The neuron as a dynamic electrogenic machine: modulation of sodium-channelexpression as a basis for functional plasticity in neurons

Neurons signal each other via regenerative electrical impulses (action pote ntials) and thus carl be thought of as electrogenic machines. Voltage-gated sodium channels produce the depolarizations necessary for action potential activity in most neurons and, in this respect, lie close to the heart of t he electrogenic machinery. Although classical neurophysiological doctrine a ccorded 'the' sodium channel a crucial role in electrogenesis, it is now cl ear that nearly a dozen genes encode distinct sodium channels with differen t molecular structures and functional properties, and the majority of these channels are expressed within the mammalian nervous system. The transcript ion of these sodium-channel genes, and the deployment of the channels that they encode, can change significantly within neurons following various inju ries. Moreover, the transcription of these genes and the deployment of vari ous types of sodium channels within neurons of the normal nervous system ca n change markedly as neurons respond to changing milieus or physiological i nputs. As a result of these changes in sodium-channel expression, the membr anes of neurons may be retuned so as to alter their transductive and/or enc oding properties. Neurons within the normal and injured nervous system can thus function as dynamic electrogenic machines with electroresponsive prope rties that change not only in response to pathological insults, but also in response to shifting functional needs.