Question #142

Which of the following best explains the notion of an absolute refractory period in nerve cells? the impracticality of applying an infinitely large stimulus
a complex relationship between Na+, K+ and Cl- conductances
persistent activation of an excess of voltage activated K+ channels
during this time is is inpossible to activate a sufficient proportion of the available voltage actived Na+ channels
during this time too large a proportion of the voltage activated Na+ channels are inactivated to permit the initiation of a new action potential no matter how large the stimulus intensity

During the absolute refractory is is impossible to initiate a new action potential, irrespective of the magnitude of the stimulus applied. A new all-or-nothing action potential will only be initiated when a critical number of voltage activated Na+ channels open such that the inward Na+ current exceeds the maximum possible outward K+ current. The number of such Na+ channels that open following a stimulus depends on the total number of channels that exist, the proportion of these that are available to open (related to the inactivation curve for a single channel) and the proportion of these that do open (related to the activation curve for a single channel). During the absolute refractory period so few channels are available to open that even an infinitely large stimulus that open all the available channels would not open a sufficient number of channels to initiate an action potential.

See A Thinking Approach to Physiology, pages 11 - 15.

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