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
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.
Summation of Action
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.