Final answer:
Serotonin leads to long-term changes at the sensory-motor synapse primarily through the activation of CREB, which can stimulate gene transcription for processes involved in synaptic plasticity like LTP and LTD. The correct option is A.
Step-by-step explanation:
The question asks about the mechanism by which serotonin leads to long-term changes at the sensory-motor synapse. One of the important mechanisms by which serotonin can affect synaptic plasticity is by the activation of CREB (cAMP response element-binding protein).
When serotonin binds to its receptors, it can activate a signaling cascade that results in the phosphorylation and activation of CREB. Once activated, CREB can then stimulate the transcription of genes that contribute to long-term potentiation (LTP) or long-term depression (LTD), processes that are crucial for learning and memory.
This typically results in structural and functional changes in the synaptic connections, such as the insertion or removal of AMPA receptors in the postsynaptic neuron. LTP is associated with a strengthening of these connections, often through the insertion of AMPA receptors, while LTD involves the weakening of connections, signaled by the removal of AMPA receptors, allowing the nervous system to prune less-used synaptic connections.
Therefore, the correct answer to the student's question is A. Activation of CREB.