PHGY 209 Lecture Notes - Lecture 18: Resting Potential, Action Potential, Multiple Sclerosis
21 Feb 2018
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Sodium channels: open via positive feedback mechanism, sodium current increase with action potential; they inactivate to contribute to the decrease in membrane potential. Open substantially at the action potential threshold, allows for the action potential to occur. It is an all or none point, if the threshold is not reached the action potential will not occur: the size of the action potential across the neuron (axon) will never change, simply move over. For a few milliseconds after the action potential, the sodium channels are inactivated and the membrane is completely unexcitable. This period is called the absolute refractory period. This occurs because the inactivated sodium channels cannot open to depolarize the axon again. This sets the absolute upper limit on how many action potentials the axon can fire in a given time: this period varies depending on the neuron. The variation can be due to the function, tuning of the neuron, and type of neuron, etc.
