Descending pathways eliciting forelimb stepping in the lateral funiculus: experimental studies with stimulation and lesion of the cervical cord in decerebrate cats.

The funicular pathways that elicit forelimb stepping were investigated with stimulation and lesion of the cervical white matter in decerebrate cats with the lower thoracic cord transected. We localized cross-sectional areas where stimulation evoked rhythmic motor-nerve discharges imitating those of stepping (fictive locomotion) in the immobilized animal, and further examined whether or not lesions made in the corresponding areas abolished actual locomotor movements. Stimulation of the C3 lateral funiculus (LF) produced fictive locomotion in the ipsilateral forelimb. The effective areas of stimulation were located separately in the dorsolateral funiculus (DLF) and in the ventrolateral funiculus (VLF), while the VLF was more effective than the DLF. Effective stimuli were pulse trains with a frequency of about 30 Hz, with a rather wide pulse duration of about 0.5 ms. Blocking axonal conduction through the lower thoracic cord by cooling reproducibly facilitated fictive locomotion in both amplitude and frequency. In the lesion experiments, forelimb locomotor movements were elicited spontaneously or by stimulation of the mesencephalic locomotor region (MLR). The locomotor movements were abolished by complete lesions of the C2-C3 LFs on both sides, but these remained when either the DLF or the VLF was intact on one side. These findings together suggested that the descending pathways for the activation of the spinal locomotor network of the single forelimb are located ipsilaterally in the DLF as well as in the VLF. Both the DLF and the VLF pathways can initiate locomotion, while the VLF pathways have a higher potential for its initiation. Lesion experiments further showed that cats can walk with both forelimbs, even though the spinal locomotor network of the single forelimb was deprived of its main descending input by unilateral lesions of the LF. However, when the unilateral lesion extended to the medial part of the LF, the bilateral walking was abolished; the limb on the lesioned side showed only rhythmic extension movements without active flexion movements, which was out of phase with the stepping movements on the intact side. This finding suggested that the medial part of the LF is important for producing flexion movements during the swing phase of stepping.