While some autonomic targets receive input from only one division of the autonomic nervous system most targets are controlled by coordinated sympathetic and parasympathetic innervation. For example, the parasympathetic division via cholinergic output, promotes adsorption of food and digestion by enhancing salivary, gastric, intestinal and pancreatic secretions by increasing motility of the gut, and by relaxing pyloric and intestinal sphincters. In contrast, the catecholaminergic output of the sympathetic division decreases the motility and secretory activity of the gastrointestinal system. The sympathetic and parasympathetic systems have opposite effects on cardiac function, sympathetic output stimulates the heart rate and contractility while parasympathetic output reduces cardiac output. Both sympathetic and parasympathetic fibers innervating the heart are tonically active allowing a very rapid and controlled responses to conditions of stress etc For example.
Norepinephrine acts primarily thru b -adrenergic receptors to modulate several ionic currents.
Finally, Norepinephrine also acts on a -adrenergic receptors to increase peripheral vascular resistance therefore increasing venous return and increasing blood pressure. The parasympathetic system acts to decrease cardiac output, this is achieved by vagal activation of parasympathetic preganglionic neurons which innervate cholinegic ganglion neurons in the heart. The result is an increase in the resting K conductance. This hyperpolarizes the cell and slows conduction through the atrioventricular node. Acetylcholine also decreases heart rate by shifting the pacemaker current and thereby increasing its threshold for activation and decreasing heart rate.