Final answer:
Increased vagal nerve impulses lead to a decrease in heart rate, as the parasympathetic system slows the heart's rate of depolarization through the release of acetylcholine.
Step-by-step explanation:
The heart rate (HR) can be influenced by various factors due to the autonomic nervous system's control. Specifically, an increase in vagal nerve impulses, which is part of the parasympathetic nervous system, will tend to decrease heart rate. The vagus nerve releases the neurotransmitter acetylcholine (ACh) which opens potassium channels in the cardiac muscle and slows the rate of spontaneous depolarization. This extends repolarization and results in a slower HR. This parasympathetic activity effectively acts like putting a 'break' on the heart's pace.
Stimulation of cardiac nerves, an increase of sympathetic nerve impulses, release of norepinephrine (NE) and epinephrine, and activation of neurons in the cardioaccelerator center all correlate with an increase in HR and are part of the sympathetic nervous system, which acts to increase the heart's work rate.
Therefore, out of the options provided, the correct factor that will tend to decrease heart rate is c. Increased vagal nerve impulses.