Cell to Cell spread
Electrical excitation (generated by the pacemaker cells) is spread cell-cell via gap junctions. These allow the free movement of ions between cells.
Interatrial pathway and the Internodal pathway. The former allows for almost simultaneous contraction of both atria due to the rapid electrical spread via gap junctions. The latter directs this excitation towards the atrioventricular node, which (because there is a layer of fibrous, non-conducting tissue between ventricles and atria) is the only point of electrical communication between the atria and ventricles.
The AV node
The atrio-ventricular node is located at the base of the right atrium, just above the junction of the atria and ventricles. The AV node is the ONLY point of contact between atria and ventricles i.e. in order from SA activity to reach the ventricles, it must go through the AV node.
The AV node cells are unique in that they are smaller in diameter and thus have a smaller conduction velocity. This has two major implications:
- The pacemaker activity of the AV node produces a slower firing rate (around 40-60/min compared with the SA node’s 70-80/min).
- However, the signal generated by the SA node, if it reaches the AV node, overrides this.
- The signal from the SA node is delayed at the AV (slow conduction) to allow atrial systole to precede ventricular systole.
Bundle of His and Purkinje fibres
After excitation reaches the AV node and atrial systole has ended, it spreads rapidly down the ventricles via specialised cells that form the Bundle of His (of which there is a left and a right bundle) and to the rest of the myocardium via similar Purkinje fibres. NB Purkinje fibres do not excite every cardiac cell- full excitation is completed by gap junction spread.