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The cells that create these rhythmical impulses are called pacemaker cells, and they directly control the heart rate. Artificial devices also called pacemakers can be used after damage to the body's intrinsic conduction system to produce these impulses synthetically.
Although all of the hearts cells possess the ability to generate these electrical impulses (or action potentials), a specialised portion of the heart, called the sinoatrial node, is responsible for the whole heart's beat.
The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiac myocytes. They possess some contractile filaments, though they do not contract.
Cells in the SA node will naturally discharge (create action potentials) at about 70-80 times/minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.
If the SA node doesn't 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 the heart's pacemaker. These cells form the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum.
The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.
Further down the electrical conducting system of the heart, the Bundle of His, the left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential if they aren't inhibited by other electrical activity. These tertiary pacemakers fire at a rate between 30-40 per minute.
Even individual cardiac muscle cells will contract rhythmically on their own.
The reason the SA node controls the whole heart, is that its action potentials are released most often, this triggers other cells to generate their own action potentials. In the muscle cells, this will produce contraction. The action potential generated by the SA node, passes down the cardiac conduction system, and arrives before the other cells have had a chance to generate their own spontaneous action potential. This is the normal conduction of electrical activity within the heart.
There are three main stages in the generation of an action potential in a pacemaker cell. Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system. This can lead to some confusion. There is no phase one or two, just phases zero, three and four.
The key to the rhythmical firing of pacemaker cells is that, unlike muscle and neurons, these cells will slowly depolarise by themselves.
As in all other cells, the resting potential of a pacemaker cell (-60mV to -70mV) is caused by a continuous outflow or "leak" of potassium ions through ion channel proteinsIn cell biology, an integral membrane protein IMP is a protein that is situated in the cell membrane and provides some sort of function for the cell. Most commonly this function is to act as a transporter for various molecules that would otherwise not be in the membraneA component of every biological cell, the cell membrane (or plasma membrane is a thin and structured bilayer of phospholipid and protein molecules that encapsulate the cell. It separates a cell's interior from its surroundings and controls what moves in a that surrounds the cells. The difference is that this potassium permeability decreases as time goes on, partly causing the slow depolarisation. As well as this, there is an slow inward flow of sodiumSodium is the chemical element in the periodic table that has the symbol Na Natrium in Latin) and atom number 11. Sodium is a soft, waxy, silvery reactive metal belonging to the alkali metals that is abundant in natural compounds (especially halite)., called the 'funny' current. This all serves to make the cell more positive.
This relatively slow depolarisation continues until the threshold potential is reached. Threshold is between -40mV and -50mV. When threshold is reached, the cells enter phase 0.
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