Non-allometric expansion and enhanced compartmentalization of Purkinje cell dendrites in the human cerebellum
Purkinje cell (PC) dendrites are optimized to integrate the vast cerebellar input array and drive the sole cortical output. Classically, PCs are stereotypical computational units. Yet, mouse PCs are morphologically diverse and those with multi-dendritic structure can receive non-canonical climbing fiber (CF) multi-innervation that confers independent compartment-specific signaling. This morphological motif is universal among human PCs, but their morphology is otherwise uncharacterized. Do human PCs exceed allometry to achieve enhanced integrative capacities relative to mouse? To answer this, we used comparative histology in human and mouse to analyze cellular morphology, parallel fiber (PF) and CF input arrangement, and regional PC demographics. Quantitatively and qualitatively distinct, human PCs: are substantially larger than predicted by cortical thickness, have increased spine density and size, commonly host multiple CFs, and exhibit previously unreported "spine clusters". They harness a horizontally oriented multi-dendritic motif to multiplex broad PF/CF arrays, which may subserve computations requiring multi-modal association.