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The refractory period in a neuron occurs after an action potential and generally lasts one millisecond. An action potential consists of three phases. Phase one is depolarization. During depolarization, voltage-gated sodium ion channels open increasing the neuron's membrane conductance for sodium ions and depolarizing the cell's membrane potential (from typ. -70mV towards a positive potential). In other words, the membrane is made less negative. After the potential reaches the activation threshold (-35mV) the depolarisation is actively driven by the neuron and overshoots the equilibrium potential of an activated membrane (+40mV). Phase two is repolarization. During repolarization, voltage-gated sodium ion channels inactivate (different than the close state) due to the now depolarized membrane, and voltage-gated potassium channels activate (open). Both the sodium ion channels closing and the potassium ion channels opening act to repolarize the cell's membrane potential back to its resting membrane potential. When the cell's membrane voltage overshoots its resting membrane potential (near -60mV), the cell enters a phase of hyperpolarization. This is due to a larger than resting potassium conductance across the cell membrane. Eventually this potassium conductance drops and the cell returns to its resting membrane potential.

The refractory periods are due to the inactivation property of voltage-gated sodium channel and the lag of potassium channels in closing. Voltage-gated sodium channels have two gating mechanisms, opens the channel with depolarization and the inactivation mechanism that closes the channel with repolarization. While the channel is in the inactive state it will not open in response to depolarization. The period when the majority of sodium channels remain in the inactive state is the absolute refractory period. After this period there are enough voltage-activated sodium channels in the closed (active) state to respond to depolarization. However, voltage gated potassium channels that opened in response to depolarization don't close as quickly as voltage gated sodium channels return to the active closed state. During this time the extra potassium conductance means that the membrane is at a higher threshold and will require a greater stimulus to cause action potentials to fire. This period is the relative refractory period.

Category:Physiology

Category:Cardiac electrophysiology

Category:Neurophysiology

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Adapted from the Wikipedia article Refractory period (physiology), under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki