Comparison of cortically and subcortically controlled motor systems. II. Distribution of anterogradely labeled terminal boutons on intracellularly filled rubrospinal neurons in rat and turtle.

The present study examined the circuitry of the red nucleus of the Sprague-Dawley rat and the freshwater pond turtle, Chrysemys picta, by using intracellular cell filling combined with anterograde tract tracing. Although both species have a well-developed cerebellorubral system, they differ in that the red nucleus of rats receives direct input from the motor areas of the cerebral cortex, whereas turtles do not. However, a direct descending projection from the hypothalamus to the red nucleus of turtles has been described. The aim of this study was to elucidate the relative functional contributions of the cerebellum and descending inputs to motor signal generation in the red nucleus. The results show that the cellular distribution of cerebellar inputs on rubrospinal neurons is similar between the rat and turtle; these projections are observed on the soma and the proximal and distal dendrites. In contrast, the hypothalamic inputs in turtles occupy mainly the more distally located dendrites, similar to the position of the cortical inputs in rats. These findings suggest that, first, the cerebellar inputs are not spatially segregated from the cortical or hypothalamic inputs in rats or turtles, as far as can be determined by light microscopy. Second, there is specificity of input from the cortex in rats and hypothalamus in turtles onto the distal portions of the dendrites. The similarity in the organizational features of the mammalian and non-mammalian cerebellorubrospinal systems has implications for interpretations of the relative roles of the cerebellum and cerebral cortex in motor control. Copyright 2000 Wiley-Liss, Inc.