is still unfinished. Electrical and chemical signaling is also the basis for the way the human senses send information to the brain.
A note on simplifications
This is an outline. Real neurons come in many shapes, some synapses are purely electrical, and signaling inside cells adds further layers. Anyone wanting more detail can consult a university-level neuroscience text. As general educational information, this article is not a guide to diagnosing or treating any condition.