| Popis: |
Much of the action of the nervous system can be ultimately associated with changes in the potential of individual neurons. Inhibition plays an essential role in neuronal interactions but it is the excitation provided by postsynaptic potentials and action potentials that generally determines neuronal output. Since excitation is usually associated with depolarization, maintained excitatory states of neurons require mechanisms of maintained depolarization. Sodium channels are one common channel type that select for ions whose electrochemical gradient produces an inward depolarizing current. These channels generally have strongly voltage-dependent inactivation mechanisms that prevent continued channel opening during depolarization. While this action prevents prolonged positive feedback action through the Hodgkin cycle, it makes these channels unavailable for maintaining a depolarized state of the channel. Calcium channels, the other large class of channels producing depolarization, exhibit both voltage-dependent and Ca2+-dependent inactivation. One type of calcium channel, the HVA class, is capable of remaining open for hundreds of milliseconds but calcium channels in general are not suited to providing current for maintained depolarization. |
