Tyrosine hydroxylase-immunoreactive nerve fibres in rat posterior longitudinal ligament

The nerve supply to the posterior longitudinal ligament (PLL) of the lumbar vertebrae has been the focus of considerable interest to gain insight into the pathogenesis of low back pain. The present study aimed to characterize the sympathetic fibres in the PLL by immuno-electronmicroscopy for tyrosine hydroxylase (TH), the rate limiting enzyme in catecholamine synthesis. The posterior central branches of the segmental lumbar arteries received numerous communicating fibres from the sinuvertebral nerve (SVN), but only shortly after their entrance to the spinal canal. The non-vessel-associated branches of the SVN formed transverse bundles, which met fibres from the opposite side in a plexus-like mid-sagittal network. As these fibres approached the midline, they gradually lost their Schwann cell cover. The free and naked fibres contained numerous terminal-like varicosities. The TH-ir and 6-hydroxydopamine (6-OH-DA) sensitive fibres were intermingled with non-TH-ir fibres. The TH-ir sympathetic fibres had no obvious target structures except for the numerous, intermingled, closely related and communicating terminal-like axons in the mid-sagittal network in contact with non-TH-ir fibres. This may represent a neuroanatomical equivalent reflecting modulatory functions, which could participate in the pathogenesis of low back pain.