Resting intracellular voltage of myocardial cell is –90mV (high intracellular K maintained by Na/K pump)
Sinoatrial node located at junction of superior vena cava and the right atrium.
Narrow originate in atria or AV node Broad originate from ventricles or from atria or AV node wirh aberrant concuction to the ventricles (LBBB or RBBB)
Re-entry is when tisue that is usually refractory is reactivated. Occurs if there is a ring of tissue that does not conduct normally. Usually the two paths of action potentials meet and die out, but if they do not meet then a circuit is set up. Abnormal pacemaker activity can occur if myocardial cells are damaged eg ischaemic heart disease also via catecholamine overactivity Delayed after-depolarisation occurs when in cardiac tissue that usually has to wait for the initiation of an action potential from pacemaker tissue, the cardiac cell automatically depolarises. This leads to a repetitive discharge (triggers ectopic beats -‘R on T phenomenon’) (related to extent of inward Ca current) caused by cardiac glycosides eg digoxin Re-entry (partial conduction block eg accessory pathways linking atria and ventricles eg Wolff-Parkinson-White syndrome) halted by drugs that prolong the refractory period
![Content ,[object Object],[object Object],[object Object],[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)