The cytoarchitecture, cytology, and synaptic organization of the basilar pontine nuclei in the rat. I. Nissl and golgi studies

Studies of the rat basilar pontine gray in Nissl‐stained frozen sections revealed the presence of four major subdivisions referred to as medial, ventral, lateral, and peduncular nuclei with respect to their position relative to the cerebral peduncle. Neurons located adjacent to the surface of, or within the cerebral peduncle were considered collectively as the peduncular nucleus. It should be emphasized that the above subdivisions are useful only for descriptive purposes and do not necessarily reflect distinct cytological similarities or differences within the identified cell groups. The dorsal peduncular (pontine) region was continuous around the lateral and medial boundaries of the peduncle with more ventral pontine areas. A distinct cluster of neurons was consistently observed near the ventromedial surface of the basilar pons, seemingly walled‐off by fascicles of pontocerebellar axons. In addition, in most animals, a cleft appeared in the peduncle and allowed direct communication between dorsal and ventral peduncular regions, while the medial lemniscus formed a partial boundary separating dorsal pontine neurons from the nucleus reticularis tegmenti pontis. Golgi studies indicated that four general varieties of pontine neurons could be distinguished, and in order of relative frequency were described as spine‐laden, pauci‐spined, pauci‐spined with long, thin dendrites, and small bipolar or unipolar cells whose axon sometimes collateralized within the pons. A wide variety of spinous processes and finger‐like dendritic protrusions, some of which formed a terminal tuft‐like arrangement, characterized pontine neuron dendrites. In addition, some dendrites of small neurons gave rise to rather long, thin, beaded processes which in certain instances emanated from an unusual triangulated branch point enlargement. Dendritic orientation (neuron shape) was sometimes determined by location, with cells in the peduncular nucleus oriented parallel to the nearest peduncular surface, while cells near the ventral pontine surface were arranged parallel to transversely directed pontocerebellar axons. Cells of medial pontine regions did not project dendrites across the midline, while neurons near the peduncular cleft sent dendrites into that space. Based on the locations of various pontine afferent termination zones as determined in previous as well as ongoing studies, it appears that some pontine neurons exhibit dendritic systems reaching into more than one afferent projection area, thereby affording them an opportunity to receive inputs from several afferent systems.