Neurons and Neurotransmission: How Cells Communicate

Every psychological event—remembering a name, flinching at a loud noise, deciding to keep reading—is carried by electrical and chemical signals across a network of roughly 86 billion neurons (Azevedo et al., 2009). Understanding how single nerve cells work makes the rest of neuroscience legible, and it is also the best antidote to oversimplified brain claims. This page walks through a neuron's structure, how it fires, what happens at the synapse, and what the major neurotransmitters actually do.

Table of Contents

1. The Neuron: Anatomy of a Signaling Cell
2. The Action Potential
3. The Synapse: Where Neurons Talk
4. The Major Neurotransmitters
5. Plasticity: How Experience Rewires Circuits
6. What the Chemistry Does Not Explain
7. Key Takeaways
8. References

The Neuron: Anatomy of a Signaling Cell

Despite enormous diversity, neurons share a four-part plan:

• Dendrites — branching antennas that receive signals from other cells.
• Cell body (soma) — integrates incoming signals and keeps the cell alive.
• Axon — the output cable; in myelinated axons, a fatty sheath (built by glial cells) lets signals jump ahead in blocks, reaching speeds around 100 meters per second.
• Axon terminals — release chemical messengers into the synapse.

Glial cells outnumber or match neurons and are far more than packing material: they build myelin, supply nutrients, and actively prune and maintain synapses.

The Action Potential

A neuron at rest holds a small electrical charge across its membrane (about −70 millivolts), maintained by pumps that push positively charged sodium out and potassium in. When incoming signals push the charge past a threshold (around −55 mV), the membrane flips: sodium rushes in, then potassium restores the balance—a self-regenerating spike called the action potential.

Two properties matter for psychology:

1. All-or-none: once past threshold, the spike is always full size. Intensity is therefore encoded in firing rate, not spike size.
2. Refractory period: briefly after firing, a neuron cannot fire again, which sets a ceiling on firing rate and enforces one-way travel down the axon.

The Synapse: Where Neurons Talk

When the action potential reaches a terminal, calcium influx triggers vesicles to spill neurotransmitter molecules across a ~20–40 nanometer gap. On the far side, receptor molecules catch them, opening or closing ion channels that make the next cell more (excitatory) or less (inhibitory) likely to fire.

The signal then must be switched off—by reuptake (the sending cell recycles the molecules) or by enzymes that break them down. Most psychiatric medications work on precisely this step: SSRIs, for example, slow the reuptake of serotonin.

The Major Neurotransmitters

No neurotransmitter has a single psychological meaning: dopamine is about prediction error and motivation, not "pleasure"; serotonin is one actor among many mood-relevant systems. Receptor types (there are many for each transmitter) make effects highly region-specific.

Plasticity: How Experience Rewires Circuits

Donald Hebb's principle—neurons that repeatedly co-activate strengthen their connection (Hebb, 1949)—became mechanistically real with studies of long-term potentiation (LTP): glutamate-driven co-activation leaves synapses permanently more responsive. Eric Kandel's work on the sea slug Aplysia showed learning producing measurable, lasting molecular changes at identified synapses (Kandel, 2001), work honored with a Nobel Prize.

This is the biology underneath memory formation, skill learning, and the brain's plasticity generally: use strengthens circuits; disuse prunes them. Tools like neurofeedback attempt to put this loop under conscious control, with evidence that remains strongest for specific applications such as ADHD research contexts rather than as a general self-optimization tool.

What the Chemistry Does Not Explain

Neurotransmitters are necessary but not sufficient explanations of mind. Two cautions:

• The "chemical imbalance" story of depression is an oversimplification. Serotonin activity differs between people in complex ways, and SSRI effectiveness does not prove that depression is low serotonin. A 2022 umbrella review found little consistent support for the simple version of the serotonin hypothesis (Moncrieff et al., 2022); modern models treat depression as multifactorial—genetic, psychological, social, and biological at once.
• Drugs that "flood the brain with dopamine" do not create specific experiences. Pharmacology changes probabilities of firing in circuits whose content is determined by wiring and experience—which is why the same stimulant can feel different depending on context and expectation.

Key Takeaways

1. A neuron receives on dendrites, integrates at the soma, transmits down a myelinated axon, and releases chemicals at terminals.
2. Firing is all-or-none; information is carried in firing rate and timing, not spike size.
3. Synapses switch off via reuptake and enzymes—and most psychiatric medications act exactly there.
4. Glutamate and GABA do most of the work; dopamine, serotonin, acetylcholine, norepinephrine, and endorphins modulate systems with psychology-wide consequences.
5. Co-active neurons strengthen their connections (Hebb, LTP)—the physical basis of learning, and a two-way street: experience changes chemistry, not just the reverse.

References

• Azevedo, F. A. C., Carvalho, L. R. B., Grinberg, L. T., Farfel, J. M., Ferretti, R. E. L., Leite, R. E. P., … Herculano-Houzel, S. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology, 513(5), 532–541.
• Hebb, D. O. (1949). The organization of behavior. Wiley.
• Kandel, E. R. (2001). The molecular biology of memory storage: A dialogue between genes and synapses. Science, 294(5544), 1030–1038.
• Moncrieff, J., Cooper, R. E., Stockmann, T., Amendola, S., Hengartner, M. P., & Horowitz, M. A. (2022). The serotonin theory of depression: A systematic umbrella review of the evidence. Molecular Psychiatry, 27(10), 3910–3919.
• Kandel, E. R., Koester, J. D., Mack, S. H., & Siegelbaum, S. A. (Eds.). (2021). Principles of neural science (6th ed.). McGraw Hill.