Does the parasympathetic nervous system increase or decrease heart rate?

Both the parasympathetic and sympathetic nervous systems innervate the heart. The parasympathetic nervous system functions in regulating heart rate through the vagus nerve, with increased vagal activity producing a slowing of heart reate. The sympathetic nervous system has an excitatory influence on heart rate and contractlitiy, and it serves as the final common pathway for controlling the smooth muscle tone of the blood vessels. Reference: Porth, C M (2007) Pathophysiology Concepts of Altered Health States, 2nd Ed. (p. 344) Lipponcott Williams & Wilkins, Philadelphia