What is the first step in the mechanism of exocytosis?

The first step in the mechanism of exocytosis involves the triggering of an action potential. This action potential is typically generated in response to a stimulus which causes a fluctuation in the membrane potential of a cell. For example, in a neuron, when an electrical signal reaches the axon terminal, it prompts the opening of voltage-gated ion channels, allowing an influx of sodium ions (Na+). This influx further depolarizes the membrane and leads to the generation of the action potential.

Once the action potential is initiated, it propagates along the axon to the presynaptic terminal, ultimately leading to the subsequent steps of exocytosis, such as the binding of calcium ions to proteins. Thus, the action potential serves as the crucial initial trigger that catalyzes the entire process of neurotransmitter release via exocytosis.