554 THE JOURNAL OF BONE AND JOINT SURGERY Sympathetic afferent units from lumbar intervertebral discs T. Takebayashi, J. M. Cavanaugh, S. Kallakuri, C. Chen, T. Yamashita From Sapporo Medical University School of Medicine, Sapporo, Japan  T. Takebayashi, MD, PhD, Orthopaedic Surgeon  T. Yamashita, MD, PhD, Professor and Chairman Department of Orthopaedic Surgery Sapporo Medical University School of Medicine, South-1, West-16, Chuo-ku, Sapporo 060-8543, Japan.  J. M. Cavanaugh, MD, PhD, Professor  S. Kallakuri, MS, Assistant Professor  C. Chen, MD, Assistant Professor Bioengineering Center Wayne State University, 818 W. Hancock, Detroit, Michigan 48201, USA. Correspondence should be sent to Dr T. Takebayashi; e-mail: takebaya@sapmed.ac.jp ©2006 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.88B4. 17194 $2.00 J Bone Joint Surg [Br] 2006;88-B:554-7. Received 15 September 2005; Accepted after revision 15 December 2005 To clarify the pathomechanisms of discogenic low back pain, the sympathetic afferent discharge originating from the L5-L6 disc via the L2 root were investigated neurophysiologically in 31 Lewis rats. Sympathetic afferent units were recorded from the L2 root connected to the lumbar sympathetic trunk by rami communicantes. The L5-L6 discs were mechanically probed, stimulated electrically to evoke action potentials and, finally, treated with chemicals to produce an inflammatory reaction. We could not obtain a response from any units in the L5-L6 discs using mechanical stimulation, but with electrical stimulation we identified 42 units consisting mostly of A-delta fibres. In some experiments a response to mechanical probing of the L5-L6 disc was recognised after producing an inflammatory reaction. This study suggests that mechanical stimulation of the lumbar discs may not always produce pain, whereas inflammatory changes may cause the disc to become sensitive to mechanical stimuli, resulting in nociceptive information being transmitted as discogenic low back pain to the spinal cord through the lumbar sympathetic trunk. This may partly explain the variation in human symptoms of degenerate discs. Lumbar intervertebral discs are recognised as one of the major sources of low back pain. Elu- cidating the sensory innervation of the lumbar intervertebral disc and its adjacent tissues is important in understanding the mechanism of discogenic low back pain. It is accepted that lumbar intervertebral discs are innervated seg- mentally, with sinuvertebral nerves branching from the nerve fibres of the lumbar spine and the rami communicantes of the corresponding levels. 1-3 Recently, it has been demonstrated in rats that the sinuvertebral nerves proceed into the paravertebral sympathetic trunk through the rami communicantes. 2,4 Several immuno- histological studies in the rat 5-7 using retro- grade neurotracer methods have shown inner- vation of the L5-L6 intervertebral discs and adjacent tissues from neurons of the L1 and L2 dorsal root ganglia, through the paravertebral sympathetic trunk. Similarly, it has been reported in neurophysio- logical studies that mechanosensitive afferent units with receptive fields in the retroperitoneal space, including ventral aspects of the lumbar intervertebral disc and psoas muscles, can run through the paravertebral sympathetic trunk. 8,9 In addition, these units responded to mechani- cal and chemical stimuli such as potassium chloride, hypertonic saline solution and brady- kinin, which may indicate the involvement of nociceptive units. In the gastrointestinal sys- tem, it has been demonstrated that sympathetic C-fibre afferents with low and high thresholds can encode nociceptive information, such as excessive distension and ischaemia. 10 Collec- tively, these studies imply that sensory infor- mation, including nociceptive stimulation from the lower lumbar intervertebral discs inner- vated by the sinuvertebral nerves, may be con- ducted by the paravertebral sympathetic trunk, via the rami communicantes, and then through the L1 and L2 dorsal root ganglia into the spi- nal cord. Although the sensory pathway from the lower lumbar intervertebral discs has been investigated both histologically and anatom- ically, the mechanisms of discogenic low back pain are still unknown. Using neurophysiological techniques, we investigated in a rat model whether sympa- thetic afferent discharges originating from lower lumbar intervertebral discs and adjacent tissues pass through L2 dorsal roots as post- ulated and examined the properties of the afferent units. Materials and Methods All the surgical procedures were reviewed and approved by the Animal Investigation Com- mittee at the institution where the experiments were carried out. The study was performed on