Which autonomic nerve fibers slow down the heart rate?

The autonomic nervous system consists of two main branches: the sympathetic and parasympathetic systems. The parasympathetic motor fibers are responsible for promoting "rest and digest" functions, which include slowing down the heart rate. Specifically, the vagus nerve, a major component of the parasympathetic system, releases acetylcholine at the heart. This neurotransmitter acts on the sinoatrial (SA) node, the heart's natural pacemaker, to decrease the rate of electrical impulses, ultimately resulting in a slower heart rate.

In contrast, sympathetic motor fibers prepare the body for "fight or flight" responses, which would increase the heart rate. Sensory fibers primarily carry information from the body to the central nervous system rather than influencing heart rate directly. Somatic fibers, associated with voluntary control of skeletal muscles, do not play a role in heart rate regulation.

Thus, the role of parasympathetic motor fibers in reducing heart rate highlights their crucial function in maintaining homeostasis and promoting a state of calm in the body.