Final answer:
Calcium ions initiate the fusion of neurotransmitter vesicles with the presynaptic membrane by activating synaptotagmin. Synaptobrevin and syntaxin form the SNARE complex, directly facilitating vesicle docking and fusion. Calmodulin is not directly responsible for neurotransmitter release but participates in diverse signaling pathways. The correct option is B.
Step-by-step explanation:
Exocytosis of neurotransmitter vesicles at chemical synapses is a meticulously orchestrated process involving several proteins and ions. When an action potential reaches the axon terminal, voltage-gated Ca²+ channels open, leading to an influx of Ca²+ ions. These ions play a crucial role in triggering the fusion of vesicles with the presynaptic membrane.
Calcium ions activate synaptotagmin, a protein that serves as a Ca²+ sensor and triggers the vesicle to merge with the presynaptic membrane. Calmodulin, another calcium-binding protein, is involved in various signaling pathways but is not directly responsible for neurotransmitter release.
The actual fusion mechanism is facilitated by a SNARE complex, primarily composed of proteins like synaptobrevin and syntaxin. Synaptobrevin is a vesicle-associated membrane protein (VAMP), and syntaxin is present on the presynaptic plasma membrane.
These proteins, along with SNAP-25, form the SNARE complex, which is directly responsible for the docking and fusion of the neurotransmitter vesicles leading to exocytosis. However, Calcium is not a cofactor for calmodulin in the process of SNARE complex assembly. It acts primarily as a trigger for the fusion of vesicles.
Therefore, while calcium plays a pivotal role in activating synaptotagmin and facilitating vesicle fusion, calmodulin plays a less direct role in neurotransmitter exocytosis. Synaptobrevin and syntaxin, as part of the SNARE complex, are vital for the mechanical fusion of vesicles with the presynaptic membrane, thereby allowing neurotransmitter release.