Which nerve serves both the respiratory and heart functions?

The vagus nerve is crucial as it is a key player in the autonomic nervous system, which controls involuntary bodily functions. This nerve extends from the brainstem throughout the body, providing extensive innervation to various organs. It has a significant role in both respiratory and cardiac functions.

In terms of respiratory function, the vagus nerve helps regulate the rate and depth of breathing by sending signals to and from the lungs and the brain. It influences bronchoconstriction and modulates mucus secretion which is essential for maintaining clear airways.

For heart function, the vagus nerve has a critical impact on heart rate through its parasympathetic control. It releases neurotransmitters that can slow down the heart rate, effectively counteracting the sympathetic nervous system's fight-or-flight responses.

The other nerves listed have distinct functions that do not overlap with both respiratory and heart control. For instance, the phrenic nerve primarily controls the diaphragm for breathing but does not have a direct role in cardiac function. The radial nerve primarily serves the muscles of the arm and hand for movement and sensation, while the facial nerve is involved in facial expressions and taste sensation rather than the autonomic control of vital functions such as respiration and heart rate.