Refractory Period (physiology) - Neuronal Refractory Period

Neuronal Refractory Period

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 typically -70 mV toward a positive potential). In other words, the membrane is made less negative. After the potential reaches the activation threshold (-55 mV), the depolarization is actively driven by the neuron and overshoots the equilibrium potential of an activated membrane (+40 mV).

Phase two is repolarization. During repolarization, voltage-gated sodium ion channels inactivate (different from the close state) due to the now-depolarized membrane, and voltage-gated potassium channels activate (open). Both the closing of the sodium ion channels and the opening of the potassium ion channels act to repolarize the cell's membrane potential back to its resting membrane potential.

When the cell's membrane voltage undershoots 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. This potassium conductance eventually drops and the cell returns to its resting membrane potential.

The refractory periods are due to the inactivation property of voltage-gated sodium channels and the lag of potassium channels in closing. Voltage-gated sodium channels have two gating mechanisms, the activation mechanism that 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 do not 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.