Final answer:
A decrease in the SNS rate decreases MAP due to reduced heart rate and vasodilation. Conversely, a decrease in the vagal rate increases MAP because it leads to an increased heart rate. These effects are mediated by the autonomic nervous system, particularly the balance between sympathetic and parasympathetic influences.
Step-by-step explanation:
The effects of a decrease in Sympathetic Nervous System (SNS) rate or a decrease in vagal rate on Mean Arterial Pressure (MAP) can be understood in the context of the autonomic control over cardiovascular function. A decrease in SNS rate while holding the vagal rate constant leads to a lowering of MAP due to reduced heart rate and vasodilation. On the other hand, a decrease in vagal rate, which is the parasympathetic activity, while keeping the SNS rate constant results in an increase in MAP due to an increase in heart rate.
Baroreceptor reflexes play a central role in these processes. High blood pressure triggers increased baroreceptor firing that initiates parasympathetic stimulation, which reduces heart rate and leads to vasodilation, thus lowering MAP. Conversely, low blood pressure reduces baroreceptor firing rate, prompting increased sympathetic stimulation that raises heart rate and causes vasoconstriction, thereby increasing MAP.
In summary, a decrease in SNS rate leads to reduced MAP, while a decrease in vagal rate leads to an increase in MAP, due to the opposing roles of the sympathetic and parasympathetic nervous systems in cardiovascular regulation.