Which is correct regarding neurotransmitters and neurotransmission?

Neurotransmitters are indeed released from the presynaptic nerve terminals as a critical step in neurotransmission. This release occurs when an action potential travels down the axon of the neuron, leading to the influx of calcium ions into the presynaptic terminal. The rise in intracellular calcium levels prompts synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane, releasing their contents into the synaptic cleft. This process is vital for the transmission of signals between neurons or from neurons to other types of cells, such as muscle or gland cells.

The other statements involve inaccuracies about the mechanisms of neurotransmission. For example, the arrival of an action potential affects the presynaptic neuron's terminal, not the postsynaptic cell, initiating neurotransmitter release rather than being triggered by it. Additionally, it’s crucial for calcium levels to rise, not drop, before neurotransmitter release can occur, as increased calcium is what signals the vesicles to release their contents. Lastly, while serotonin and dopamine play important roles in neurotransmission, they are not classified as the primary neurotransmitters in the autonomic nervous system (ANS), which primarily utilizes acetylcholine and norepinephrine instead.