Fine Structure of the Mesencephalic Nucleus of the Trigeminal Nerve in the Rat

The mesencephalic nucleus of the trigeminal nerve of the male adult rat was studied under electron microscopes and the following results were obtained.1. The neuronal cytoplasm is characterized by a network of Nissl substance composed of rosette-like polysomes and free ribosomes rather than granular reticula. Numerous Golgi complexes are situated apart from the cell nucleus, accompanying several lysosomal dense bodies. Lipofuscin-like dense bodies also often appear at a distance from the Golgi complexes.2. Some neurons of the mesencephalic nucleus exhibit soma-somatic junctions with an intercellular space of about 110-300Å. These junctions are not considered temporary or vestigial structures, because of their frequent occurrence and the linear array of mitochondria. It might be questioned whether this type of junction acts as an electrical synapse in the central nervous system of the rat.3. Excepting the soma-somatic junctions, the neuronal surface shows irregular projections (or crests) and depressions. In an extreme case, a dendrite is deeply infolded within the cleft of the cytoplasm formed by soma spines (crests), but the significance of this structure is unknown.4. The afferent terminals form axo-somatic synapses on the irregular surface of the perikarya. We classified these synaptic terminals into S-type and F-type (UCHIZONO, BODIAN) according to the forms of their synaptic vesicles and discussed their possible origins.