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-motoneurons
1 Department of Biomedical Engineering and Center for Neuroscience, University of Alberta, Edmonton, Canada
2 Division of Neuroscience and Mental Health, Imperial College London, London, United Kingdom
* To whom correspondence should be addressed. E-mail: rgaunt{at}ualberta.ca.
Microstimulation within the motor regions of the spinal cord is often assumed to activate motoneurons and propriospinal neurons close to the electrode tip. However, previous work has shown that intraspinal microstimulation (ISMS) in the gray matter activates sensory afferent axons as well as 
-motoneurons (MNs). Here we report on the recruitment of sensory afferent axons and MNs as ISMS amplitudes increased. Intraspinal microstimulation was applied through microwires implanted in the dorsal horn, intermediate region and ventral horn of the L5-L7 segments of the spinal cord in four acutely decerebrated cats, two of which had been chronically spinalized. Activation of sensory axons was detected with electroneurogram (ENG) recordings from dorsal roots. Activation of MNs was detected with electromyogram (EMG) recordings from hindlimb muscles. Sensory axons were nearly always activated at lower stimulus levels than MNs irrespective of the stimulating electrode location. Electromyogram response latencies decreased as ISMS stimulus intensities increased, suggesting that MNs were first activated transsynaptically and then directly as intensity increased. Intraspinal microstimulation elicited antidromic activity in dorsal root filaments with entry zones at least 17 mm rostral and caudal to the stimulation sites. We posit that action potentials elicited in localized terminal branches of afferents spread antidromically to all terminal branches of the afferents and transsynaptically excite MNs and interneurons far removed from the stimulation site. This may help explain how focal ISMS can activate many MNs of a muscle, even though they are distributed in long thin columns.
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