Sensory and sympathetic innervation of cervical facet joint in rats.

Objective: To explore the patterns of innervation of cervical facet joints and determine the pathways from facet joints to dorsal root ganglions (DRGs) in order to clarify the causes of diffuse neck pain, headache, and shoulder pain.

Methods: Forty-two male-Sprague-Dawley rats, weighing 250-300 g, were randomly divided into three groups: Group A (n=18), Group B (n=18), and Group C (n=6). Under anesthesia with intraperitoneal pentobarbital sodium (45 mg/kg body weight), a midline dorsal longitudinal incision was made over the cervical spine to expose the left cervical facet joint capsule of all the rats under a microscope. The rats in Group A underwent sympathectomy, but the rats in Group B and Group C did not undergo sympathectomy. Then 0.6 microlitre 5% bisbenzimide (Bb) were injected into the C1-2, C3-4 and C5-6 facet joints of 6 rats respectively in Group A and Group B. The holes were immediately sealed with mineral wax to prevent leakage of Bb and the fascia and skin were closed. But in Group C, 0.9% normal saline was injected into the corresponding joint capsules. Then under deep re-anesthesia with intraperitoneal pentobarbital sodium (45 mg/kg body weight), C1-C8 left DRGs in all rats and the sympathetic ganglions in Group B were obtained and the number of the labeled neurons was determined.

Results: Neurons labeled with Bb were present in C1-C8 DRGs in both Group A and Group B, and sympathetic ganglions in Group B. In the C1-2 and C3-4 subgroups, labeled neurons were present from C1 to C8 DRGs, while in C5-6 subgroups they were from C3 to C8. The number of Bb(+) neurons after sympathectomy was not significantly different in the injected level from that without sympathectomy. But in the other levels, the number of Bb(+) neurons after sympathectomy was significantly less than that without sympathectomy.

Conclusions: The innervation of the cervical facet joints is derived from both sensory and sympathetic nervous system, and DRGs are associated with sympathetic ganglions through nerve fibers outside the central nerve system.