# Neuron Synapse and Neurotransmitter Release

> A synapse is the junction where one neuron passes a message to the next cell. An electrical signal reaching the axon terminal triggers vesicles to release chemical messengers, which cross a tiny gap and open channels on the receiving cell.

Web page: https://cellnaut.com/topics/neuron-synapse-transmission

![Teaching illustration: Neuron Synapse and Neurotransmitter Release](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/neuron-synapse-transmission-teaching.jpg)

## Labelled parts

1. **Presynaptic axon terminal**: The swollen end of the sending neuron's axon, packed with vesicles.
2. **Synaptic vesicles**: Small membrane sacs that store neurotransmitter molecules.
3. **Mitochondrion**: Supplies the ATP needed to refill vesicles and run ion pumps.
4. **Vesicle fusion site**: Where a vesicle merges with the membrane and releases its contents by exocytosis.
5. **Synaptic cleft**: The narrow gap, about 20 nm wide, between the two cells.
6. **Neurotransmitters**: Chemical messengers that diffuse across the cleft in a fraction of a millisecond.
7. **Postsynaptic membrane**: The receiving cell's membrane, which carries the receptors.
8. **Receptor channels**: When the transmitter binds, these ion channels open and change the receiving cell's voltage.

## Key points

- Calcium entering the terminal makes vesicles fuse with the membrane and release neurotransmitter.
- Neurotransmitters cross the synaptic cleft by diffusion, which is fast because the gap is only about 20 nm wide.
- Binding to receptor channels changes the voltage of the receiving cell, so the signal can continue or be inhibited.

## Check yourself

**1. What triggers synaptic vesicles to release their contents?**

- A. Calcium ions entering the terminal
- B. Oxygen leaving the cell
- C. DNA replication
- D. Light

**Answer:** A. Calcium ions entering the terminal - An arriving action potential opens calcium channels, and calcium triggers vesicle fusion.

**2. How do neurotransmitters cross the synaptic cleft?**

- A. Active transport by pumps
- B. Diffusion
- C. They are carried by red blood cells
- D. They are not released

**Answer:** B. Diffusion - The gap is so narrow that simple diffusion is fast enough.

## Related lessons

- [Cell Communication and the Neuron](https://cellnaut.com/learn/cell-communication)

## See it in 3D

- [Neuron](https://cellnaut.com/studio/neuron)