The myocardium is a smooth muscle which allows it to function autonomically. The muscle cells are connected together by intercalated disks and are composed of sarcomeres (actin and myosin filaments). Mechanically, contraction of sarcomeres occur when there is an influx of calcium (Ca++) ions into the myocardial cells that bind to troponin proteins on the actin filaments. This results in a release of ATP which produces shortening of the sarcomeres and muscle contraction.
A resting polarized action potential exists between the interior and the exterior of the cell membrane based on concentrations of cations on either side of the membrane. Potassium (K+) is the predominant cation intracellularly, and sodium (Na+) and calcium (Ca++) concentrations tend to be higher extracellularly. The resting action potential (-80mV) is maintained by various mechanisms along the cell membrane. Membrane proteins pump Na+ out of the cell and K+ into the cell. Intracellular anions that attract Na+ and K+ are too large to exit across cell membranes. Along the cell membrane are calcium channels which allow the influx of Ca++ during depolarization but are closed to calcium influx during the resting potential. Calcium ions are transported out of cells by pumps.
Stimulation of the cell membrane inactivates Na+ pumps allowing the influx of Na+ ions. Simultaneously, K+ ions move out of the cell. As the net membrane charge changes (-80mV → -35mV), calcium channels are opened facilitating the influx of Ca++ ions. (The net membrane charge is now +30mV). The action potential for myocardial contraction occurs through 4 phases:
- rapid depolarization due to Na+ influx
- plateau during which Ca++ influxes to complete depolarization
repolarization through closure of Ca++ channels
- reactivation of Na+, Ca++, and K+ pumps restoring the negative action potential across the cell membrane
The calcium influx during depolarization promotes muscle contraction.
Conduction System in the Heart
An "electrical impulse" travels across the myocardium of the heart depolarizing the resting potentials of the cell membrane as it travels across the cell. This impulse initiates in the Sinoatrial (SA) node and moves down specific pathways across the myocardium. The SA node is situated in the right superior wall of the right atrium. Through automaticity, it generates 60-100 impulses/minute. The inherent rate is generally 70 impulses/minute. However, certain factors may influence heart rate. Sympathetic (beta1-adrenergic) stimulation increases heart rate. Parasympathetic stimulation—cholinergic through the vagus nerve—slows heart rate.
From the SA node, the impulse travels across the right atrium along internodal tracts and across the left atrium through the Bachmann's bundle. As the impulse travels across the atria, it stimulates atrial depolarization and contraction.
The impulse arrives at the atrioventricular (AV) node located in the right interatrial septum above the tricuspid valve's septal leaflet. The AV node slows conduction from the atria into the ventricles to allow ventricles to fill with blood from atrial contraction. If the SA node and the atria fail to produce any impulse, the AV node has an inherent automaticity of 40-60 impulses/minute. However, as long as the AV node receives an impulse from higher up the conduction pathway, it will function as transmitter to the ventricular conduction pathways.
The conduction pathways through the ventricles begin with the common Bundle of His immediately leaving the AV node near the annulus of the tricuspid valve. The Bundle of His splits into the right and left bundle branches along the ventricular septum. The right bundle branch delivers the impulse across the right ventricle. The left bundle branch splits into the left anterior fascicle (which covers the anterior left ventricle) and the left posterior fascicle (which covers the posterior and inferior surfaces of the left ventricle).
At the end of the bundle branches are Purkinje fibers which stimulate depolarization of the ventricular myocardium. Should no impulse be transmitted from the bundle branches to the Purkinje fibers, they can initiate an impulse—through their own automaticity—with an inherent rate of < 40 impulses/minute.
Properties of the Myocardium
Myocardial muscle cells have unique properties.
- Excitability is the potential for a cell to depolarize and form an action potential when stimulated.
- Automaticity is the ability of a myocardial cell to initiate an impulse to begin the depolarization process.
- Conductivity is the cell's ability to conduct an impulse across the membrane and extend it to adjacent cells.
- Refractoriness is the cell's inability to generate another impulse during the periods of depolarization and repolarization.