Neurons do not touch each other; a tiny gap called the synapse separates them. Most signals cross it as chemicals released from the axon terminal.
Chemical synapse
When the impulse reaches the terminal, calcium enters and triggers the release of neurotransmitter into the gap. The transmitter binds receptors on the next cell, then is broken down, taken back up or diffuses away.
Excitatory and inhibitory
Some transmitters make the next neuron more likely to fire and others less likely. The receiving neuron adds up all its inputs and fires only if the total reaches threshold.
Neuromuscular junction
Motor neurons release acetylcholine onto skeletal muscle, which makes the muscle contract. Myasthenia gravis, neuromuscular blocking drugs and some poisons act at this junction.
Drugs at the synapse
Many medicines work by copying, blocking or prolonging a transmitter. Examples are opioids at opioid receptors, SSRIs blocking serotonin reuptake and benzodiazepines boosting GABA.
| Neurotransmitter | Main effect | Clinical link |
|---|---|---|
| Acetylcholine | Muscle contraction, parasympathetic effects | Myasthenia gravis, organophosphate poisoning |
| Norepinephrine | Sympathetic fight or flight | Vasopressors, beta blockers |
| Dopamine | Movement, reward | Parkinson disease, antipsychotics |
| Serotonin | Mood, sleep, gut | SSRIs, serotonin syndrome |
| GABA | Main inhibitory transmitter | Benzodiazepines, alcohol |
| Glutamate | Main excitatory transmitter | Seizures, nerve cell injury after stroke |
At the bedside
Naloxone reverses an opioid overdose by pushing opioids off their receptors, so breathing returns within minutes. It can wear off before the opioid does, so keep watching breathing and level of consciousness.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).