The direct spinal area of the inferior olivary nucleus: an electron microscopic study.

Identification of the direct spinal areas (portions of the dorsal and medial accessory nuclei) within the opossum inferior olivary complex was accomplished by mapping the location of the terminal degeneration by the Fink-Heimer technique subsequent to cervical cord lesions. Following similar lesions, sampling of these same regions for electron microscopic study was assured by examination of transversely oriented, 1 mu plastic sections prior to thin sectioning. The first evidence of electron dense axon terminals was found at a survival time of 24 hours. At survival times of 36, 48 and 72 hours, degenerating presynaptic profiles shrink, become irregular in shape and are totally or partially surrounded by glial processes. Spinal terminals average 1-2 mu in their greatest dimension, contain round, clear synaptic vesicles and generally contact small diameter (0.4--1.8 mu) dendritic shafts or occasional spiny appendages. The spiny dendritic appendages make up the central core of the olivary glomeruli and these juxtaposed dendritic processes exhibit gap junctions. At longer survival times (5, 7 and 9 days) many presynaptic profiles with either round or pleomorphic synaptic vesicles remain normal in appearance and contact dendritic shafts or the spiny appendages within glomeruli. Afferents from other sources (possible including intrinsic neurons) must terminate within the direct spinal portion of the nuclear complex to account for the numerous axon terminals which retain normal morphology after such long survival times.