Morphology of the Golgi-impregnated Lugaro cell in the rat cerebellar cortex: a reappraisal with a description of its axon.

We present a detailed description of the somatodendritic and axonal features of the Golgi-impregnated Lugaro cell in the rat cerebellar cortex. This neuron, present throughout the cerebellum, is characterized by a fusiform cell body located at the border between the granular and Purkinje cell layers, horizontal bipolar dendrites extending in the parasagittal plane and an axon projecting exclusively to the molecular and granular layers. A quantitative analysis of 77 Lugaro cells confirms the somatodendritic homogeneity of this cell type. Computer reconstructions showed that the bipolar dendrites spread like an horizontal X centered by the cell body, implying that Lugaro cells are arranged in a flat lattice just beneath the Purkinje cell layer. Fifty-seven Lugaro cells with sufficiently impregnated axons displayed a common pattern of organization with, nevertheless, a certain variability of the axonal trajectory. Some axons projected directly into the molecular layer with a profuse plexus spreading just above the parent cell. Other axons first traveled downwards into the granular layer, sometimes even reaching and passing into the white matter. They always hooked back to reascend through the granular layer and also finally terminate in the molecular layer, near and sometimes at a distance from the parent cell. In a few samples, one or two remote axon collaterals were seen to extend longitudinally in the lower molecular layer for a few hundred microns, in the same direction as the parallel fibers. In all cases a few collaterals projected into the granular layer. In view of its dense afferentation by Purkinje cell recurrent collaterals and its profuse inhibitory projection in the molecular layer, the Lugaro cell could act as a feedback interneuron on the corticocerebellar output.