NERVOUS SYSTEM ::
Propagation of the action potential ::
A propagated action potential is a wave of excitation passing up or down through an axon At any one time only a protion of the axon is depolarized This local depolarization gives rise to a passive spread of current passing up and down in the adjacent regions of the axon threshold is reached and there is generation of action potential in a new region The propagation of the action potential is unidirectional because regions which have just been excited are refractory
factors affecting nerve conduction
The action potential mechanism for the propagation of action potential is relatively slow there are a number of local currents that help in faster spread of action potential Conduction velocity in principle can be enhanced by increasing the membrane space constant and by minimizing the membrane time constant the intracellular resistance (Ri) is dependent on fibre diameter because in larger fibre more ions are available to carry the current whereas the extracellular resistance (Ro) is not normally a limiting factor because of the large volume of extracellular fluid Thus increase in fibre diameter (thus decreasing Ri and increasing the space constant ) can increase the conduction velocity example is giant nerve fibre (lmm diameter ) of squid whose conduction velocity is 25 m/sec as compared to other nerve fibres (14uM diameter ) whose conduction velocity is just lm/sec
Myelination also increases the conduction velocity by increasing membrane resistance and hence the membrane space constant without increasing the time constant in addition to this myelination also restricts membrane current and the generation of action potential at the nodes of Ranvier the passing of an action potential from node to node is called saltatary conduction Because of myelination there is higher metabolic efficiency due to reduced flux of ions and hence the expenditure of energy required to restore ionic composition is reduced
Lastly conduction velocity may also be affected by cold anoxia compression and drugs
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