mechanism of pacemaker
Answers
The cells that create these rhythmic impulses, setting the pace for blood pumping, are called pacemaker cells, and they directly control the heart rate. ... In most humans, the concentration of pacemaker cells in the sinoatrial (SA) node is the natural pacemaker, and the resultant rhythm is a sinus rhythm.
This is a common misunderstand- ing. A pacemaker does not actually beat for the heart, but delivers en- ergy to stimulate the heart muscle to beat. Once someone stops breathing, his body can no longer get oxygen and the heart muscle will die and stop beating, even with a pacemaker.
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The proper expression and function of the cardiac pacemaker is a critical feature of heart physiology. The sinoatrial node (SAN) in human right atrium generates an electrical stimulation approximately 70 times per minute, which propagates from a conductive network to the myocardium leading to chamber contractions during the systoles. Although the SAN and other nodal conductive structures were identified more than a century ago, the mechanisms involved in the generation of cardiac automaticity remain highly debated. In this short review, we survey the current data related to the development of the human cardiac conduction system and the various mechanisms that have been proposed to underlie the pacemaker activity. We also present the human embryonic stem cell-derived cardiomyocyte system, which is used as a model for studying the pacemaker. Finally, we describe our latest characterization of the previously unrecognized role of the SK4 Ca2+-activated K+ channel conductance in pacemaker cells. By exquisitely balancing the inward currents during the diastolic depolarization, the SK4 channels appear to play a crucial role in human cardiac automaticity.
The pacemaker activity of a cell can be defined as its ability to spontaneously and cyclically generate an electric signal. To achieve this, the membrane potential of the SAN cell needs to slowly depolarize to a threshold, which is a specific voltage required for the generation of an AP. This depolarization, called “Diastolic Depolarization” (DD) or “pacemaker depolarization”, results from a small net inward current flowing across the cell membrane and is the key feature of cardiac automaticity. DD occurs during diastole at the end of an action potential and is responsible for triggering the next action potential. Without diastolic depolarization, the heart would simply be unable to beat spontaneously. A crucial requirement for rhythmic automaticity is the existence of inward currents at diastolic potentials and a subtle dynamic integration of the sarcolemmal ion channels, transporters and Ca2+ cycling proteins.
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