Sassen and Zimmerman[1] recorded (R) compound
action potentials during nerve trunk stimulation (S) while
applying an electric current at the blocking site (B). The
changes in the Compound Action Potential (CAP) with increasing
current strengths is shown in the figure. As the current
increases, the amplitude of the primary peak diminishes and
the arrival of the CAP is delayed. Although a conclusion
could be made that some action potentials are blocked, the
effect could be due to destructive summation of differentially
delayed action potential peaks.
‘A’ shows the CAP recorded from the Sural Nerve; II and III label
the components from the Group II and II fibers. ‘B-F’- Recording
of the CAP with application of polarizing currents of strength shown on right
at the electrodes at B. Conduction distance of 58 mm.
Blocking Action Potential
Propagation with Direct Currents.
Campbell and Woo [2] explored the effect
of imposing a DC “blocking” stimulus (B) between
a stimulating electrode (S) and a recording electrode (R).
They used two recording sites, one on the peripheral nerve
(recording a compound action potential; not shown in the
figure) and one on a dorsal root filament (recording a “single” fiber
response). They found that fast conducting action potentials
were successively reduced in the compound action potential
as the DC “block” amplitude was increased. They
also noticed the base line of the peripheral nerve recording
appeared noisier than when the DC “block” was
off. Recordings from the dorsal root filament showed that
the effect of the DC “block” was to cause repetitive
firing of the nerve fiber that lasted for 10’s of seconds,
yet single stimuli could still come through the “blocking” region.
After some time, 10’s of seconds, true block could
be effected but the compound action potential was not a reliable
indicator of conduction block
‘DC’ Block
Casey and Blick [3] have shown
that anodal polarization of peripheral nerve causes a decrease
in the conduction velocity when action potentials propagate
through the “blocked” region. The decrease is
fiber diameter dependent and can be fitted to a curve that
shows the change is approximately 10% of the original velocity.
[1] Sassen, M. and Zimmerman, M. 1973 Pflügers Archive 341, 179-195.
[2] Campbell and Woo, (1966) Bul. of the Los Angeles Neurological Societies,
Vol 31, No. 2, pp 63-71
[3] Casey and Blick (1969) Brain Research, vol 13, pp155-167