  The objective of this work was to determine whether application of
subthreshold currents to the peripheral nerve increases the excitability of the
underlying nerve fibres, and how this increased excitability would alter neural
activity as it propagates through the subthreshold currents. Experiments were
performed on two Romney cross-breed sheep in vivo, by applying subthreshold
currents either at the stimulus site or between the stimulus and recording
sites. Neural recordings were obtained from nerve cuff implanted on the
peroneal or sciatic nerve branches, while stimulus was applied to either the
peroneal nerve or pins placed through the lower hindshank. Results showed that
subthreshold currents applied to the same site as stimulus increased excitation
of underlying nerve fibres (p < 0.0001). With stimulus and subthreshold
currents applied to different sites on the peroneal nerve, the primary CAP in
the sciatic displayed a temporal shift of -2.5 to -3 us which agreed with
statistically significant changes in the CAP waveform (p<0.02). These findings
contribute to the understanding of mechanisms in myelinated fibres of
subthreshold current neuromodulation therapies.
