Effect of Currents on Compound Action Potentials.

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.

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

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