Compound Action Potentials: Problems when Testing for Block

The compound action potential is often use as an indicator of neural transmission and can yield erroneous interpretations. A compound action potential (CAP) is a signal recorded from a nerve trunk made up of numerous axons. It is the result of summation of many action potentials from the individual axons in the nerve trunk. A CAP may be initiated on a peripheral nerve by an electrical stimulus applied to the nerve at some point at a distance from the recording site. The latency between the application of the stimulus and the onset of the compound action potential is a function of the distance between the recording site and the site of stimulation. In the figure, the initial biphasic spikes are the signal artifact from the stimulator (S). As the recording site (R) is moved further away from the stimulation point, the features of the CAP change. The different conduction velocities of the axon population results in a shift in time of the amplitude peaks as the recording site moves further from the stimulation site.
Cooling a nerve and the application of ac and dc electric currents have been reported to effect “block” of large fibers before small nerve fibers. These conclusions were based on the changes in the compound action potential where the high amplitude short latency signal in the compound action potential was observed to diminish, before the long latency signal was affected, as the magnitude of the “blocking” agent was increased. Many investigators interpreted these findings to mean that cooling, ac or dc potentials could be applied to effect selective block of large nerve fibers. Subsequent to these observations other investigators have observed slowing of the propagation velocity by varying amounts as an action potential passed through the so called “blocking” zone, which gives rise to slight variations in the time of arrival of the action potentials at a recording electrode. Such a slight change can give rise to diminished recorded signal and the illusion that a particular population was blocked.

The amplitude of the recorded compound action potential is a summation of the individual action potentials from the different axons. When the waves pass the recording site in phase they add constructively and display a higher peak. On the other hand, when they are out of phase, they add destructively. In the figure, waves 'a' and 'b' add to produce 'c'. In the left panel constructively when in phase, and in the right panel, destructively when out of phase. As a result, action potentials could be traveling on a number of peripheral nerve fibers and no compound action potential be recorded if they sum destructively or if timing differences do not permit constructive summation. This latter point is important to keep in mind when drawing conclusions about 'block' based on the amplitude of a compound action potential.