Final answer:
The characteristic clinical sign of parasympathetic nervous system stimulation is a reduced heart rate, as it slows down heart rate through the action of the neurotransmitter acetylcholine.
Step-by-step explanation:
The clinical sign characteristic of the parasympathetic nervous system stimulation is a reduced heart rate. Parasympathetic stimulation, primarily through the vagus nerve (cranial nerve X), releases the neurotransmitter acetylcholine (ACh) which acts on the heart by slowing down the heart rate. This occurs because ACh opens potassium ion channels, leading to a slower rate of spontaneous depolarization and extending repolarization, which increases the time before the next heart contraction occurs. The parasympathetic nervous system is often considered the 'rest and digest' part of the autonomic nervous system, as it brings the body back to homeostasis after sympathetic activation. Contrary to bronchodilation, reduced digestion, and vasodilation, which are signs of sympathetic activation, the parasympathetic system decreases the heart rate, stimulates digestion, and causes other effects that conserve energy and promote maintenance of the body's regular functions.