The paratrigeminal nucleus. I. Neurons and synaptic organization.

The paratrigeminal nucleus, a diffuse collection of neurons on the lateral medullary surface, lies embedded in the fibres of the restiform body, ascending spinocerebellar tract and the spinal tract of the trigeminal nerve. Its rostrocaudal extent is approximately 1500--2000 micrometer in the rat, 2800--3500 micrometer in the rhesus monkey, and 5000--6000 micrometer in the human, where it is a well-defined nucleus. In Golgi preparations the neuronal somata are generally fusiform, ranging from 8--15 micrometer in width and 15--25 micrometer in length. Electron microscope studies show that their polar dendritic processes are thick and long and they intertwine in bundles among islands of cells or myelinated fibres. The perikarya have a high nucleus to cytoplasm ratio and the scanty cytoplasm is conspicuous for its paucity of organelles and its plethora of polysomal arrays. The neuropil is complex and contains a heterogeneous population of axonal varicosities displaying markedly different axoplasmic structures. Many different types of large granular vesicle-containing axons prevail. These axons engage in axo--somatic, axo--dendritic, axo--spinous and axo--axonic synapses with the processes of the cells within the nucleus.