Intradural Communication of L3 and L4 Ventral Nerve Rootlets of Spinal Nerves

Abstract

Normally, the nerve roots within the dural sec never communicate with one another. Even the fusion of the ventral and dorsal nerve roots to form the spinal nerve occurs outside the dural sac. In dissecting the spinal cords of 54 cadavers, we found one abnormal case in which the ventral nerve rootlet of the L3 spinal nerve gave a branch which joined with the L4 nerve rootlet within the dural sac. This communication consisted only of motor fibers. This pattern is different from patterns of distribution of normal spinal nerves, which are described by their segmentation and peripheral distribution patterns, both of which occur outside the dural sac and comprise of both motor and sensory fibers. Several causes for this abnormality have been proposed. Firstly, some neurons of the L4 anterior horn may have interposed with the L3 anterior horn cells. These neurons may have given axons and which joined with the L3 spinal nerve, then split back to join with the L4 spinal nerve, to ultimately supply the myotome of the L4 somite. Secondly, the fusion of myotomes of several somites to form one specific muscle may have developed sooner than its nerve and may have influenced the communication between the L3 and L4 nerve rootlets within the dural sac. An ultrastructural study of the communication segment by TEM clearly reveals a bundle of myelinated nerve fibers which are all peripheral nerves with their Schwann cells covered externally.