A study of the morphology of the large reticulospinal neurons of the lamprey ammocoete by intracellular injection of Procion yellow.

47 reticulospinal neurons of the lamprey brain-stem were stained by intracellular injection of Procion yellow, so that their morphology and dendro-architecture could be determined. The vagal, bulbar, Mauthner, isthmic and mesencephalic groups could be distinguished by the shape and size of their somata and the distribution of their dendrites. However, they shared the following features: the somata of all these cells were relatively large (up to 80 micrometer largest diameter); the dendrites were extensively distributed in the brain-stem in an open-field arrangement; the initial segment expanded abruptly at the soma junction but tapered gradually to form a narrow axon, which joined the medial longitudinal fasciculus in the brain-stem.