The Axon Collaterals of Purkinje Cells

Abstract

A. The Distribution and Mode of Termination The recurrent collaterals of Purkinje axons take their origin from the proximal as weil as more remote parts of the axon. Thus, they can reach closely neighboring regions of the cortex within the same folium as weil as in adjacent and even in rather remote folia. According to the classical description of CAJAL (1911) the recurrent collaterals are myelinated and aseend in oblique direction through the deeper strata of the granular layer. Upon reaching the upper stratum of this layer they bend into a tangential direction, and by branching profusely they give rise to a dense plexus situated beneath the layer of the Purkinje cell bodies. This plexus is referred to generally as the "infraganglionic plexus". Branches arising either directly from the Purkinje cell axons or from the infraganglionic plexus, aseend to the Ievel above the Purkinje cell bodies, where another tangential plexus of myelinated fibers is established: the so-called "supraganglionic" plexus. The course of the fibers in the infraganglionic plexus is predominantly parallel to the trans-versal plane of the folium, but the supraganglionic plexus in the molecular layer is longitudinally orientated (Fig. 101). Under the EM the fibers, especially of the upper plexus, can be recognized easily from their myelin sheaths (Fox, SIEGESMUND and DuTTA, 1964), there being no other myelinated fibers in the molecular layer except the rare Cajal-Smirnov fibers. Using the degeneration technique, FREZIK (1963) could show that, while there are very few if any recurrent Purkinje collaterals of the vermis reaching beyond the immediate vicinity of their origin in the longitudinal direction of the folium, in the transverse plane of the folia quite considerable distances can be bridged by these connexions. For example, in the sagittal plane of the vermis, recurrent Purkinje axon collaterals could be traced from lobe VII B of LARSELL as far as lobe VA. Essentially similar results were obtained by EAGER (1963 b, 1965) who found that the majority of association systems is of relatively short interfolial character with some predominance of connexions in the sagittal plane. Long association pathways have been traced from anterior paravermal regions to caudal folia of the homolateral crus II and from crus II a crossed pathway to the contralateral crus II and paramedian lobule. lt is a remarkable observation of EAGER (1965) that instead of being distributed throughout the folium, degenerationwas confined to a surface area corresponding roughly in position to the area of damaged cortex giving rise to the pathway. A similar quasi point-to-point relationship between cortical areas and those to which they send association fibers could be observed also in our own material. 178 J. C. Eccles et al., The Cerebellum as a Neuronal Machine