Nonlocomotor and locomotor hindlimb responses evoked by electrical microstimulation of the lumbar cord in spinalized cats

As a preliminary step to using intraspinal microstimulation (ISMS) for rehabilitation purposes, the distribution of various types of hindlimb responses evoked by ISMS in spinal cats ( T 13) is described. The responses to ISMS applied through a single electrode was assessed, before and after an intravenous injection of clonidine (noradrenergic agonist), using kinematics and electromyographic recordings in subacute (5-7 days, untrained) or chronic (3-5 wk trained on a treadmill) spinal cats. ISMS was applied in the dorsal, intermediate and ventral areas of segments L-3-L-7, from midline to 3 mm laterally. Uni- and bilateral non-locomotor responses as well as rhythmical locomotor responses were evoked. In the subacute cats, ipsilateral flexion was elicited in the dorsal region of L-3-L-7, whereas ipsilateral extension was evoked more ventrally and mainly in the caudal segments. Dorsal stimuli could induce ipsilateral flexion followed by ipsilateral extension. Sites inducing bilateral flexion and bilateral extension were similarly distributed to those evoking ipsilateral flexion and extension in the rostrocaudal axis but were evoked from more medial sites. Ipsilateral flexion with crossed extension was evoked from intermediate and ventral zones of all segments and lateralities. Unilateral ipsilateral locomotion was rarely observed. Contralateral locomotion was more frequent and mainly evoked medially, whereas bilateral locomotion was evoked exclusively from dorsal regions. With some exceptions, those distribution gradients were similar in the four conditions ( subacute, chronic, pre- and postclonidine), but the proportion of each response could vary. The distribution of ISMS-evoked responses is discussed as a function of known localization of interneurons and motoneurons.