Relation between putative afferent axons and the glia limitans in rat motor roots

The law of Magendie states that ventral roots channel efferent axons from the spinal cord to the periphery, while dorsal roots channel afferent axons from the periphery into the spinal cord. As primary afferent C-fibres occur in mammalian ventral roots, this law has been questioned. However, other observations suggest that ganglionic axons do not enter the spinal cord via ventral roots. The present paper examined, by double labelling immunohistochemistry, the relation between putative peripheral afferents and the PNS/CNS transition in the trigeminal motor root and in selected spinal ventral roots of the rat. The afferents were labelled with antibodies against vasoactive intestinal polypeptide, substance P or calcitonin gene-related peptide. The glia limitans at the PNS/CNS transition was defined with antibodies against glial fibrillary acidic protein. The results showed that no immunoreactive axons occurred in the trigeminal motor root. However, in all ventral roots examined, labelled axons were frequently observed. While some of these ended blindly, looped or branched in the rootlets, others shifted to the pia mater. Immunoreactive axons crossing the glia limitans at the PNS/CNS transition were not observed. Thus, the results obtained support the law of Magendie.