Nerve cell refractory period

Generally, it is always only a very small part of the membrane that is depolarized during an action potential. The process can therefore be repeated again after a short refractory period, when the nerve cell is stimulated again. Conduction of the action potential on the surface of the nerve cell is based on the fact that the local increase in the membrane potential causes neighboring voltage-gated ion channels to open, so that the membrane stimulation spreads over the whole cell in the form of a depolarization wave. 

The excitable membrane of nerve axons, like the membrane of cardiac muscle (see Chapter 14) and neuronal cell bodies (see Chapter 21), maintains a resting transmembrane potential of -90 to -60 mV. During excitation, the sodium channels open, and a fast inward sodium current quickly depolarizes the membrane toward the sodium equilibrium potential (+40 mV). As a result of this depolarization process, the sodium channels close (inactivate) and potassium channels open. The outward flow of potassium repolarizes the membrane toward the potassium equilibrium potential (about -95 mV) repolarization returns the sodium channels to the rested state with a characteristic recovery time that determines the refractory period. The transmembrane ionic gradients are maintained by the sodium pump. These ionic fluxes are similar to, but simpler than, those in heart muscle, and local anesthetics have similar effects in both tissues. 

Besides the existence of a threshold for stimulation, excitable systems are characterized by the existence of a refractory period during which the response to further stimulation is reduced or even totally absent (Fitzhugh, 1961). As shown in fig. 5.33, the cAMP signalling system of D. discoideum shares this property. Represented in this figure is the ratio Aic/Aiii of the maxima of two successive peaks of intracellular cAMP, as a function of the time interval separating the second stimulus from the maximum of the first response, which defines time zero. For 4 min, no cAMP synthesis can be elicited by the second stimulus. This phase defines an absolute refractory period. Thereafter, the response to the second stimulus increases gradually, until the second maximum reaches the value of the first, after some 15 min. This second phase defines a relative refractory period. The two types of refractory period are known in other excitable systems, for example in nerve cells (Fitzhugh, 1961). 

Reduction, of number of variables, 18,200 to two-variable system, 183,213 to three-variable system, 203,234 Refractory period absolute, 212,213 in nerve cells, 212 relative, 212,213... 

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