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In acute experiments performed in decerebrated and spinalized (T 13 ) cats, an intraspinal injection of clonidine, a noradrenergic agonist, restricted to mid-lumbar segments L 3 -L 4 , can induce hindlimb locomotion, whereas yohimbine, a noradrenergic antagonist, can block spinal locomotion, and a second spinal lesion at L 4 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 T 13 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 L 2 or rostral L 3 levels, spinal locomotion was maintained; when the second lesion was performed at caudal L 3 or L 4 , 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 L 4 , perineal stimulation induced hyperextension of the hindlimbs but no locomotion. Considering that the main motoneuron pools of the hindlimbs are caudal to L 4 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. I N T R O D U C T I O NAfter a complete spinal transection at the lowest thoracic level, several animal species are capable of walking with the hindlimbs over a treadmill , a behavior that is essentially generated by a spinal "central pattern generator" (CPG) (Grillner 1981). The question studied here concerns the segmental distribution of this locomotor network and whether it is regionalized or distributed throughout the spinal cord of the cat. In the in vitro neonatal rat, the thoracolumbar cord constitutes a key area for locomotor pattern generation (Cazalets 2000) although the sacral cord also has autonomous rhythmogenic capabilities (Cazalets and Bertrand 2000a;Lev-Tov et al. 2000). Other works also suggest a regionalized organization of rhythmogenicity based on the segmental distribution of various serotonin (5-HT) receptors (Jordan and Schmidt 2002). Chronic lesions of the gray matter at the level of L 2 by kainic acid results in paraplegia in adult rats (Magnuson et al. 1999). Other studies in which spinal lesions and neurotransmitters were also used on various spinal segments suggest that the rhythmogenic potential is distributed throughout the spinal cord, with rostral lumbar segments being more excitable (Kjaerulff and Kiehn 1996).It was suggested that, in cats, the rostral lumbar segments L 3 -L 5 play a leading role during scratching but that rhythmogenicity was present in lower L ...

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