| Intraspinal microstimulation (ISMS) is a novel functional electrical stimulation technique for the restoration of locomotion after spinal cord injury. Through stimulation of the ventral gray matter of the lumbar spinal cord ISMS has elicited a series of functional results such as weight bearing stepping movements, near normal recruitment order of motoneurones and fatigue resistant standing. Furthermore, ISMS elicits activation of axons and fibres in passage at lower thresholds than motoneurones. Together these data suggest that ISMS relies on fibre in passage systems and not primarily on the activation of cell bodies directly. This project sought to investigate the fibre in passage systems involved in the effective spread of activation of ISMS, specifically comparing the contribution of afferent and propriospinal fibres to the effective spread of ISMS.;In the animals with intact afferents, 47% of the cells recorded from responded to ISMS. The majority of the cells (87%) exhibited an increase in firing rate in response: to ISMS; 73% of the total within 3ms (short latency excitation). In the deafferented model, only excitation was observed. Given our stimulation parameters, the absolute current spread was approximately 0.5mm. However, cellular responses up to 10mm from the stimulation site were observed in the intact model and 6mm in the deafferented model. These results show that ISMS does not rely solely on the afferent projections for the effective spread of current but that propriospinal and descending projections play an active role. Furthermore, these data suggest that, as anticipated, ISMS works primarily through the fibre in passage systems for its functional outcomes. |
