Routes of putative afferent axons in rat lumbosacral ventral roots and pia mater.

Unmyelinated sensory axons occur in mammalian ventral roots. On this basis the law of Magendie has been questioned. With few exceptions, the current ideas on the routes that these axons might follow have been based on electron microscopic examination of transverse sections, and evidence obtained through electrophysiological experiments or tracer studies. The purpose of the present study is to examine putative afferent rat ventral root axons, which have been visualized directly by immunohistochemical labelling. After immunolabelling of axons containing substance P or calcitonin gene-related peptide in longitudinal sections from the ventral roots L4-S1, and in wholemount pia mater-rootlet preparations, fluorescent fibres were searched for in the microscope. The results show that some immunolabelled axons end blindly in the root, while others shift to the pia mater, or make U-turns. Some axons make irregular 'zigzag' deviations or branch. No immunoreactive axons were seen entering the spinal cord via the ventral roots. These observations support the view that the law of Magendie is valid.