An electron microscope study of synaptic organization in the lateral reticular nucleus of the medulla oblongata in the cat.

Synaptic organization in the lateral reticular nucleus (LRN) was investigated electron microscopically in the cat. The number of synaptic knobs encountered in a survey of 69 somatic profiles cut through the nucleolar plane varied from 0 to 20 per profile. In 5564 mum of cell perimeters analyzed the number of synaptic knobs per 100 mu was 5.2, ranging from 0 to 22 in each somatic profile. About 46% of the axosomatic synaptic knobs were filled with round vesicles, and 54% with pleomorphic ones. Out of 1424 axodendritic synaptic knobs, about 63% were filled with round vesicles, and 37% with pleomorphic ones. After placing lesions in the spinal cord, frontal cortices or red nuclear areas, electron-dense degenerated synaptic knobs were observed in the LRN. In the cats with spinal lesions degenerated knobs with the active zones of the synapses were found both on somatic and dendritic profiles. On the other hand, in the cats with cortical or rubral lesions degenerated knobs with the active zones were encountered only upon dendritic profiles. These degenerated knobs found in the present study never exceeded more than 3% of the total synaptic population in the areas examined. Thus, the axon terminals of fibers arising from the spinal cord, frontal cortices and red nuclear areas constituted only a small fraction of the total axonal endings in the LRN.