If you are a regular reader of this column you will remember that last time various types of muscle were discussed.

The main subject was skeletal muscle and I would like to apologise for the abrupt end to the last column or rather the lack of one that I understand was due to a printing error.

This week's column is going to focus on another type of muscle, namely cardiac. It is sincerely hoped that this one won't just suddenly stop! This is a highly specialised muscle type unique to the heart.

Each minute the heart of a resting adult pumps about five litres of blood, which is approximately the person's total blood volume.

Each day this works out to at least 7,200 litres per day. During strenuous exercise the rate of pumping may increase to 20 or 30 litres per minute. This amazing effort uses five per cent to ten per cent of the body's total energy budget.

The adaptations of the heart to the body's changing needs are complex and it has therefore been difficult to construct a mechanical heart with anything like the refinement of the human heart.

The force of the contractions within the ventricles is largely self-adjusting. There are three main types of myocardial cells that are, myocardial muscle, pacemaker cells and conducting cells.

The myocardial cells are the muscle cells that generate the force of the contraction in the ventricle and atria. The heart has a high degree of intrinsic regulation.

The pacemaker cells are the ones that create the action potential or nerve impulse that in turn initiates the heart beat and helps to regulate the heart rate. The conducting cells are the specialised cells that co-ordinate the contraction within the myocardium.

A clever characteristic of cardiac cells is that they are electrically coupled to one another so one action potential initiated by a single cardiac fibre can activate a contraction in the whole heart.

The pacemaker cells that initiate the nerve impulse are present in two main areas of the heart.

One is the sinoatrial node (SA) and is located near the junction of the right atrium and the major vein entering the heart, the superior vena cava.

Secondly, the atrioventricular node (AV) is positioned at the base of the two atria. The conducting cells determine the routes by which the impulses pass.

There are conducting fibres that travel from the SA node and create a contraction in the two atria.

The SA node relays with the AV node cells by way of the internodal fibres. These have some great names in my opinion.

The fibres that make up the tracks are called Purkinge fibres.

The Purkinge fibres make up the bundle of His that travels down from the AV node coordinating the contraction of the two ventricles. It really is an elegant system.

If a heart was removed from the body and maintained under the right conditions it would continue to beat.

When the intrinsic rhythm of the SA nodes and the AV nodes do not function well, disorders known as arrhythmias can arise.

One cause is when the action potential or nerve impulse fails to travel successfully from the SA node to the AV node.

This is called AV block and the block can be partial or complete.

In complete or third degree block AV block, the atria continue to beat at the rate set by the SA node but the ventricles beat at the slower rate of the AV node.

An AV block creates disco-ordination between the activities of the atria and the ventricles and slows the heart rate.

It is not necessarily life threatening because the ventricles can still fill adequately.

Pacemakers can assist a patient's heart with its own regulation. The fitting of a pacemaker is now a fairly straightforward procedure.

Some are set to regulate the heart constantly whilst others only go to work and create an electrical discharge when the heart delays or fails to create an intrinsic beat.

Another interesting procedure is that of cardioversion. This is a technique used when the heart is beating irregularly and involves putting the patient under a general anaesthetic.

A strong electric shock is applied to the heart and the heart is then 'reset' with the correct rhythm. This is similar to the scenes we are all familiar with on hospital dramas when the casualty is defibrillated.

There are some simple ways to look after your heart muscle.

Eat well and exercise. Don't smoke, drink or eat to excess. Boring possibly, but effective! If you have any questions or suggestions for future columns please contact me at Ilkley Chiropractic Clinic on 605060.