1. In neurology, a term applied to certain nerve-cells in the cerebellum, reticular membrane, and olfactory bulbs which exhibit a staining of the nucleus only when treated by the Nissl method used to highlight important structural features of neurons (named for Franz Nissl [1860-1919], German neurologist).
2. The cell of a nerve with an easily staining nucleus.

1. A nerve cell whose nucleus is deeply stainable while its body is not.
2. A nerve cell body having little or no Nissl substance (granular endoplasmic reticulum and ribosomes) visible but a nucleus that stains intensely.