Excitation of Heart - Circulation
The heart has an inherent capacity to contract rhythmically without any external stimulus. Proof of this is obtained when we see that the heart in a developing chick embryo begins to beat even before any nerve has reached it and also heart muscle cells grown in culture contract rhythmically without any external stimulus. The electrical activity begins in a pacemaker region of the heart. Pacemaker cells are capable of spontaneous activity and may be either neurous as in many invertebrate hearts or muscle cells. If the heartbeat is initiated in a neuron, the pacemaker is called neurogenic pacemaker and if the heartbeat is initiated in muscle cell then the pacemaker is known as myogenic pacemaker.
Based on the type of pacemaker, hearts are classified as neurogenic or myogenic. The contraction of mammalian heart begins in a small piece of embryonic type muscle located where the vena cava enters the right atrium. This is known as sinoatrial node (S-A node). The contraction spreads to the two atria, and after a slight delay to the atrioventricular node (A-V node). From here it spreads rapidly to the tip of the ventricles through special muscle fibres, from where it spreads more slowly up the walls of the ventricles. This arrangement allows the contraction to begin at the tip of the ventricles and to spread upwards to squeeze out blood in the most efficient way. It also ensures that both ventricles contract simultaneously.