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.
‘AC 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.
[1] Woo and Campbell (1964) Bul. of the
Los Angeles Neurological Societies, Vol 29, No. 2, pp 87-94
[2] McNeal, Bowman, and Momsen (1973) Advances in External Control of Human Extremities,
Yugoslav Committee for Electronics and Automation, Belgrade, pp 35 - 46