What mechanism is primarily responsible for the neurotransmitter release at the synaptic cleft?

The mechanism primarily responsible for neurotransmitter release at the synaptic cleft is exocytosis. This process involves the fusion of synaptic vesicles, which contain neurotransmitters, with the presynaptic membrane in response to an increase in intracellular calcium ion concentration. When an action potential reaches the axon terminal, voltage-gated calcium channels open, allowing calcium ions to flow into the terminal. The influx of calcium triggers the synaptic vesicles to move towards the membrane and fuse with it, releasing their contents into the synaptic cleft through exocytosis.

This release of neurotransmitters is crucial for communication between neurons, as these chemicals then bind to receptors on the postsynaptic neuron, resulting in the transmission of the neural signal. The specificity and rapid nature of this process underscore its significance in neurotransmission.