What type of autonomic nerve fibers directly slow the heart?

The correct response highlights the role of parasympathetic motor fibers in regulating heart rate. These fibers are part of the autonomic nervous system, which operates involuntarily to control various bodily functions.

Parasympathetic fibers, particularly those that arise from the vagus nerve, release acetylcholine, which binds to receptors on the pacemaker cells of the heart located in the sinoatrial node. This binding slows down the heart rate by decreasing the rate of action potentials generated by these cells. Consequently, the overall effect of parasympathetic activation is to promote a state of rest and relaxation in the body, often termed "rest and digest."

In contrast, sympathetic motor fibers, which are part of the same autonomic system, primarily serve to increase heart rate and contractility when the body is under stress or in "fight or flight" situations. Voluntary motor fibers are responsible for the control of skeletal muscle movements and do not play a direct role in heart rate regulation. Sensory nerve fibers, on the other hand, carry information from the body to the central nervous system but do not directly control heart rate. Thus, the function of parasympathetic motor fibers in reducing heart rate makes it the accurate choice for this question.