Structural characteristics of connections between medial descending systems and spinal neurons

Abstract

Structural and ultrastructural changes in the medial part of the ventral horn were studied in segments of the cat spinal cord following destruction of the ventral column at the level C1–C2. Analysis of results obtained by the Fink — Heimer method showed that degenerating preterminals occur mainly in Rexed's lamina VIII and also in ventromedial zones of lamina VII. Preterminals of descending pathways of the ventral column are also found in the intermediate nucleus of Cajal (central part of lamina VI) and in the ventromedial motor nucleus. Fewer of these preterminals are present in the thoracic and, in particular, in the lumbar segments. Staining by the Hollander — Vaaland method revealed degenerating myelinated axons of small diameter (3–5 µ), evidently collaterals of descending fibers entering the gray matter, in lamina VIII. Degenerative changes in myelinated axons may be manifested either as marked condensation and shrinking or as the appearance of numerous neurofilaments, polymembraneous structures, and cytolysomes. Degeneration also affects axon terminals (axo-dendritic, axo-somatic, and axo-axonal) with spherical or flattened synaptic vesicles. Counting the relative numbers of intact terminals of the various types and their comparison with the corresponding figures for normal animals shows that most connections of descending fibers with spinal neurons are axo-dendritic in character. No degenerating terminals were found on the soma of the "dark" neurons or their processes.