What type of nerve fibers slows the heart rate through autonomic innervation?

The correct response indicates that parasympathetic motor fibers are responsible for slowing the heart rate through autonomic innervation. The autonomic nervous system consists of two main branches: the sympathetic and the parasympathetic systems.

When activated, the parasympathetic nervous system primarily utilizes the neurotransmitter acetylcholine, which acts on the heart's sinoatrial (SA) node, often referred to as the heart's natural pacemaker. This interaction causes a decrease in heart rate. The vagus nerve, which is part of the parasympathetic system, plays a significant role in this process by transmitting signals that inhibit the SA node’s activity.

In contrast, sympathetic motor fibers generally promote an increase in heart rate and cardiac output by releasing norepinephrine, which enhances the firing rate of the SA node and stimulates the myocardium. Somatic motor fibers primarily control voluntary muscle movement rather than directly regulating heart function. Spinal motor fibers are also related to motor control and not directly involved in autonomic heart rate regulation.

Thus, the role of parasympathetic motor fibers in decreasing heart rate through acetylcholine release effectively illustrates their key function in autonomic innervation of the heart.