The cardiac muscle has a unique ability to depolarize and contract without the intervention of the nervous system. Even in the event that all the nerve connections to the heart are cut off, the heart will still continue to rhythmically beat.
The Intrinsic Conduction System
The activity of the heart is independent yet it is coordinated. This activity is attributed to the existence of gap junctions and also because of the heart’s own conduction system. The heart’s in-house intrinsic cardiac conduction system is composed of specialized cardiac cells whose main function is to distribute and initiate impulses throughout the heart. With this conduction system in place, the heart is able to contract and depolarize in a sequential, orderly manner. With all these mechanisms in place, the heart beats as one coordinated unit.
Action Potential Initiation
The cardiac muscle is unique in itself, but the autorhythmic cells found in the heart is nothing short of spectacular. Unlike contractile cells that are not stimulated of the heart, these cells have an unstable resting membrane potential. The autorhythmic cells compose the intrinsic conduction system and continuously and drifts as it depolarizes.
The innate characteristic of having unstable resting potential triggers the rhythmic contraction of the heart. This spontaneous change in the membrane potentials are called pacemaker potentials or sometimes referred to as prepotentials. The pacemaker potentials initial the action potential and spreads all over the heart.
The pacemaker potential is the product of changes that happen in the ion channels found in the sarcolemma. Potassium channels closes during hyperpolarization in relation to the end of an action potential. As the potassium channel closes, the sodium channels slowly open up. The sudden influx of sodium changes the balance between sodium entry and potassium loss thereby making the membrane interior more positive. At a certain point, the calcium channels then opens up which permits the entrance of calcium found in the extracellular space. To summarize the cell chemistry, the surge of calcium influences the rising phase of the action potential and in turn reverses the membrane potential.
Sequence of Excitation
The sinoatrial node, the AV bundle, AV node and Purknje fibers all contain autorhythmic cardiac cells. The SA node is has a monumental task in hand and that is to generate impulses for almost 75 times in every minute. No other region or mechanism of the conduction system can go head to head with the SA node; hence it is the one that sets the tone or pace of the beating heart. The SA node is also known as the cardiac pacemaker and the rhythm that the cardiac pacemaker makes is known as the sinus rhythm.
The atrioventricular node, when compared to the SA node conducts impulses more slowly. There is a 0.1 second delay in the impulse of the AV node, which allows the atria to complete its contraction and then followed by the ventricle. The atrioventricular bundle is also called the bundle of His. The bundle of His is the only electrical connection between the atria and the ventricle.