Reflex and functional responses to distal tibial/medial plantar nerve electrical stimulation in the treadmill-locomoting spinal cat

Low intensity electrical stimulation of the distal tibial nerve innvervating the plantar foot with pulse trains promotes extensor activity in reduced animal preparations when delivered during the extensor phase. Since enhancing extensor activity could potentially improve weight support after spinal cord injury (SCI), we evaluated if electrical stimulation of the distal tibial nerve or its medial plantar branch enhances/prolongs extensor activity in a locomotor-trained feline model of SCI. With stimulation trains at levels activating large diameter afferents (low-threshold cutaneous/muscle group I afferents) we found no effects on the activity of the extensors and flexors during walking, whereas stimulation levels activating smaller fibers (higher-threshold cutaneous/ muscle group II) were found to increase swing height for stimuli delivered in late stance/early swing. A further increase in stimulus intensity to levels recruiting noxious afferents terminated ongoing stance and produced a flexor withdrawal response of the paw. Muscle reflex responses to low intensity single-pulse stimuli during walking showed a short-latency inhibition in extensor muscles followed by a longer latency excitatory response when stimulation was delivered during stance, and limited responses when delivered during swing. Flexors showed an early-latency excitatory response followed by a weak inhibitory response for stimulation delivered throughout most of the walking cycle. These reflex responses to single pulse stimulation agreed with prior studies. The absence of extensor activity enhancement with low-intensity train stimulation of the distal tibial nerve during stance observed in other preparations may be partially explained by changes in spinal neuronal properties and locomotor networks in chronic spinal cats.