Distribution of external cuneate nucleus afferents to the cerebellum: II. Topographical distribution and zonal pattern‐an experimental study with radioactive tracers in the cat

Small injections of tritiated leucine and the autoradiographic method were used to demonstrate efferents from restricted portions of the external cuneate nucleus (NCE) to the cerebellum. Sites of injection were analyzed by reference to the distribution of primary muscle afferents in NCE. On transverse sections, the silver deposits form longitudinal bands that, in certain regions, are packed together and label the entire surface of the granular layer; in other parts, they are separated by empty longitudinal bands. The longitudinal deposits are not continuous in the rostrocaudal direction. On the basis of the distribution of the longitudinal bands, 14 zones have been described for lobules II‐VI, and 6 zones were recognized in lobules I, VIII, and the paramedian lobule. Afferents from NCE are distributed topographically. Regions of the nucleus receiving axial and neck muscles project mainly to vermal regions of lobules I‐III, and to parts of lobules VIII and IX. Regions receiving afferents from forelimb muscles send their fibers preferentially to the vermian region of lobule V, to paravermian regions of lobules IV‐VI, to parts of lobules VIII and IX, and to the paramedian lobule. These distributions in several respects are in agreement with the somatotopical maps of the cerebellum. However, other features support a “mosaic” arrangement: efferents from a region of NCE are distributed over several distinct sites of the cortex and efferents from different parts of the nucleus also converge to neighboring cortical regions.