Sympathetic Nervous System

At rest, the resting membrane potential of sinus node is -60 mV and the threshold for generation of impulse is -40 mV. The resting heart rate is approximately 70-75 bpm.

Upon sympathetic stimulation, the activation of β1 receptors in sinus node by neurotransmitter noradrenaline increases the resting membrane potential from -60 mV to -50 mV. This allows the threshold of the ion channels (-40 mV) to be reached more quickly (more rapid depolarisation), thus increasing the depolarisation frequencies. In other words, the heart rate increases (E.g.: 120 bpm)

In addition, sympathetic stimulation elsewhere in the myocardium promotes increased Ca2+ transport into the contractile muscle cells. The heart is then able to contract both faster and more powerfully, and consequently squeeze out more blood per heartbeat.