Cytoarchitecture and cytology of the lateral reticular nucleus in the rat.

The cytoarchitecture and cytology of the rat lateral reticular nucleus (LRN) was studied in serial sections of paraffin embedded tissue stained with cresyl violet. Cell outlines and nuclear outlines were drawn in the transverse plane and the nucleus was serially reconstructed. The LRN in the rat begins at a point just caudal to the caudal limit of the inferior olivary nucleus and extends to the mid-olivary level. The nucleus can be subdivided into a predominantly small-celled parvocellular division ventrally, a predominantly large-celled magnocellular division dorsomedially, and a subtrigeminal division dorsolaterally containing predominantly medium-sized cells. At rostral levels the nucleus comprises two parts, a medial principal portion and a lateral subtrigeminal division. Measurements of neuronal diameters yielded size distributions which confirmed the predominance of large cells in the magnocellular division, small cells in the parvocellular division and medium-sized cells in the subtrigeminal division. The neurons display multipolar, triangular, piriform and fusiform somata. All types show a range in size from small to large. The larger cells have abundant Nissl bodies which are coarse in nature and voluminous cytoplasm. The smaller cells have poorly developed Nissl bodies and scant amounts of cytoplasm.