5.7 Neurons

Neuron structure

A neuron is a nerve cell that transmits information. Dendrites receive signals from other neurons; the cell body (soma) processes them; the axon carries the signal away, often insulated by a fatty myelin sheath that speeds conduction. At the end, axon terminals release chemical messengers into the gap called the synapse.

The action potential

A neuron fires when stimulation exceeds its threshold, triggering an action potential — a brief electrical impulse that travels down the axon. Neurons follow the all-or-none principle: they either fire completely or not at all. After firing, there's a brief refractory period before the neuron can fire again.

Synaptic transmission

When an impulse reaches the axon terminal, it triggers release of neurotransmitters into the synaptic gap. These chemicals bind to receptor sites on the receiving neuron's dendrites, either exciting or inhibiting it. Leftover neurotransmitter is cleared via reuptake, where the sending neuron reabsorbs it, ending the signal.

Key terms

Neuron

A nerve cell; the basic building block of the nervous system that transmits electrical and chemical signals.

Action potential

A brief electrical impulse that travels down an axon when stimulation exceeds threshold.

All-or-none principle

A neuron either fires completely or does not fire at all — no partial firing.

Synapse

The tiny gap between the axon terminal of one neuron and the dendrite of the next, where neurotransmitters are released.

Reuptake

The reabsorption of excess neurotransmitter by the sending neuron, stopping the signal.

Try an AP-style question

An excitatory neurotransmitter makes the receiving neuron…

1. more likely to fire
2. unable to fire
3. less likely to fire
4. fire backward