How do pacemaker cells work

Closure of ion channels causes ion conductance to decrease. As ions flow through open channels, they generate electrical currents that change the membrane potential. In the SA node, three ions are particularly important in generating the pacemaker action potential. The role of these ions in the different action potential phases are illustrated in the above figure and described below:.

It is important to note that action potentials described for SA nodal cells are very similar to those found in the atrioventrcular AV node. AV nodal action potentials also have intrinsic pacemaker activity produced by the same ion currents as described above for SA nodal cells.

Cardiovascular Physiology Concepts Richard E. What is a pacemaker? The generator is a small battery-powered unit. It produces the electrical impulses that stimulate your heart to beat. The generator may be implanted under your skin through a small incision. The generator is connected to your heart through tiny wires that are implanted at the same time. The impulses flow through these leads to your heart and are timed to flow at regular intervals just as impulses from your heart's natural pacemaker would.

Some pacemakers are external and temporary, not surgically implanted. Why do I need one? Your doctor may recommend a pacemaker to make your heart beat more regularly if: Your heartbeat is too slow and often irregular. Your heartbeat is sometimes normal and sometimes too fast or too slow. How does it work? It replaces the heart's defective natural pacemaker functions. 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 depolarize by themselves. As in all other cells, the resting potential of a pacemaker cell mV to mV is caused by a continuous outflow or "leak" of potassium ions through ion channel proteins in the membrane that surrounds the cells.

The difference is that this potassium permeability decreases as time goes on, partly causing the slow depolarization. As well as this, there is a slow inward flow of sodium , called the funny current , as well as an inward flow of calcium. This all serves to make the cell more positive.

This relatively slow depolarization continues until the threshold potential is reached. Threshold is between mV and mV. When threshold is reached, the cells enter phase 0. Though much faster than the depolarisation caused by the funny current and decrease in potassium permeability above, the upstroke in a pacemaker cell is relatively slow compared to that in an axon.

The SA and AV node do not have fast sodium channels like neurons, and the depolarisation is mainly caused by a slow influx of calcium ions.

The funny current also increases. The calcium is let into the cell by voltage-sensitive calcium channels that open when the threshold is reached. The calcium channels are rapidly inactivated, soon after they open.

Sodium permeability is also decreased. Potassium permeability is increased, and the efflux of potassium loss of positive ions slowly repolarises the cell. Hormonal effects can also affect the heart. Noradrenaline and adrenaline also called Norepinephrine and Epinephrine respectively are both released into the bloodstream by the adrenal medulla.

They have the same action on heart rate as direct sympathetic stimulation. Template:WikiDoc Sources Cardiology. This article is about the natural pacemaker in the heart. For the article about the medical device that simulates the function, see artificial pacemaker.

Categories : Cardiology Electrophysiology. Cookies help us deliver our services. By using our services, you agree to our use of cookies. Namespaces Home Page Discussion. Views Read View source View history Help. WikiDoc Resources for Cardiac pacemaker. Unlike the sympathetic mechanism, the heart will slow quite soon after vagal stimulation.

Acetylcholine ACh is released from the vagus nerve endings, and binds to muscarinic receptors on the pacemaker cells.