What is the effect of parasympathetic (vagal) stimulation on the heart?

• A. It decreases heart rate (negative chronotropy) and has a minor effect on contractility
• B. It increases heart rate and contractility
• C. It has no effect on heart rate but increases contractility
• D. It increases heart rate and decreases contractility

Answer: (A)

Parasympathetic (vagal) stimulation slows the heart by acting on the SA and AV nodes. Acetylcholine released onto muscarinic receptors increases potassium conductance and decreases calcium influx in nodal cells, making the cells hyperpolarized and less likely to fire quickly. This slows the rate of spontaneous depolarization in the SA node and reduces conduction speed through the AV node, producing a lower heart rate (negative chronotropy) and slower AV conduction. The ventricular muscle has little parasympathetic innervation, so contractility is minimally affected; if there is any change, it is small. So the primary effect is a slower heart rate with only a minor influence on contractility.