What effect do muscarinic receptors have on the heart rate?

Muscarinic receptors, specifically those of the M2 subtype, primarily have an inhibitory effect on heart rate. When activated by the neurotransmitter acetylcholine (ACh), these receptors are located in the heart and work to decrease the rate of cardiac pacemaker activity. This results in a slower heart rate.

The stimulation of muscarinic receptors leads to a series of intracellular events that cause hyperpolarization of the cardiac cells, mainly those in the sinoatrial (SA) node, which is responsible for setting the rhythm of the heart. This hyperpolarization makes it less likely for the cardiac cells to reach the threshold for an action potential, thus resulting in a decreased heart rate.

While the correct answer suggests that muscarinic receptors stimulate smooth muscle and slow heart rate, it’s crucial to note that the slowing of the heart rate is the most significant effect directly related to muscarinic receptor activation in this context. The statement regarding smooth muscle is true in the broader context of muscarinic activity throughout the body but is somewhat tangential to the heart’s primary response to these receptors.

Therefore, muscarinic receptors indeed slow heart rate rather than accelerate it, and their activation does involve effects on smooth muscle in other areas, but the