Mechanisms that Affect the Heart Rate
Two different mechanisms affect the heart rate. Nerve impulse to the pacemaker region and hormonal influences. A branch of the vagues nerve of parasympathetic origin slows the heartbeat by releasing acetylcholine while another nerve of sympathetic origin releases epinepherine and norepinepherine at the pacemaker end to increase the heartbeat. Epinepherine is also a hormone that is released from the adrenal medulla into the blood. It accelerates heartbeat.
This you know is a well documented part of the fight-or-flight reaction. Apart from the vagus and sympathetic nerves, the pacemaker can be affected by temperature. This is observable when we have fever. Increased temperature increases the heart rate and fall in body temperature slows the heart rate.