Convergence pattern of cortical and interposital influences on rubrospinal neurons of the cat.

In adult semi-chronically implanted sedated cats discharges of single rubrospinal (RS) neurons were tested for selective activation of movement-evoking foci within pericruciate cortex (CX; area 4) and anterior division of cerebellar interpositus nucleus (IN). It was found that a very high incidence of neuronal responses was obtained only when stimulating IN and CX foci which controlled the same joint with respect to that moved from activation of the rubral foci including the recorded neurons. In these cases very frequent convergence phenomena were observed by activating IN and/or CX foci. Analysis of response patterns showed that RS neurons included in a focus controlling a given muscle were excited by IN foci for the same muscle (agonist foci) and inhibited from IN foci for the antagonist muscle (antagonist foci). In contrast, the same RS neurons were inhibited by agonist CX foci and excited by antagonist CX foci. Such an organization suggests that the motor cortex plays a competitive role in modulating the action of the interposito-rubrospinal system.