<P> Sinus node dysfunction describes an irregular heartbeat caused by faulty electrical signals of the heart . When the heart's sinoatrial node is defective, the heart's rhythms become abnormal--typically too slow or exhibiting pauses in its function or a combination, and very rarely faster than normal . </P> <P> Blockage of the arterial blood supply to the SA node (most commonly due to a myocardial infarction or progressive coronary artery disease) can therefore cause ischaemia and cell death in the SA node . This can disrupt the electrical pacemaker function of the SA node, and can result in sick sinus syndrome . </P> <P> If the SA node does not function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker . </P> <P> The sinoatrial node was first discovered by a young medical student, Martin Flack, in the heart of a mole, whilst his mentor, Sir Arthur Keith, was on a bicycle ride with his wife . They made the discovery in a makeshift laboratory set up in a picturesque farmhouse in Kent, England, called Mann's Place . Their discovery was published in 1907 . </P>

When does the sinoatrial node receive a stimulus to initiate a heart beat