Biological Systems MCAT Practice Exam

The release of acetylcholine at the neuromuscular junction is primarily triggered by the influx of calcium ions. When an action potential reaches the presynaptic terminal of a neuron, voltage-gated calcium channels in the membrane open, allowing calcium ions to flow into the neuron. The increase in intracellular calcium concentration is a critical signal that stimulates synaptic vesicles containing acetylcholine to move towards the presynaptic membrane.

Once the vesicles reach the membrane, they undergo a process known as exocytosis, where they fuse with the membrane and release acetylcholine into the synaptic cleft. This released acetylcholine then binds to receptors on the postsynaptic muscle cell, leading to muscle contraction. The specificity of calcium in this process is crucial, as other ions like sodium, potassium, and chloride do not play the same role in triggering the exocytosis of neurotransmitters in this context.