An anatomical investigation of the corticopontine projection in the primate ( Macaca fascicularis and Saimiri sciureus)—II. The projection from frontal and parietal association areas

Abstract

In continuation of a previous study on the corticopontine projection from sensorimotor areas of the cerebral cortex in primates, the projection from frontal and parietal association areas has been investigated with degeneration and autoradiographic tracing techniques. Since the connexions are ipsilateral the projection from peri-Rolandic areas was also studied on the contralateral side of the same animals as a control. It was found that association areas have only a modest contribution to the corticopontine system as compared to the massive projection from sensorimotor areas. Of those association areas studied in the present experiments, area 5 of the parietal cortex and the premotor cortex contributed most corticopontine fibres. The prefrontal cortex and area 7 were found to have only very weak connexions. It is evident, especially from autoradiographic studies, that the corticopontine system has an intricate organization, with one particular cortical area projecting to multiple small target zones in the pontine nuclei interdigitating with target zones or ‘patches’ receiving fibres from other cortical areas. The distribution of these zones may be rather widespread, especially those receiving afferents from the motor cortex, thus providing ample opportunities for integration of motor command signals with sensory, mainly somatosensory, signals. The fact that the pontine nuclei receive chiefly signals from primary cortical areas, rather than from high-order association areas, may be taken to indicate that the cortico-ponto-cerebellar system subserves rapid corrections of movements and ‘triggered actions’, a proposition which is also supported by behavioural studies in monkeys subjected to reversible cooling of the dentate nucleus.