Spinal axon collaterals of vestibulo-spinal tract neurons identified by intracellular injection of horseradish peroxidase (HRP)

Abstract

The branching patterns of axons of lateral vestibulo-spinal tract neurons (LVSTNs) wereinvestigated in the cervical cord of the cats. After electrophysiological identification of axons of LVSTNs, HRP was injected iontophoretically into single axons. The branching patterns of 47 single axons were reconstructed in three dimensions from 100μm thick serial sections stained by the Heavy Metal Method. Many LVSTNS had multiple axon collaterals at different cervical segments (maximum 7 collaterals). Each axon collateral had a narrow rostro-caudal extension, usually less than 1.0mm in length. In the upper cervical cord (C1-C4), axon terminals were observed in the gray matter of laminae VIII and IX of Rexed, and they made direct contact with the proximal dendrites and cell bodies of motoneurons. In the lower cervical cord (C5-C8), axons of LVSTNs were divided into two types; medial and lateral types. Medial type: This was the most popular type (65%). Most of the axon branches projected to laminae VII and VIII. Some of them projected directly into the contralateral gray matter through the anterior commissure. Lateral type: The axon branches of this type projected to lamina IX and made direct contact with the proximal dendrites and cell bodies of motoneurons in this lamina.