Refractory period (physiology)

Action potential

Refractory Period (Physiology)

The refractory period in physiology refers to a time interval during which an organ or cell is unable to repeat a particular action, or (more precisely) 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 excitation. This concept is critical in understanding the function of neurons and muscle cells, as well as the physiological basis of certain medical conditions.

Types of Refractory Periods

There are two main types of refractory periods: the absolute refractory period and the relative refractory period.

Absolute Refractory Period

During the absolute refractory period, a second stimulus, no matter how strong, will not evoke a second action potential. This period ensures that each action potential is a separate, all-or-nothing event and enforces one-way transmission of nerve impulses. In neurons, this period lasts approximately 1-2 ms.

Relative Refractory Period

The relative refractory period follows the absolute refractory period. During this time, a stronger than usual stimulus is required to evoke an action potential. This period is associated with the phase of membrane potential known as hyperpolarization (in neurons). It allows the neuron to return to its resting state and is ready for the next action potential, albeit with a higher threshold.

Physiological Importance

The refractory period is crucial for the proper functioning of many physiological systems. In the nervous system, it ensures the unidirectional flow of nerve impulses and contributes to the coding of stimulus intensity by the frequency of action potentials. In the cardiac system, the refractory period of cardiomyocytes prevents the occurrence of tetanus (a condition in which muscles are contracted continuously), which is vital for the heart's pumping action.

Clinical Significance

Alterations in the refractory period are associated with various medical conditions. For example, in epilepsy, the refractory period may be shortened, leading to the abnormal and excessive firing of neurons. In the heart, prolonged refractory periods can contribute to arrhythmias, as they may disrupt the normal sequence of cardiac excitation and contraction.

See Also

• Action potential
• Nerve impulse
• Membrane potential
• Hyperpolarization (biology)
• Cardiomyocytes