Blocking Action Potentials with Alternating Currents.

Woo and Campbell [1] explored the effect of imposing a AC “blocking” stimulus (20 kHz) between a stimulating electrode (S) and a recording electrode. They used two recording sites, one on the peripheral nerve to record compound action potentials (B, not shown in the figure) and one on a dorsal root filament to record “single” fiber responses (R). They found that fast conducting action potentials were successively reduced in the compound action potential as the AC “block” amplitude was increased. They also noticed the base line of the peripheral nerve recording appeared noisier than when the AC “block” off. Recording from the dorsal root filament showed that the effect of the AC “block” was to cause repetitive firing of the nerve fiber and that single stimuli could still come through the “blocking” region. However, after some time true block could be effected but the compound action potential was not a reliable indicator of conduction block.

McNeal et al. [2] working on the peripheral motor nerve serving medial gastrocnemius, employed two recording sites, one on the peripheral nerve (recording a compound action potential) and one in the muscle (recording EMG). An “AC “blocking” stimulus was applied to the peripheral nerve distal to the nerve recording electrode and a stimulating electrode was placed proximal to the “blocking” electrode. Their results show that “blocking” stimuli of 600 Hz and greater, yield similar results and that the “block” is occurring at the neuromuscular junction, (probably because of transmitter depletion) and not at the site of the “blocking” electrode. The “block” is not complete because the force does not go to zero. Occasional action potentials must be transmitted across the junction.
Caveat!
Beware that in studies employing maximal muscle force as an indicator before the “block” stimulus is turned on, the true effect of the “block” stimulus may be masked, i.e. a large burst of activity that occurs at the initiation of the “block” cannot cause an increase in muscle force and therefore obscures a transient increase in nerve activity.