Definition
- Synaptic transmission is transfer of a nerve impulse from a presynaptic neuron to a postsynaptic cell across a synapse, via presynaptic and postsynaptic mechanisms.
- Synaptic delay is the time from calcium influx to transmitter release.
Synaptic delayabout 200 μs
Presynaptic Mechanisms
- Step 1 Neurotransmitter vesicles concentrated at the active zone undergo docking (membrane attachment) and priming (readiness to discharge).
- Step 2 The arriving action potential depolarizes the presynaptic terminal membrane.
- Step 3 Depolarization opens voltage-gated calcium channels, and calcium enters the terminal through the active zone.
- Step 4 Rising intracellular calcium triggers calcium-mediated exocytosis, fusing vesicles with the presynaptic membrane.
- Step 5 Neurotransmitter is released into the synaptic cleft in a quantized amount, called Dale's phenomenon, and diffuses passively to the postsynaptic membrane.
- Calcium-triggered fusion occurs through contraction of microfilaments in the dense tuft, which moves vesicles to the membrane and helps discharge their contents.
- Kiss-and-run discharge:
- Content is released through a small hole that reseals rapidly; the main vesicle stays intracellular.
- Some vesicles are quickly recovered by endocytosis and refilled locally, short-circuiting the endocytic process.