Origin and Conduction of heart beat:
The heart beat starts in a tiny island of tissues present in the upper region of right auricle, called the sino-auricular node or sino-atrial node (S.A.). It is located near the opening of large vein. S.A. acts as the pace maker of the heart. The heart impulses generated here radiate out from the heart as a wave and cause each muscle fibre, through which they pass, to contract. A similar node is present in the partition dividing the right and left auricles. A bundle of fibres originates from the auriculoventricular node (AV). This is known as Bundle of His or Purkinje fibres.
The impulse after passing through the AV node passes along bundle of HIS, which happens to be the only direct muscular communication between auricles and ventircles. High in the interventricular septum the bundle of HIS divides into right and left bundle branches which are distributed respectively to the right and left ventricle. The bundle branches terminate in thin fibres called the Purkinje fibres through which the impulses reach the ventricular muscles.
The sino-auricular node sends out intermittent impulses. Each impulse initiates a wave of contraction, which passes through both the auricles and causes their contraction. When this wave of contraction alongwith the impulse reaches the auriculo-ventricular node, it is also’stimulated. The impulses from it make the ventricles contract.
While the ventricles contract the auricles relax. This sequence is repeated again and again. The whole system- the SA node, the AV node, bundle of HIS and the purkinje fibre systems constitute the conducting system of the heart. These are composed of modified cardiac muscle cells. The stimulus for the contraction of auricles and ventircles is electrical.
This sequnece of electrical discharge and the phases of the cardic cycle may be studied with the help of an instrument called the Electrocardiogram. This instrument can measure the electrical impulses of the heart muscles and give out the wave forms on a strip of paper and this is called the Electrocardiograph.