Phases of a Cardiac Cycle

Topics: Heart, Electrical conduction system of the heart, Cardiac anatomy Pages: 4 (1324 words) Published: October 11, 2010
Describe the Cardiac Cycle

The Cardiac cycle is a description of the activities that take place in the period from the start of one heart the next. The conducting system autonomically controls the heart beat impulses it is also called the cardiac conduction system or the nodal system. It is made up of six main components the sinoatrial node (SA-Node), internodal paths, atrioventricular node (AV-Node), atrioventricular bundle (AV bundle) or bundle of His, bundle branches and Purkinje fibres. Contractile cells make up about ninety nine percent of the cardiac muscle tissue on the walls of the aria and ventricles of the heart. The conducting cycle begins at the (SA-Node) when the Vagus nerve stimulates an action potential in the sinoatrial node. This leads to the stimulation of the contractile cells which are stimulated autonomically by Purkinje fibres.

The sinoatrial node (SA-Node) is made up of pacemaker cells and therefore is know as the hearts natural pace maker functioning to maintain a normal heart rate. Stimuli such as exercise, stimulant drugs, or hormones can change the rate of the SA-Node. The SA-Node is attached to the upper posterior wall of the right atrium near the superior vena cava. The SA-Node has specialized muscle cells which generate spontaneous action potentials which takes about fifty milliseconds to spread to the atria-ventricular node, or AV node. These electrical impulses are generated at the SA-Node one hundred to one hundred and ten per minute. After fifty milliseconds when the action potential moves through the atria surface via the internodal pathways to reach the AV node. During the transition of the action potential moving through the internodal path conducting cells stimulate the contractile cells of the atria.

The AV node is found in the inferior-posterior region of the right atrium. After the impulse has left the internodal path the impulse starts to slow down this is because the AV node cells have a smaller diameter...
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