What is one effect of muscarinic receptors in cardiac tissue?

Muscarinic receptors in cardiac tissue are primarily associated with the parasympathetic nervous system, which plays a crucial role in regulating heart rate and overall heart function. When activated, muscarinic receptors lead to a decrease in heart rate, a process known as negative chronotropy. This occurs because stimulation of these receptors results in increased activity of the vagus nerve, which releases acetylcholine. Acetylcholine binds to muscarinic receptors in the sinoatrial (SA) node of the heart, slowing the rate of depolarization and consequently reducing the heart rate.

In addition to influencing heart rate, muscarinic receptor activation can also lead to a decrease in contractility in the atrial muscle, although this effect is less pronounced in the ventricles. Therefore, the primary effect of muscarinic receptor activity in cardiac tissue is to reduce the heart rate, making the chosen answer correct.