Final answer:
Acetylcholine is released into the synaptic cleft when an action potential reaches the axon terminal of a presynaptic neuron, leading to the opening of calcium channels and the movement of ACh-containing vesicles to the presynaptic membrane.
Step-by-step explanation:
Acetylcholine (ACh) is released into the synaptic cleft under the influence of an action potential and the subsequent influx of calcium ions.
During neurotransmission, ACh is synthesized and encapsulated in vesicles within the presynaptic neuron. When an action potential reaches the axon terminal, voltage-gated calcium channels open, allowing calcium ions (Ca²⁺) to enter. The presence of Ca²⁺ prompts synaptic vesicles to bind to the presynaptic membrane, causing the release of ACh into the synaptic cleft. Once in the synaptic cleft, ACh diffuses and binds to ACh receptors on the postsynaptic membrane. This binding triggers the opening of ligand-gated ion channels, leading to depolarization. The neurotransmitter is quickly broken down by acetylcholinesterase (AChE) to prevent prolonged activation of the muscle or neural pathway.