Final answer:
Serotonin modulates neurotransmission at sensory-motor synapses by enhancing presynaptic release but does not inhibit postsynaptic receptors, block neurotransmitter synthesis, or decrease synaptic vesicle formation. The correct option is A.
Step-by-step explanation:
Serotonin (5-HT) plays a significant role in modulating neurotransmission at the sensory-motor synapse. It can affect neurotransmission by facilitating or inhibiting the release or action of other neurotransmitters. Serotonin is synthesized from the amino acid tryptophan and is stored in vesicles within the presynaptic neuron.
Upon release, 5-HT can interact with autoreceptors to regulate its own synthesis and release. After its release, serotonin principally acts through various serotonin receptors, most of which belong to the G-protein-coupled receptor superfamily, that can either inhibit or stimulate different intracellular pathways.
Specifically, serotonin can enhance presynaptic release of neurotransmitters by acting on certain receptors that stimulate the presynaptic neuron to release more neurotransmitters into the synaptic cleft.
However, serotonin does not block neurotransmitter synthesis, decrease synaptic vesicle formation, nor directly inhibit postsynaptic receptors—it modulates the effect of neurotransmitters already present.