The sympathetic division of the autonomic nervous system is well know for mediating the "fight or flight" response. Sympathetic nerve signaling to the heart increases the heart rate by increasing phase 4 depolarization in pacemaker cells of the sinoatrial node.
Sympathetic nerve ending release norepinephrine onto pacemaker cells. Epinephrine reaches the heart by way of the blood. When the sympathetic division is stimulated, the adrenal glands release epinephrine. Both norepinephrine and epinephrine activate beta-adrenergic receptors on pacemaker cells which causes cyclic AMP levels to rise. Cyclic AMP activates funny channels..
See also: parasympathetic division.
Overview of the ANS: Merck Manual