Axonal projection of descending pathways responsible for eliciting forelimb stepping into the cat cervical spinal cord

Summary The descending pathways responsible for eliciting forelimb stepping are located in the lateral funiculus (Yamaguchi 1986). In order to determine into which spinal segments the descending pathways project and to know the projections and functions of the other descending system, the ventral funicular pathways, we placed various lesions in the cervical spinal cord of decerebrate cats with the lower thoracic cord transected and studied their effects on forelimb stepping evoked by stimulation of the midbrain locomotor region. (1) The lateral funiculus was transected on one side. The operation removes descending input to all the segments caudal to the lesion. Experiments with serial transections from the caudal to rostral segment revealed that stepping activity of the limb on the lesioned side is reduced when the lesion is placed at the level between the C6 and C7 segment and then between C5 and C6. A slight reduction of activity was also observed after a lesion placed between C7 and C8. (2) Consistently, bilateral transection of the lateral funiculus at the level between C5 and C6 abolished stepping movements of both forelimbs. (3) The cervical cord was split in the parasagittal plane through the dorsal root entry. The operation removes the descending input to the segment in which the lesion is placed. The parasagittal lesions from the C1 to C6 did not abolish stepping activity, although a lesion placed between C5 and C6 could slightly affect stepping. The results, (1)–(3) suggest that the lateral funicular pathways project into the spinal segments mainly at the C6–C7 level with some rostrocaudal extension into C5 and C8. (4) Complete transections of the medial part of the spinal cord cut extensor bursts short and raised stepping frequency. Nevertheless, if the lesion at C1–C5 spared the ventromedial part of the ventral funiculus, it did not result in such high-frequency stepping or in weakened extensor activity. In the case of segments caudal to C6, medial transections which spared the corresponding region could result in such stepping. It is suggested that the pathways descending through the ventromedial part of the ventral funiculus in the rostral segments provide extensor activity during stepping. They may change their course in the more dorsal part of the ventral funiculus below the C6 and presumably project into the grey matter of more caudal segments.