Final answer:
The brainstem directly regulates autonomic fiber activity through several nuclei, including the cardiovascular center, solitary nucleus, and dorsal motor nucleus of the vagus nerve.
Step-by-step explanation:
The brainstem directly regulates autonomic fiber activity. Several brainstem nuclei are important for the visceral control of major organ systems. The cardiovascular center, located in the medulla, regulates cardiac function and blood pressure. Other brainstem nuclei, such as the solitary nucleus and the dorsal motor nucleus of the vagus nerve, contribute to the parasympathetic and sympathetic control of the heart rate and other autonomic functions.