An electron microscopic study of the direct spino-olivary projection to the inferior olivary nucleus in the rat.

Prior to the electron microscopic identification of spino-olivary terminals, the distribution of the direct spino-olivary projection was determined at the light microscopic level. Lesions were placed in the cervical or thoracic spinal cord in 7 rats. After a 7 day survival, animals were perfused, and transverse sections through the medulla were processed according to the Fink-Heimer technique. Terminal degeneration was found in the caudal third of the medial accessory olive, and in the lateral half of the dorsal accessory olive throughout its length. In order to determine the ultrastructural characteristics of direct spino-olivary terminals, lesions were placed in the cervical spinal cord of a further 4 rats. After a 2 or 3 day survival period, the animals were processed for electron microscopy. The portions of the inferior olivary nucleus in which degeneration had been noted in the Fink-Heimer studies were sampled and examined in the electron microscope. The majority of degenerating spino-olivary terminals underwent an electrondense change. Most degenerating terminals contained round, clear vesicles, but a few were found to contain predominantly flattened vesicles. Degenerating terminals contacted small dendritic profiles or spines, neither of which were associated with synaptic clusters.