2.2.4

Describe the intrinsic and extrinsic regulation of heart rate and the sequence of excitation of the heart muscle.

• The electrical impulse is generated by the sinoatrial node (SA node), with is called the “pacemaker”. This sets the rate of depolarization to the whole heart.
• The internodal tract then carries the impulse throughout the atria walls and the atria contracts.
• It then travels across the atria to the atrioventricular node (AV node), where the impulse is delayed to allow filling of the ventricle.
• The AV bundle (or bundle of His) carries the impulse for the atrium down through the septum
• The purkinje fibers then carry the impulse up the walls of the ventricle, which results in the contraction of the ventricles.

The sympathetic and parasympathetic branches of the automatic nervous system also influence heart rate.

A number of other factors, including hormones (adrenaline), ion concentration, exercise, and change in body temperature can influence heart rate.