Forelimb movements in the cat; kinetic features and neuronal control

Abstract

The role of defined descending pathways in the control of forelimb movements in cats was investigated in behavioural experiments. The movements studied were "target-reaching" (i.e. the movement used to reach for food) and "food-taking" (i.e. the movement used for grasping a piece of food and for bringing it to the mouth). The interesting feature of these movements is that the neuronal systems mediating the descending commands to motoneurones have previously been investigated. The command for target-reaching can be mediated by propriospinal neurones (PNs) in the C3-C4 segments and food-taking depends on interneurones in the forelimb segments (C6-Th1). The descending control of food-taking was investigated with serial lesions. The movement is normally abolished after a lesion of the dorsal part of the lateral funiculus in C5/6 interrupting the input from the cortico- and rubrospinal tracts (CST/RST) to the forelimb segments. If a similar lesion was made serially in two sessions with an initial lesion transecting the CST with minimal RST damage, the movement was not abolished when the remaining rubrospinal fibres were transected one month later; it was performed in the first postoperative test. Food-taking could now be abolished by a ventral lesion in C2. It is suggested that rubrospinal fibres remaining after the first lesion induced ventral reticulospinal fibres to mediate the command for the movement. The C3-C4 PNs receive input from the cortico-, rubro-, tecto- and reticulospinal tracts. The hypothesis that the subcortical systems may update a cortical command en route to motoneurones was investigated in cats trained to perform reaching to different targets. The position of the wrist was recorded with a SELSPOT system. If the position of the target was shifted during the movement, a correction was initiated with short latency (60-70 ms). A transection of the input from the CST and RST to interneurones in the forelimb segments so that the movements depended solely on the C3-C4 PNs did not appear to affect the corrections, but an additional ventral C2 lesion prolonged the response latency. A lesion transecting axons of the C3-C4 PNs changed the kinematics of the corrections with ataxia in the initial postoperative period and enhanced brake of protraction when the ataxia had subsided. It is postulated that the command for switching of target-reaching can be mediated via the C3-C4 PNs and it is suggested that fast switching of target-reaching may be evoked via the input from ventral tecto- and tecto-reticulospinal pathways; other routes are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)