To investigate the mechanism of the inhibitory effect of dorsal column stimulation (DCS) on the monosynaptic reflex, excitability of the Ia fiber terminal was measured by Wall's method before, during and after epidural spinal cord stimulation. The experiment was performed under general anesthesia on five normal cats and five cats which had undergone hemisection of the lower thoracic cord more than three weeks before the experiment. Bipolar silver-ball electrodes were placed epidurally on the midline of the thoracic cord, caudal to the hemisected site. During and after DCS, increase of the excitability of the Ia fiber terminal was observed in both normal and hemisected cats, suggesting that the presynaptic inhibition at the Ia fiber terminal plays an important role in inhibition of the monosynaptic reflex. Moreover, this excitability change was maintained more than ten minutes after cessation of DCS, compatible with a clinical observation that the inhibitory effect of DCS on spasticity often continues after turning off the stimulating system. However the mechanism of this after-effect is unknown.
dorsal column stimulation
primary afferent depolarization