Cardiac Control (Myogenic hearts) summary

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Myogenic Hearts The spontaneous rhythmic beating of the heart is the result of electrical activity that originates from a pacemaker. This electrical activity spreads throughout the heart causing it to contract. In vertebrates, the pacemaker cells are located in the sinoatrial (SA) node in the upper region of the right atria. The pacemaker cells spontaneously depolarise and generate action potentials (AP) that then spread throughout the heart. Although the activity of the pacemaker can be influenced by nervous input, it does not need any input in order to depolarise. A myogenic heart is one in which the pacemaker cells are specialised muscle cells. The electrical activity (action potential) generated in the SA node spreads in an orderly fashion throughout the heart. First, the wave of depolarisation (action potentials) spreads throughout both atria. The cells in the atria are large and therefore have a low resistance to the flow of electrical current. This allows the wave of depolarisation to pass very quickly throughout the atria. A fibrous septum between the atria and the ventricles prevents the spread of electrical activity from the atria to the ventricles at all but one location the atrioventricular (AV) node). The wave of depolarisation that swept through the atria converges at the AV node. Conduction through the AV node is slow due to the fact that the cells in the AV node are quite small and have a large resistance to the flow of current. This slow transition through the AV node is important because it allows time for the atria to fully contract before the wave of depolarisation hits the ventricles causing them to contract. Since contraction of the atria is (in part) involved in the filling of the ventricles with blood, it is important to allow the atria to completely contract and eject blood into the ventricles before the ventricles pump it out through the aorta or the pulmonary artery. From the AV node, the wave of depolarisation moves down the Bundle of His and into the right and left branch bundles. The Bundle of His and the branch bundles are located in a fibrous septum that separates the right and left ventricle. As the electrical activity spreads down the branch bundles there is no depolarisation (or contraction) of the cardiac muscle itself. The electrical activity remains in the branch bundles within the fibrous septum. The wave of depolarisation leaves the branch bundles and enters Purkinjie fibres. The Purkinjie fibres move upward into the cardiac muscle. The wave of depolarisation passes from the Purkinjie fibres into the muscle causing the muscle cells to depolarise and the ventricles to contract. Since the wave of depolarisation moves from the bottom (apex) of the heart upward, contraction of the ventricles occurs from the bottom to the top of the ventricles. This is important because the vessels leaving the ventricles (aorta and pulmonary artery) are at the top of the ventricles. This pattern of contraction means that blood is being forced upward in the ventricles toward the blood vessels. The cells in the heart are electrically joined (or coupled) together by gap junctions. Within the gap junction there is a channel that connects one cell to the next cell. This means that electrical activity can directly pass from one cell to the next without the need of a neurotransmitter. This
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