The normal heart rate is called the basal heart rate, and this can deviate from the normal by external signals from the nervous and hormonal system. The nervous system controls the heart rate involuntarily by medulla oblongata. With this brain stem, two nerves, sympathetic and parasympathetic, are connected. These nerves can release chemical neurotransmitters to increase or decrease the heart rate. The SAN causes contraction in the left and right atria of the heart by transmitting electrical impulses. Then these impulses travel down the purkyne tissues to contract left and right ventricles.
· The sympathetic nerve produces noradrenaline that can bind to receptors on Sino atrial nodes (SAN) in the heart, hence causing more rhythmicity.
· The parasympathetic nerve releases acetylcholine to decrease heart rate by decreasing rhythmicity.