Principles of organization of the monkey corticopontine projection

In order to elucidate the role of the cerebellum in motor control, various aspects of cerebrocerebeUar relations have been studied by physiological methods during recent years~,a,9,~L To a large extent such studies have been carried out in primates, since the parts of the cerebellum connected to the cerebral cortex are much more developed in primates than in lower mammals, and the diversity of skilled movements influenced from the cerebellum is greater. However, the anatomy of cerebrocerebellar connections has mainly been studied in the cat, and it cannot be taken for granted that principles of organization are identical in cat and primates. The present study is an attempt to analyze the corticopontine projection which is the first link in the massive cortico-ponto-cerebellar projection. In the cat this projection when studied by axonal degeneration techniques exhibits a complicated pattern of divergence and convergence, such that each cortical region as a rule projects to several well restricted pontine regions which often have the form of bands or columns. Furthermore, each pontine region often receives fibres from more than one cortical area 5-7. Whether these principles of organization also apply to the monkey corticopontine projection cannot be decided from the literaturO 5-17,2°. In 33 adult rhesus monkeys (Macaca mulatta) cortical lesions of most parts of the cerebral cortex not buried in sulci were made under Nembutal anaesthesia by suction or by removing the pia. Most of the animals have been used in other studies n-13. Since the corticopontine projection from all regions studied was found to be almost entirely ipsilateral, bilateral lesions were used in many cases, facilitating comparisons between projections from different areas with regard to quantity and sites of termination. Most of the lesions were limited to one functional or cytoarchitectonic a area. After a survival time of 3-15 days, the animals were perfused in deep Nembutal anaesthesia with saline followed by 10 ~ formalin. The exact extent and site of the lesions were checked microscopically. Frozen sections through the pons were cut coronally (in a few cases transversely) and stained according to the Nauta-Gygax 19, Fink-Heimer 1° or Wiitanen method2a; in some instances all 3 methods were employed on alternate sections giving identical results with regard to site of terminations.