Which ion entry triggers neurotransmitter release at the presynaptic terminal?

• A. Sodium
• B. Calcium; enters presynaptic terminal to trigger neurotransmitter release.
• C. Chloride
• D. Potassium

Answer: (B)

Calcium entry into the presynaptic terminal triggers neurotransmitter release. When an action potential arrives, voltage-gated calcium channels open and calcium rushes into the terminal, raising local intracellular Ca2+ levels. This Ca2+ binds to sensor proteins like synaptotagmin, which drives the SNARE-mediated fusion of synaptic vesicles with the presynaptic membrane, causing exocytosis of neurotransmitter into the synaptic cleft. Sodium’s role is to depolarize the membrane to propagate the action potential, not to directly trigger release. Chloride and potassium influence shaping membrane potentials and recovery, but do not directly initiate vesicle fusion for release.