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A refractory period, in physiology, is a period of time during which an organ or cell is incapable of performing a particular action.
Refractory period in biology
Biologically, a refractory period is the amount of time it takes for an excitable membrane to be stimulated and then be ready for stimulus again.
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. Depolarization, where voltage-gated sodium channels open increasing the neuron's membrane conductance for sodium ions and depolarizes the cells membrace potential. Repolarization, where voltage-gated sodium channel inactivation, due to the now depolarized membrane potential, and voltage-gated potassium channel opening both act to repolarize (or hyperpolarize) the cell's potential membrane back to its resting membrane potential. After-hyperpolarization occurs when the cell's membrane voltage overshoots its resting membrane potential, following repolarization, due to a still higher than resting potassium conductance across the cell membrane. Eventually this potassium conductance drops and the cell returns to its resting membrane potential.
The refractory period is due to the inactivation property of voltage-gated sodium channels. Voltage-gated sodium channels have two gating mechanisms, one that opens the channel with depolarization and the inactivation mechanism that closes the channel with depolarization. Channel opening with depolarization is faster than inactivation, thereby allowing an initial entry of sodium ions in to the cell. But eventually, all the sodium channels will close with sustained depolarization. The only way to de-inactivate voltage-gated sodium channels is to hyperpolarize the membrane for a sustained period of time. The time between one action potential and when not enough of the voltage-gated sodium channels are de-inactivated and able to generate a new action potential in response to stimulus is called the absolute refractory period. The time between absolute refractory period and when all the sodium channels are de-inactivated is called the relative refractory period. During the relative refractory period, a stimulus can excite an action potential, but it must be stronger than the minimum stimulus required to elicit and action potential at rest.
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, 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, so 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.
Refractory period in cardiac physiology
References & Bibliography
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