How Does the Brain Work? How Neurons Send Signals
Someone across a noisy room says your name, and before you have decided anything, your head is already turning. The whole event takes a fraction of a second. In that time, a sound wave became an electrical signal, the signal was passed from cell to cell, it was recognized as your name among all the other noise, and a command went out to the muscles of your neck.
That chain is the answer to how the brain works. The brain is not one machine doing one job. It is roughly 86 billion neurons passing signals to one another, and everything it does, from turning your head to remembering a face, comes from the way those signals are sent, received, and combined.
An electrical signal runs along each neuron
A neuron at rest is like a small, charged battery. Its inside is slightly negative compared with the outside, held that way by pumps and channels in its membrane that control the flow of charged particles such as sodium and potassium.
When a neuron is stimulated strongly enough, channels in its membrane snap open, sodium rushes in, and the charge flips for about a millisecond. That brief spike is an action potential. It does not fade as it travels. Instead, each patch of membrane triggers the next, so the spike regenerates itself all the way down the axon, the neuron’s long output fiber.
An action potential is all or nothing. A neuron either fires a full spike or none at all, so it signals the strength of a stimulus mainly by how often it fires, not by making bigger spikes. Speed depends on the fiber. In axons wrapped in myelin, the signal jumps between gaps in the insulation and can travel at more than 100 meters per second, while thin, unwrapped fibers carry signals far more slowly.
At the synapse, the message becomes chemical
Most neurons do not actually touch. Between the end of one neuron and the next lies a gap far too small to see, the synapse. When an action potential arrives at the end of the axon, it causes tiny packets to release chemical messengers, called neurotransmitters, into that gap.
The neurotransmitter molecules drift across and attach to receptors on the receiving cell, much like a key fitting a lock. This whole handoff takes roughly a millisecond. Then the messenger is cleared away, either broken down or pulled back into the cell that released it, so the synapse is ready for the next signal. A small number of synapses skip the chemical step and pass current directly through channels that link two cells, but chemical synapses are by far the most common in the brain.
Each neuron adds up its messages before firing
A single neuron may receive input from thousands of others at once, and not all of that input says the same thing. Some signals are excitatory: they nudge the receiving neuron closer to firing. Others are inhibitory: they push it further away. Glutamate is the brain’s main excitatory messenger, and GABA is its main inhibitory one.
The neuron adds these pushes and pulls together at every moment. Only if the total crosses a threshold does it fire its own action potential. This is where the brain’s real computing happens, and keeping the two forces in proportion, called excitation-inhibition balance, matters for healthy function. Other messengers, such as dopamine and serotonin, act more like volume controls. Rather than switching single neurons on or off, they adjust how whole groups of neurons respond.
Neurons work in circuits and networks
One neuron firing means little on its own. Meaning comes from patterns across many cells. Neurons that are linked together form circuits, and circuits in different regions link into large networks through bundles of nerve fibers, including the corpus callosum that joins the two halves of the brain.
Reading this sentence is a good example. Visual areas detect the shapes of the letters, language areas turn them into words and meaning, memory systems connect those words to what you already know, and attention networks keep you on the page. No single region reads. The network does. (For what these networks accomplish, see what the brain does.)
The connections keep changing
The brain’s wiring is not fixed like the circuits in a device. When two connected neurons are repeatedly active together, the synapse between them tends to grow stronger, and connections that go unused can weaken. This ongoing adjustment, called synaptic plasticity, is the cellular basis of learning and memory.
The work continues at night. During sleep, the brain keeps controlling breathing and heart rate, and it also appears to replay and stabilize some of what was learned during the day.
This is also the main reason the brain is not really like a computer. A computer runs fixed hardware at enormous speed, one step after another. The brain runs on much slower signals but processes vast amounts of information in parallel, mixes all-or-nothing spikes with gradual chemical signals, and rewires itself as it goes. It does all of this on roughly the power of a dim light bulb.
This article is for general education and does not replace advice from a qualified clinician.
References
- National Institutes of Health. Information about the Brain. NIH Curriculum Supplement Series. NCBI Bookshelf — https://www.ncbi.nlm.nih.gov/books/NBK20367/
- Caire MJ, Reddy V, Varacallo MA. Physiology, Synapse. StatPearls. NCBI Bookshelf — https://www.ncbi.nlm.nih.gov/books/NBK526047/
- Grider MH, Jessu R, Kabir R. Physiology, Action Potential. StatPearls. NCBI Bookshelf — https://www.ncbi.nlm.nih.gov/books/NBK538143/
- Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd ed. Sinauer Associates; 2001. NCBI Bookshelf — https://www.ncbi.nlm.nih.gov/books/NBK10799/
Written by: MedMaru Editorial Team
Reviewed for medical accuracy by:
S. Chang, KMD
