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Bavidra kulendrarajah Describe the generation of an action potential in smooth and cardiac muscle The cardiac muscle is myogenic as it can generate its own action potential at a constant rate which leads to the heart pumping in a regular rhythm. The cardiac action potential originates in a specialised knot of myocytes known as the sinoatrial node which is found in the right atrium at the upper end of the crista terminalis. In these myocytes the membrane potential depolarises spontaneously and once threshold is reached an action potential is generated which propagates to other myocytes within the heart which are connected by gap junctions. As the SAN is responsible for generating the action potential at a regular rate of about 60-100 beats per minute it is often referred to as the pacemaker, however its activity can be modulated by inputs of the autonomic nervous sytem; the vagus nerve decreases the heart rate whereas sympathetic nerves increase the heart rate. Once the action potential is generated in the SAN it is then propagated to the right and then the left atrium within about 40ms. The impulse is then propagated to the atrio-ventricular node which is found in the base of the right atrium. The impulse is prevented from spreading to the ventricles due to the atrioventricular ring made of non conduction fibrous tissue. In order for the impulses to reach the ventricles the impulses are propagated through the atrioventricular bundle of HIS which originates from AV node and divides into a left and right branch either side of the interventricular septum. This bundle then terminates as purkinje fibres which conduct action potential to the ventricular myocytes and causes simulataneous contraction in both ventricles from the apex upwards. The action potentials generated in different regions of the heart vary in their initiation time, shape and duration. This occurs because the heart is a heterogenous organ where the myocytes in different regions are specialised for specific functions and this results in the cells having a differing permeability to different ions due to them having a distinct set of ion channels. Another factor affecting action potential is that different regions of the heart have differing membrane capacitance. Myocytes that are found in the SAN, AVN are specialised in generating action potentials, whereas atrial and ventricular myocytes are specialised in conduction and the myocytes that make up the bundle of his and purkinje fibres are specialised in conducting the impulses. Sino-atrial node The heart has three intrinsic pacemaking tissues which are the SA, AV nodes and the purkinje fibres. The key function of these tissues is to spontaneously depolarise the membrane to threshold and generate action potentials at a regular rhythm. The primary pacemaker is the SAN node as it generates the highest frequency of action potentials and it is these that are propagated around the heart. However if the myocytes in the SAN are damaged, the role of setting the heart rate is taken over by the AVN node. The maximum diastolic potential that occurs during phase 4 (occurs after repolarisation) is roughly between -60mv and -70mv. In comparison to the atrial and ventricular myocytes the resting membrane potential is less negative and this is due to the large influx of sodium ions through the sodium ion channels. However the resting membrane potential is not stable and it gradually depolarises to a threshold of about -55mv. The main reason why the
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