Polarization, membrane, action potential

Membranes create action potentials by actively changing their permeabilities to ions such as sodium and potassium. A comparison of the results of Equation 19.8 and Equation 19.9 shows a swing of about 100 mV between the resting and excited states (polarized to depolarized). The change of permeability from resting to excited values and then back again allows the membrane to generate an action potential. 

Action potential A rapid change in the polarity of the voltage of a cell membrane from negative to positive and back to negative a wave of electrical discharge that travels across a cell membrane. 

Neurons constitute the most striking example of membrane polarization. A single neuron typically maintains thousands of discrete, functional microdomains, each with a distinctive protein complement, location and lifetime. Synaptic terminals are highly specialized for the vesicle cycling that underlies neurotransmitter release and neurotrophin uptake. The intracellular trafficking of a specialized type of transport vesicles in the presynaptic terminal, known as synaptic vesicles, underlies the ability of neurons to receive, process and transmit information. The axonal plasma membrane is specialized for transmission of the action potential, whereas the plasma... 

Fig. Id represents the ionic changes and reversal of polarity of the membrane when the nerve is stimulated. Na+ ions enter the membrane ahead of the electrical charge and K+ ions pass out at the peak of the potential reversal.1 Fig. le shows how the ionic interchange is related to the action potential (or magnitude of polarity change). It must be stressed that the actual percentage changes of concentration are very small indeed. The exact nature of the restoration of the original concentration of ions is not completely known. Obviously a source of energy is required, and this is considered to be derived from the metabolism of the cell.

Reduction, neutralization, or change in direction of polarity. 2. Transient reduction in the membrane potential. See Action Potential Hyperpolarization... 

With respect to membranes, an increase in the polarization of a membrane. See Depolarization Action Potential... 

These equations offer an adequate basis for the development of the negative membrane potential of 70 to 90 mV. Excitation as a process characterizing nerve and muscle cells is associated with a transient reduction or abolition of this membrane potential, and in some cases with a temporary "overshoot" or reversal of its polarity. Just as for the membrane potential, these major but transient perturbations in the production of action potentials have been adequately modeled in dynamics of ionic equilibria by Hodgkin and Huxley (2). 

The important point to note is that once an action potential has fired at the neck of the axon, it has reversed the polarity of the membrane at the point. This in turn has an effect on the neighbouring area of the axon and depolarizes it beyond the critical threshold level. It too fires an action potential and so the process continues along the whole length of the axon (see Fig. A2.5). 

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