Which type of nerve fibers primarily dominate at rest affecting heart rate?

The correct choice reflects the fact that parasympathetic fibers are predominantly active at rest and play a crucial role in managing heart rate. The parasympathetic nervous system, particularly through the vagus nerve, exerts a calming influence on the heart. When the body is at rest, these fibers release acetylcholine, which slows down the heart rate by increasing the time it takes for the electrical impulses to travel through the heart's conduction system.

While sympathetic fibers are responsible for increasing heart rate during activities such as exercise or stress, they are not the primary influence at rest. Cardioaccelerator fibers are part of the sympathetic system that specifically contributes to heart rate acceleration, and they become more prominent during times of increased demand for oxygen and nutrients. Vagal fibers are a subset of the parasympathetic fibers that specifically innervate the heart, but the term "parasympathetic fibers" encompasses a broader range of neural influences on heart function at rest. Therefore, the strong presence of parasympathetic fibers at rest is what primarily affects the heart rate, leading to a lower resting heart rate and allowing the body to conserve energy.