Use Play to run the whole transmission cycle, or Step to advance one stage at a time. Drag to rotate the scene and click any structure — vesicle, calcium channel, neurotransmitter, or receptor — to learn what it does.
Your brain has about $86$ billion neurons, and they constantly pass messages to one another. But neurons almost never touch. Where one neuron meets the next there is a microscopic gap called the synaptic cleft, only about $20$–$40\,\mathrm{nm}$ wide — thousands of times thinner than a hair. An electrical impulse cannot simply jump that gap, so the neuron does something clever: it turns the electrical signal into a chemical one, sends the chemical across, and the next cell turns it back into electricity.
When the impulse (the action potential) arrives at the axon terminal, it opens voltage-gated calcium channels. Calcium ions ($\mathrm{Ca^{2+}}$) are far more concentrated outside the cell, so they flood in. That sudden rise in internal calcium is the master switch: it makes synaptic vesicles — little bubbles each holding thousands of neurotransmitter molecules — fuse with the membrane and spill their contents into the cleft. No calcium, no release.
The neurotransmitter diffuses across the cleft in well under a millisecond and binds receptors on the postsynaptic membrane. Each receptor is shape-specific, like a lock and key, and opens an ion channel. If that channel lets in $\mathrm{Na^{+}}$, the next cell is excited (pushed toward firing); if it lets in $\mathrm{Cl^{-}}$ or out $\mathrm{K^{+}}$, the cell is inhibited. Finally the signal must switch off: transmitter is pumped back (reuptake), broken down by enzymes, or diffuses away, resetting the synapse for the next impulse. The whole round trip adds only a $\approx 0.5$–$1\,\mathrm{ms}$ synaptic delay.
Press Play and watch the order: spike → calcium in → vesicle fusion → release → receptor opening → clearance. Switch the Signal type to Inhibitory and step again — the same release machinery now quiets the next cell, proving the effect is set by the receptor, not the transmitter alone. Turn off Calcium ions and step: with no calcium signal, nothing is released.
| Stage | What happens | Key molecule |
|---|---|---|
| 1. Impulse arrives | Action potential travels down the axon and reaches the presynaptic terminal | Membrane voltage |
| 2. Calcium enters | Voltage-gated Ca²⁺ channels open; Ca²⁺ rushes into the terminal | Ca²⁺ |
| 3. Vesicles fuse | Calcium triggers vesicles to dock and fuse with the membrane (exocytosis) | SNARE proteins |
| 4. Release | Neurotransmitter is dumped into the synaptic cleft | Neurotransmitter |
| 5. Receptors open | Transmitter binds postsynaptic receptors; ion channels open | Receptor + ions |
| 6. Clearance | Transmitter is removed by reuptake, enzymes, or diffusion; synapse resets | Transporters / enzymes |
Calcium is kept extremely low inside resting neurons — roughly $10{,}000$ times lower than outside. That steep gradient means that the instant the voltage-gated channels open, calcium surges in and its concentration near the membrane spikes sharply. Sensor proteins on the vesicles (synaptotagmin) detect that spike and, working with the SNARE fusion machinery, pull the vesicle membrane into the cell membrane so the contents spill out. Because the calcium signal is so localised and so fast, release happens within a fraction of a millisecond of the impulse arriving.
The same release process can either encourage or discourage the next neuron from firing, and which one depends entirely on the postsynaptic receptor. Glutamate receptors usually open sodium-permeable channels, depolarising the cell toward its threshold (an excitatory postsynaptic potential, EPSP). GABA receptors open chloride channels, pushing the cell away from threshold (an inhibitory postsynaptic potential, IPSP). A single neuron constantly adds up thousands of EPSPs and IPSPs, and only fires its own action potential if the excitatory inputs win — this summation is the basic arithmetic of the nervous system.
A synapse is the junction where one neuron passes a signal to another cell. At a chemical synapse the two cells do not actually touch — they are separated by a tiny gap, the synaptic cleft, about $20$–$40$ nanometres wide. The signal crosses that gap as a chemical messenger called a neurotransmitter rather than as electricity.
Key takeaway: a synapse is a communication junction where an electrical signal becomes a chemical message and back again.When an action potential reaches the axon terminal it opens voltage-gated calcium channels. Calcium ions rush in, and that calcium signal makes synaptic vesicles fuse with the membrane and dump neurotransmitter into the cleft. The transmitter diffuses across and binds receptors on the next neuron, opening ion channels there. So the message crosses as a chemical, not as a spark.
Key takeaway: calcium entry links the electrical spike to chemical release — it is the trigger.Synaptic vesicles are tiny membrane bubbles inside the axon terminal, each packed with thousands of neurotransmitter molecules. When calcium enters, vesicles dock and fuse with the presynaptic membrane and release their contents by exocytosis. Each vesicle is one "quantum" of transmitter, so the signal is released in packets.
Key takeaway: vesicles store and release neurotransmitter in discrete packets through membrane fusion.Receptors work by shape, like a lock and key. A neurotransmitter only opens a receptor whose binding site matches its shape, so acetylcholine fits acetylcholine receptors but not others. Whether the receiving cell is excited or inhibited depends on which ion channel that receptor controls — sodium-type channels excite, chloride or potassium-type channels inhibit.
Key takeaway: specificity comes from molecular shape matching, and the effect depends on which ions the channel passes.The signal must be switched off quickly, so neurotransmitter is cleared from the cleft in three ways: it is pumped back into the sending neuron (reuptake), broken down by enzymes, or it simply diffuses away. This reset lets the synapse fire again rapidly. Many drugs, such as SSRIs, work by blocking reuptake so the transmitter lingers.
Key takeaway: rapid clearance by reuptake, enzymes, and diffusion ends each signal and resets the synapse.A chemical synapse is one-directional because only the presynaptic terminal has vesicles full of neurotransmitter and only the postsynaptic membrane has the matching receptors. The sending side can release but cannot receive, and the receiving side can detect but cannot release back across the same synapse.
Key takeaway: vesicles on one side and receptors on the other make chemical transmission a one-way street.At an electrical synapse the cells are joined by gap junctions, so current flows directly and almost instantly from one cell to the next, in either direction. A chemical synapse adds a brief delay (about $0.5$–$1$ millisecond) because it must release and detect a transmitter, but in return it can amplify, switch, integrate many inputs, and be either excitatory or inhibitory.
Key takeaway: electrical synapses are faster and bidirectional; chemical synapses are far more flexible.