Retrograde analyses of spinothalamic projections in the macaque monkey: input to the ventral lateral nucleus

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in monkeys following variously sized injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input from different sets of STT cells. This report focuses on STT input to the ventral lateral nucleus (VL), where prior anterograde tracing studies identified dense or moderately dense STT terminations. Large and very large injections in VL produced large numbers of labeled cells predominantly in laminae V and VII (more than half as many as from massive injections in the entire thalamus). Medium-sized and small injections in VL labeled STT cells almost exclusively in laminae V and VII, in segments consistent with the coarse mediolateral VL topography; few or no cells were labeled in lamina I. All injections labeled the deep cerebellar nuclei (see accompanying article: Evrard and Craig, 2008). Notably, even the most anterior injection in VL that produced dense pallidal labeling still labeled both STT and deep cerebellar cells. These observations indicate that VL receives STT input originating from laminae V and VII neurons that may be coextensive with its cerebellothalamic input. These findings support the role of laminae V and VII STT cells in sensorimotor integration and suggest a significant ongoing influence on both motor and premotor thalamocortical function. Together with the preceding observations of selective STT projections to other thalamic regions, these results provide compelling evidence that the primate STT consists of anatomically and functionally differentiable components.