Mechanosensitive afferent units in the lumbar posterior longitudinal ligament

Study Design. The mechanosensitive afferent units in the lumbar posterior l ongitudinal ligament were investigated in an animal model using an electrop hysiologic technique. Objectives. The objectives of this study were to identify the mechanosensit ive receptive fields in the lumbar posterior longitudinal ligament and to i nvestigate their distribution and characteristics. Summary of Background Data. The lumbar posterior longitudinal ligament has a nerve network originating from the sinuvertebral nerve. These fibers are thin, and most of their terminals are free nerve endings. Some immunohistoc hemical studies have indicated that they are immunoreactive to calcitonin g ene-related peptide and/or substance P, suggesting a nociceptive function. Most of these studies investigated morphologic aspects, and there have been few studies employing electrophysiologic techniques to examine mechanosens itive units. Methods. We used 13 adult cats. They were anesthetized and then laminectomy was performed. The L5 and L6 dorsal rootlets were draped over a recording electrode. To investigate the receptive fields in the posterior longitudina l ligament, afferent impulses were evoked by mechanical stimulation with a glass probe. When the receptive fields were located, they were stimulated e lectrically to obtain conduction velocity and were Stimulated with a set of 17 nylon filaments to determine their mechanical thresholds. Results. Thirteen units were identified in the lumbar posterior longitudina l ligament. The majority of the units were located around the intervertebra l disc level of the posterior longitudinal ligament. The mean mechanical th reshold was 47.04 +/- 15.25 g. According to the conduction velocities of th e units, 12 units were classified into Group III (0.5-2.5 m/sec) and one un it into Group IV (2.5-20 m/sec). Conclusion. Mechanosensitive units classified into Group III or Group IV an d with a high mechanical threshold (>7.0 g) were thought to act as nocicept ive units. All units identified in this study satisfied these criteria. Our result suggests that afferent fibers from the lumbar posterior longitudina l ligament have a principally nociceptive function.