17 Refractory periods

An action potential takes place within approximately 2 milliseconds (Figure 17.1). While an action potential is in progress, another one cannot be initiated. This effect is referred to as the refractory period. There are two phases of the refractory period: the absolute refractory period and the relative refractory period. During the absolute refractory period, another action potential will not start. This is because of the inactivation gate of the voltage-gated Na⁺ channel. Once the Na+ channel is back to its resting conformation, a new action potential could be started during the hyperpolarization phase, but only by a stronger stimulus than the one that initiated the current action potential.

Absolute refractory period

The absolute refractory period is a brief period during which a neuron is unresponsive to any additional stimuli, regardless of their strength. This refractory period occurs immediately after the initiation of an action potential and is crucial for maintaining the orderly transmission of signals in the nervous system. The absolute refractory period is primarily a consequence of the inactivation of voltage-gated sodium channels. These channels are responsible for the rapid influx of sodium ions that depolarize the neuron’s membrane and generate an action potential. Following depolarization, the voltage-gated sodium channels undergo a conformational change, entering an inactivated state where they are temporarily unable to reopen. This inactivation prevents the influx of additional sodium ions, making it physiologically impossible for the neuron to generate another action potential during this brief but critical period. The absolute refractory period ensures the directionality of signal propagation and prevents the occurrence of overlapping action potentials, contributing to the precision and reliability of neuronal communication.

Relative refractory period

The relative refractory period is a critical concept in neurophysiology that refers to a specific time interval during which a neuron, having just undergone an action potential, exhibits increased resistance to being triggered for another action potential. Unlike the absolute refractory period, which is an absolute restriction on firing, the relative refractory period is characterized by a heightened threshold for excitation. During this phase, a stronger-than-usual stimulus is required to elicit a subsequent action potential. This refractory period is primarily attributed to the prolonged opening of voltage-gated potassium channels and the slow closure of voltage-gated sodium channels, which collectively contribute to the temporary decrease in membrane excitability. The relative refractory period plays a crucial role in regulating the frequency and timing of neuronal firing, preventing excessive and uncontrolled activity in the nervous system while allowing for precise signaling and information processing.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 12.