The junction where one neuron passes its message to another: the presynaptic terminal, the 20-nanometre cleft, neurotransmitter release and the receptors waiting on the other side.
Neural Signaling
Beginner
The Synapse
The Editors
November 25, 2025
2 min read
In plain English
A synapse is the meeting point between two neurons. The first neuron releases chemical messengers into a tiny gap, and the second neuron catches them with receptors. Synapses are where learning happens and where most brain drugs act.
Beginner level: No background needed. Everyday language, everyday examples — the place to start.
Concepts to know first
Go deeper next
A synapse is a specialised junction where a neuron passes a signal to another cell — another neuron, a muscle fibre or a gland. The word, coined by Charles Sherrington in 1897, means "clasping together". Estimates of the number of synapses in a human brain run to the order of a hundred trillion or more.
Three parts
The presynaptic terminal — the swollen end of an axon branch — is packed with tiny membrane bubbles called synaptic vesicles, each holding thousands of neurotransmitter molecules. The synaptic cleft is a gap of about 20 to 40 nanometres, far too small to see in a light microscope. The postsynaptic membrane on the receiving cell is studded with receptors held in place by a dense protein scaffold.
The sequence
An action potential arrives at the terminal. Voltage-gated calcium channels open and calcium rushes in. Calcium triggers vesicles to fuse with the membrane and spill their transmitter into the cleft. The transmitter diffuses across in a fraction of a millisecond, binds to receptors, and opens channels or triggers chemical cascades in the receiving cell. Finally the transmitter is cleared — pumped back into cells by transporters, broken down by enzymes, or diffusing away — so the synapse is ready for the next signal.
Excitatory and inhibitory
Synapses that make the receiving neuron more likely to fire are excitatory (most use glutamate). Those that make it less likely are inhibitory (most use GABA or glycine). The balance between the two is essential: too much excitation can cause seizures; too much inhibition, unconsciousness.
Where learning lives
Synapses are not fixed. They strengthen, weaken, appear and disappear with experience — synaptic plasticity, thought to be the main way the brain stores memories. And because synapses use chemicals, they are where most psychoactive drugs and many medicines act.
Everyday examples & things to try
Analogy
Throwing a ball across a gap
Picture two people on either side of a narrow ditch. One throws a handful of tennis balls (neurotransmitter); the other has mitts shaped to catch only tennis balls (receptors). Afterwards, a cleaner collects the stray balls (reuptake) so the next throw is clear.
Real world
Where drugs act
Antidepressants block the 'cleaner' for serotonin, caffeine blocks a receptor for adenosine, and benzodiazepines make GABA receptors catch more strongly. Almost all of them work at synapses.
Key takeaways
- A synapse has a presynaptic terminal, a ~20–40 nm cleft and a postsynaptic membrane with receptors.
- Calcium entry triggers vesicles to release neurotransmitter.
- Synapses can be excitatory or inhibitory and change with experience.
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