The McGill Physiology Virtual Lab

Compound Action Potential

Refractory period

For a short period after the passage of an impulse, the threshold for stimulation is raised, such that if a nerve is stimulated twice in quick succession, it may not respond to the second stimulus.





The absolute refractory period is the brief interval after a successful stimulus when no second shock, however maximal, can elicit another response. Its duration in mammalian A fibres is about 0.4 ms; in frog nerve at 15oC it is about 2 ms. The absolute refractory period is followed by the relative refractory period, during which a second response can be obtained if a strong enough shock is applied.
Thus, the refractoriness of a nerve after conducting an impulse sets an upper limit to spike frequency. During the relative refractory period, two impulses traversing a long length of nerve must be separated by a minimal interval if the second spike is to be full sized.
The inactivation of Na+ channels and the delayed increase in K+ conductance are the underlying ionic conductance changes which can explain the refractory period.

The diagram above shows the Time relations of the events during the propagation of the action potential (intracellular recording), the ionic movements (Na+ in and K+ out) and the membrane permeability.



The output of the stimulator is set to "Twin" instead of "single" pulse. In this mode, the stimulator sends out pairs of pulses of identical intensity, at intervals set by the delay knob. As a result, the first stimulus coincides with the start of the sweep.
Click here to view an experimental recording of the refractory period determination


In the above experimental recording, the nerve was crushed between the proximal recording electrodes. Why?
The above experimental recording showed the refractory period of  an average A-alpha fibre. How would the refractory period compare with that of an A-beta fibre?
How would you demonstrate the refractory period of an average A-beta fibre?

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