EFFECT OF SPINAL-CORD LESIONS ON FORELIMB TARGET-REACHING AND ON VISUALLY GUIDED SWITCHING OF TARGET-REACHING IN THE CAT

Cats were trained to reach to an illuminated tube placed horizontally at shoulder level and retrieve food with the forepaw. The trajectory o f an infrared light emitting diode, taped to the wrist dorsum, was rec orded with a SELSPOT-like recording system. Movement paths and velocit y profiles were compared before and after lesions: (1) in dorsal C5, t ransecting cortico-and rubrospinal pathways to the forelimb segments s o that the cats could only use the C3-C4 propriospinal neurones (PNs) to command reaching, (2) in the ventral part of the lateral funicle in C5, transecting the axons of C3-C4 PNs so that the cats had to use ci rcuitry in the forelimb segments to command reaching. Comparison of tr ajectories and velocity profiles before and after lesion 1 did not rev eal any major qualitative change. After lesion 2, the last third of th e movement was fragmented with separate lifting and protraction. Switc hing of target-reaching occurred when illumination was shifted to anot her tube during the ongoing movement. The switching latency measured f rom the time of illumination shift to the earliest change in movement trajectory had a minimal value of 50-60 ms. Short latencies were prese nt after lesion 1 as well as lesion 2 which suggest that fast switchin g mediated by the C3-C4 PNs and the interneuronal system in the foreli mb segments is controlled in parallel by the brain. In order to test a hypothesis that fast switching depends on the tectospinal and tecto-r eticulospinal pathways (the tecto-reticulo-spinal system) a ventral le sion was made in C2 aiming at interrupting these pathways. Large ventr al C2 lesions tended to block conduction in the more dorsally located rubrospinal (less in corticospinal) axons probably due to compression during surgery. When conduction in the rubrospinal tract was completel y interrupted by a ventral C2 lesion which also completely transected the axons of the tecto-reticulo-spinal system, then there was a prolon gation of the switching latency with 10-20 ms. After a similar large v entral lesion with remaining conduction in the rubrospinal tract the s witching latencies were unchanged. It is postulated that fast visually governed switching does not depend on the tecto-reticulo-spinal syste m alone but on more dorsally located pathways, presumably the rubrospi nal tract, either acting alone or together with the tecto-reticulo-spi nal system. It is further postulated that the delayed switching after interruption of conduction both in the rubrospinal tract and the tecto -reticulo-spinal system depends on the corticospinal tract. Visual con trol of rubrospinal and of corticospinal neurones is considered. It is postulated that target-reaching normally depends on signals in the co rtico-and rubrospinal tracts and mechanisms for co-ordination of activ ity in them as required during switching is discussed in view of the f indings now reported. (C) 1997 Elsevier Science Ireland Ltd.