Final answer:
Parasympathetic stimulation slows heart rate by releasing the neurotransmitter acetylcholine (ACh) via the vagus nerve. ACh opens potassium ion channels, which slows depolarization and extends repolarization, resulting in a lower heart rate. Decreasing parasympathetic stimulation allows the heart rate to increase.
Step-by-step explanation:
Parasympathetic stimulation originates from the cardioinhibitory region with impulses traveling via the vagus nerve (cranial nerve X). The vagus nerve sends branches to both the SA and AV nodes, and to portions of both the atria and ventricles. Parasympathetic stimulation releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction. ACh slows HR by opening chemical- or ligand-gated potassium ion channels to slow the rate of spontaneous depolarization, which extends repolarization and increases the time before the next spontaneous depolarization occurs. Without any nervous stimulation, the SA node would establish a sinus rhythm of approximately 100 bpm.
Since resting rates are considerably less than this, it becomes evident that parasympathetic stimulation normally slows HR. This is similar to an individual driving a car with one foot on the brake pedal. To speed up, one need merely remove one's foot from the break and let the engine increase speed. In the case of the heart, decreasing parasympathetic stimulation decreases the release of ACh, which allows HR to increase up to approximately 100 bpm. Any increases beyond this rate would require sympathetic stimulation.
Figure 19.33 illustrates the effects of parasympathetic and sympathetic stimulation on the normal sinus rhythm.