Refractory period

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A refractory period, in physiology, is a period of time during which an organ or cell is incapable of repeating a particular action.

[edit] Refractory period in biology

Biologically, a refractory period is the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation.

[edit] Action potentials in Neurons

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 (usually from -70mV towards 0). Phase two is repolarization. During repolarization, voltage-gated sodium ion channels inactivate (close) 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 (generally -70mV), 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, one 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 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 lower threshold and will require a greater stimulus to cause action potentials to fire. This period is the relative refractory period.

[edit] Refractory period in Sex

In sexual intercourse, the refractory period is a recovery phase after male ejaculation during which it is physiologically impossible for almost all men to experience sustained erection and additional ejaculations or orgasms. The penis may be hypersensitive and further sexual stimulation may even feel painful during this time frame.

The refractory period varies widely between individuals and across species, ranging from minutes to hours. An increased infusion of the hormone oxytocin during ejaculation is believed to be chiefly responsible for the refractory period and the amount by which oxytocin is increased may affect the length of each refractory period. Another chemical which is considered to be responsible for this effect is prolactin, which represses dopamine, which is responsible for sexual arousal.

Unlike men, some women do not experience a refractory period immediately after orgasm and in many cases are capable of attaining additional, multiple orgasms through further stimulation (mainly oral-genital stimulation or through masturbation). The female sexual response is more varied than that of men, and there are many women who experience clitoral hypersensitivity after orgasm, which effectively creates a refractory period. These women may be capable of further orgasms, but the pain involved in getting there makes the prospect undesirable.

[edit] Refractory period in cardiac physiology

As in nerve cells, cardiac cells have ion currents that flow into and out of the cell. The flow of ions translates into a change in the voltage of the inside of the cell relative to the extracellular space. As in nerve cells, this characteristic change in voltage is referred to as an action potential. Unlike nerve cells, the cardiac action potential duration is closer to 100 ms (with variations depending on cell type, autonomic tone, etc.). After an action potential initiates, the cardiac cell is unable to initiate another action potential for some duration of time (which is slightly shorter than the "true" action potential duration). This period of time is referred to as the refractory period.

Classically, the cardiac refractory period is separated into an absolute refractory period and a relative refractory period. During the absolute refractory period, a new action potential cannot be elicited. During the relative refractory period, a new action potential can be elicited under the correct circumstances.nl:Refractaire periode de:Refraktärphase