BC Biomedical NIBP-1010 Bedienungsanleitung PDF herunterladen (Seite 21)

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We distinguish between two essential types of action potentials:

1) Action potential with fast depolarization

This is typical namely for the cells of the working myocardium, see Fig. 2.5. When

reaching the threshold value of the stimulus, the fast Na

channels open and these

cations flow from the extracellular area quickly inside the cell. Fast speed of the flow

is given by the concentration and electrical gradient and the high amount of the Na

channels. 1-2 ms later, the channels are inactivated and remain in this state until the

late repolarization. At this moment, the electrical gradient is switched, too. Partial

repolarization is characteristic for the decrease of the membrane potential by 5-10 mV,

and is caused by the closing of the fast Na

channels and a short-term opening of the

channels (K

flowing outside from the cell). During the plateau stage, the

membrane potential remains positive (10-15 mV) and the movement of ions through

the cell’s membrane remains relatively balanced. The channels for K+ are partially

open (K+ flowing outside from the cell), and thanks to the membrane potential’s value

being near to zero, also the Ca

channels open (Ca

flowing inside). When the flow

of the cations outside from the cell prevails due to the inactivation of the Ca

channels, the membrane potential decreases, the permeability for the K

ions

increases, and the resting membrane potential is restored.

2) Action potential with slow depolarization

This is typical for the cells of the SA and the AV nodes, see Fig. 2.5. It is mostly

different in less negative and non-constant value of the resting potential, which slowly

approximates the trigger level. This phenomenon is called a spontaneous diastolic

depolarization and the cells can thus function as the source of stimuli. Another

difference is represented by the speed of the depolarization. It is lower, because it is

subject to the opening of the slow specific Ca

channels (as opposed to the fast Na

channels). As the sodium cations do not participate in this phenomenon, the stage of

the partial repolarization is missing [2.3], [2.4].

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BC Biomedical NIBP-1010 Bedienungsanleitung Seite 21