Input-output organization of the sensorimotor striatum in the squirrel monkey

The basal ganglia receive massive inputs from the neocortex and send outputs that exert both inhibitory and disinhibitory control over parts of the frontal cortex and brainstem. Between these basal ganglia inputs and outputs lies the striatum, which receives most of the cortical afferents and projects to the basal ganglia output nuclei--the globus pallidus and substantia nigra. To analyze this system we conjointly labeled, in squirrel monkeys, sensorimotor cortical inputs to the striatum and striatal outputs to the globus pallidus. Anterograde tracers were injected into the motor (MI) and somatosensory (SI) cortical body maps, at sites determined by electrophysiological stimulation and recording. Retrograde tracers were stereotaxically injected into the external and internal pallidal segments (GPe and GPi). We found that multiple dispersed modules ("matrisomes") in the putamen that all received inputs from single body-part representations in sensorimotor cortex could, in turn, send convergent outputs to single sites in the pallidum. This divergence-reconvergence pattern was found for both GPe and GPi sites, and for inputs from both SI and MI cortex. Thus, information from a single functional region in the cortex can be split up at the striatal stage only to be brought back together in the pallidum. The temporary divergence may increase lateral interactions between sensorimotor matrisomes, as well as between matrisomes and striosomes. One function of striatal modularity may thus be to set up an associative network in the striatum, which might contribute to sensorimotor learning. We also found that some sets of matrisomes did not receive strong sensorimotor inputs, even though they projected to regions of GPe and GPi that are near the sensorimotor-recipient zones described above. Thus, the matrisomal system may sort MI/SI inputs and other inputs before transfer to paired regions of GPe and GPi.