Final answer:
The parasympathetic nervous system activates the 'rest-and-digest' responses through ACh and mACh receptors, with GPCRs playing a central role in transducing the signals into cellular responses. These mechanisms involve signaling pathways such as cAMP, influencing physiological processes like heart rate and digestion, and can be modulated by specific drugs.
Step-by-step explanation:
The roles of the parasympathetic nervous system, acetylcholine (ACh), muscarinic acetylcholine receptors (mACh), G protein-coupled receptors (GPCR), potassium (K), and cyclic adenosine monophosphate (cAMP) are critical aspects of human physiology. The parasympathetic nervous system is part of the autonomic nervous system responsible for the 'rest-and-digest' activities that occur when the body is at rest. It counterbalances the sympathetic nervous system's 'fight-or-flight' responses.
All preganglionic fibers in the autonomic system release ACh, activating ganglionic neurons with nicotinic receptors—ligand-gated ion channels causing depolarization. Postganglionic parasympathetic fibers also release ACh, but the receptors are muscarinic receptors, which are GPCRs. These receptors do not exclusively cause depolarization; instead, they can trigger various intracellular responses, often involving second messengers like cAMP, which can modulate cellular activities such as muscle contraction or glandular secretion.
Each target cell in the organs affected by the parasympathetic system has muscarinic receptors that can be stimulated by parasympathomimetic drugs to enhance cholinergic effects or blocked by anticholinergic drugs to inhibit these effects. These processes regulate key physiological functions such as heart rate, digestion, and glandular secretion, ensuring the body maintains homeostasis.