Final answer:
A decrease in SNS rate leads to a decreased heart rate, while a decrease in vagal rate results in an increased heart rate, assuming the other remains constant in each scenario.
Step-by-step explanation:
To assess the effects of altering the Sympathetic Nervous System (SNS) rate and the vagal rate on heart rate, it is important to first understand their standard functions. The sympathetic nerves increase heart rate, while parasympathetic nerves, mainly the vagus nerve, decrease heart rate. A decrease in SNS rate, while holding the vagal rate constant, results in a reduced release of norepinephrine, which leads to a decrease in heart rate. Conversely, a decrease in vagal rate reduces the release of acetylcholine (ACh) and increases spontaneous depolarization, which leads to an increase in heart rate.
Therefore, the correct answer is that a decrease in SNS rate while holding the vagal rate constant decreases heart rate, and a decrease in vagal rate while holding the SNS rate constant increases heart rate.