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Mid-Lumbar Segments Are Needed for the Expression of Locomotion in Chronic Spinal Cats

Centre de Recherche en Sciences Neurologiques, Department of Physiology, Pavillon Paul-G.-Desmarais, Université de Montréal, Montreal, Quebec, Canada

Submitted 1 September 2004; accepted in final form 2 January 2005

In acute experiments performed in decerebrated and spinalized (T₁₃) cats, an intraspinal injection of clonidine, a noradrenergic agonist, restricted to mid-lumbar segments L₃–L₄, can induce hindlimb locomotion, whereas yohimbine, a noradrenergic antagonist, can block spinal locomotion, and a second spinal lesion at L₄ can abolish all locomotor activity. In the present study, we investigated whether the abolition of locomotion after this second spinal lesion was due to an acute spinal shock or to the functional disconnection of the rostral and caudal lumbar segments. In seven cats, first spinalized at T₁₃ and having recovered treadmill locomotion, a second transection was performed at lower lumbar levels. Video and electromyographic recordings were used to evaluate locomotor performance. Results show that after a second transection at L₂ or rostral L₃ levels, spinal locomotion was maintained; when the second lesion was performed at caudal L₃ or L₄, all locomotor activity was abolished even after several weeks of attempted locomotor training; vigorous fast paw shakes (FPS) were observed in all cases; and after an intraperitoneal injection of clonidine in cats with a second transection below L₄, perineal stimulation induced hyperextension of the hindlimbs but no locomotion. Considering that the main motoneuron pools of the hindlimbs are caudal to L₄ and are still functional after the second spinal transection, as evidenced by the presence of FPS, we conclude that the mid-lumbar spinal segments are essential for the specific expression of spinal locomotion but not necessarily for other rhythmic motor patterns.

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