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Depolarization occurs in the four chambers of the heart: both atria first, and then both ventricles. The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which corresponds to the P wave on an electrocardiogram.
[1] [2] This means that all atrial cells can contract together, and then all ventricular cells. Different shapes of the cardiac action potential in various parts of the heart Rate dependence of the action potential is a fundamental property of cardiac cells and alterations can lead to severe cardiac diseases including cardiac arrhythmia and ...
Electrical waves track a systole (a contraction) of the heart. The end-point of the P wave depolarization is the start-point of the atrial stage of systole. The ventricular stage of systole begins at the R peak of the QRS wave complex; the T wave indicates the end of ventricular contraction, after which ventricular relaxation (ventricular diastole) begins.
This means that at low levels of recruitment, the force increment due to recruitment is small, whereas in forceful contractions, the force increment becomes much larger. Thus the ratio between the force increment produced by adding another motor unit and the force threshold at which that unit is recruited remains relatively constant.
A typical action potential begins at the axon hillock [41] with a sufficiently strong depolarization, e.g., a stimulus that increases V m. This depolarization is often caused by the injection of extra sodium cations into the cell; these cations can come from a wide variety of sources, such as chemical synapses, sensory neurons or pacemaker ...
These cells will be initiating action potentials and contraction at a much lower rate than the primary or secondary pacemaker cells. The SA node controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest.
In a normal muscular contraction, approximately 100-200 acetylcholine vesicles are released causing a depolarization that is 100 times greater in magnitude than a MEPP. This causes the membrane potential to depolarize +40mV (100 x 0.4mV = 40mV) from -100mV to -60mV where it reaches threshold.
It is usually due to ion channels in the cell membrane that spontaneously open and close (e.g. If channels in cardiac pacemaker cells). When the membrane potential reaches depolarization threshold an action potential (AP) is fired, excitation-contraction coupling initiates and the myocyte contracts. [citation needed]