The central-peripheral transitional regions of cranial nerves. Oculomotor nerve.

Oculomotor nerve rootlets varied more markedly in size and in transitional zone length and form than those of any other nerve studied to date. However, they could be classified into four main types, each of which was associated with a characteristic type of central-peripheral transitional zone. Type 1 rootlets emerged from the brainstem through a tongue-shaped elevation of the glia limitans. This type of central-peripheral transition is found elsewhere only in a minority of abducent rootlets (Fraher et al. 1988). Type 2 rootlets contained long segments made up entirely of central nervous tissue and were the only motor rootlets so far described to contain a segment of this kind. Type 3 rootlets were the commonest and the largest. Before leaving the brainstem they ran laterally on its surface as the emergent rootlet segments, forming plexuses with one another. These rootlets contained a unique wedge-shaped type of central tissue projection. Over its entire length one surface of this coincided with the rootlet surface and a thin tapering strip of peripheral nervous tissue extended proximally for a considerable distance into the emergent rootlet segment. Type 4 rootlets emerged from the brainstem surface obliquely and contained a glial fringe.