Conducting Systems

These cells initiate and conduct electrical impulses within the heart to produce myocardial contraction. The system comprises of:

 THE SINOATRIAL NODE
Located at the junction of the superior vena cava and the right atrium. There are numerous automatic nerve endings in the node with parasympathetic fibres derived from the right vagus nerve. Blood supply is via the right coronary artery in approx 60% of people and the left in approx 40% of people.

 THE ATRIOVENTRICULAR JUNCTION
Comprises of the AV node and the Bundle of His. Its function is to delay the cardiac impulse to allow ventricular filling. Predominately supplied by the right coronary artery

The bundle of his is supplied by the left anterior descending coronary artery.

THE BUNDLE BRANCHES
The bundle of his bifurcates to form right and left bundle branches. These extend subendocardially along both sides of the intraventricular septum. The left bundle branch fans out and forms two fascicles, the left anterior and left posterior fascicle.

At the end of the bundle branches are the purkinje fibres which embed within the ventricular myocardium.

 CORONARY CIRCULATION
These vessels originate from the sinuses of valsalva

RIGHT CORONARY ARTERY
Runs forward to the atrioventricular groove and gives off a small branch to the SA node. It goes downwards and around the inferior margin to supply the right ventricular wall. It then winds around the heart to the posterior aspect and the posterior descending coronary artery.

THE LEFT CORONARY ARTERY
This branches at the AV grove to form the left anterior descending and the circumflex branch.

THE NERVE SUPPLY
The heart can beat even if removed from the body, however it is well supplied with sympathetic and parasympathetic nerve fibres which can modify the functions by changing the heart rate or strength of cardiac contraction.

CARDIAC ELECTROPHYSIOLOGY
Electrolyte concentrations within the cardiac cells and extra cellular fluid are a major contributing factor for electrical stimulation of the heart. The ions predominately involved in this initiation process are sodium potassium and calcium. The predominant intracellular ion is potassium whereas the predominant extra cellular ions are sodium and calcium.

In the normal resting phase the myocardial cell membrane is -90mV. The electrical difference across the cell membrane is maintained by the sodium pump, which transfers potassium ions into the cell five times faster than sodium ions. Should an electrical stimulus reach the cell membrane the permeability is altered allowing a change in ionic concentrations and depolarisation of the cell.