Final answer:
Ca++ influx causes the synthesis of nitric oxide in neuronal varicosities by activating the enzyme nitric oxide synthase, which then converts L-arginine into nitric oxide.
Step-by-step explanation:
Calcium ion (Ca++) influx plays a crucial role in various neuronal functions, including the synthesis of nitric oxide (NO) in neuronal varicosities. When an action potential reaches the axon terminal, it causes the opening of voltage-gated calcium channels. The influx of Ca++ into the cell then activates an enzyme called nitric oxide synthase (NOS). This enzyme synthesizes nitric oxide from the amino acid L-arginine. Nitric oxide acts as a signaling molecule and plays an important role in processes such as vasodilation, neurotransmission, and synaptic plasticity.
Therefore, the influx of Ca++ leads to the activation of nitric oxide synthase, leading to the synthesis of nitric oxide. This directly addresses the question of how Ca++ influx causes the synthesis of NO in neuronal varicosities, making option (a) Activates nitric oxide synthase the correct answer.