Final answer:
Serotonin receptor antagonists primarily affect the GI tract/vomiting centre and involve nicotinic receptors for a drug influencing both divisions of the autonomic nervous system.
Step-by-step explanation:
Serotonin receptor antagonists primarily affect the GI tract/vomiting centre. These receptors, located on the cell membranes of neurons in the GI tract, regulate gene expression through second messenger cascades, playing a significant role in the timing of the passage of food through the gastrointestinal system. Additionally, serotonin is associated with a sense of well-being, and disruptions in serotonin signaling can lead to gastrointestinal symptoms like an "upset stomach" or irritable bowel syndrome. The parasympathetic nervous system directly controls the digestive system, with serotonin as a key neurotransmitter in this regulation.
A drug affecting both divisions of the autonomic nervous system would typically bind to or block nicotinic receptors. These receptors are found on the neurons that form synapses in both the sympathetic and parasympathetic branches of the autonomic nervous system.