Motor Programs: Trajectory Versus Stability

Abstract

Two subcortical systems converge on motor cortex and brainstem nuclei (Fig. 1). The basal ganglia receive from most or all of cerebral cortex (Kemp and Powell, 1971). Ablation prevents or slows movement, suggesting roles in the initiation and continuation of movement trajectories, and single unit studies have shown relationships to a variety of movement parameters, including direction, velocity/amplitude, pattern and force of muscular activity (Crutcher and Delong, 1984; Delong et al. 1983; Delong and Strick 1974; Georgopoulos et al. 1983a, b). By contrast, the cerebellum receives from a more restricted portion of cerebral cortex, including sensorimotor cortex and parts immediately adjacent in frontal and parietal lobes, and from other portions of the motor apparatus including spinal cord, reticular and vestibular nuclei (Bloedel and Courville 1981; Brodal 1978). Single unit studies had shown that lateral cerebellum fires before movement and even before motor cortex toward which it projects (Thach 1975, 1978), which seems to suggest a role in the programming of the initiation and the direction of trajectory. More medial portions fire later (Strick 1978; Thach 1978) and code for pattern and force of muscular activity (Thach 1978) and movement velocity (Burton and Onoda 1977, 1978; Soechting et al. 1978), which seems to suggest the continuous control of ongoing trajectory (Allen and Tsukahara 1974; Evarts and Thach 1969). Yet cerebellar ablation does not abolish movement trajectory, but instead gives rise to a variety of movement instabilities (Dow and Moruzzi 1958; Holmes 1939). This has suggested specific roles in control of movement and postural stability, possibly acting on reflex pathways (Gilman 1969; Gilman et al. 1971; Granit et al. 1955; Higgins and Glaser 1964; Higgins et al. 1962; Merton 1953; Soechting et al. 1978; Terzuolo et al. 1973; Vilis and Hore 1980; Terzuolo and Viviani 1974). Recent work (Schieber and Thach 1984a, b) on cerebellar unit discharge in monkeys performing trained pursuit tracking movements helps resolve apparent inconsistencies in cerebellar ablation and unit data, and at least in this task, suggests an exclusive role in the control of movement stability.