Abstract
Cerebellothalamic neurons were investigated using retrograde labeling from single or multiple injections of cholera toxin b and fluorescent dextrans (green, purple, red) in the posteroventral part of the ventral lateral thalamic nucleus (VLpv; Olszweski’s VPLo), the main source of thalamic input to primary motor cortex. Iontophoretic or pressure injections were guided by microelectrode recordings in anesthetized cynomolgus monkeys. Large injections filling VLpv (and adjacent nuclei) produced abundant retrograde labeling in all of the contralateral deep cerebellar nuclei. Small injections at VLpv sites responsive to “tap” of contralateral hindlimb, forelimb, or face labeled groups of neurons in the anterior two-thirds of the dentate nucleus and in the anterior and posterior interposed nuclei with a general anteroposterior topography (with subcomponents) in each nucleus: hindlimb anterior and ventral, face posterior and dorsal, and forelimb in between hindlimb and face. Simultaneous injections of multiple tracers in VLpv (one tracer per body region with no overlap between injections) confirmed the general anteroposterior somatotopography but also revealed considerable intermingling between cells labeled with different tracers. Interestingly, only 3 of cells were double labeled. This organization contrasts with the concept of a segregated pathway linking separate portions of the deep cerebellar nuclei with separate portions of thalamus and cortex. Rather, these observations resemble the pattern of complex topography that has been observed physiologically in the primary motor cortex. These data are consistent with the idea that muscle synergies are represented already in the anatomical organization of cerebellothalamic projections by a general topographic framework in which regions of intermingling provide the basis for movement coordination of different parts of the body.
