1Spine Research Laboratory, Bioengineering Center, Wayne State University, Detroit, Michigan
Study Design: To establish a methodology for the neurophysiologic study of mechanoreceptors in the cervical facet joint capsule.
Objectives: To test a custom designed miniature dual bipolar electrode for recording the neural activity in cervical dorsal roots. To determine if the neural activity from different receptors in the capsule can be differentiated using this methodology.
Summary of Background Data: Injury to cervical facet joint capsules has been regarded as an important source of whiplash pain, but no neurophysiologic study has been performed to demonstrate or characterize sensory nerve function in the capsule.
Methods: Nineteen goats weighing 34 to 55 kg were used under general anesthesia. A C4–C6 laminectomy was performed to expose the C6 nerve root. Custom designed miniature dual bipolar electrodes were used to record neural activity in the left C6 branches. Electrical and mechanical stimuli were used to evoke receptor activity in the dorsal aspect of the C5/6 capsule. Conduction velocities (CVs) of evoked units were determined by electrical stimulation and dual-bipolar-electrode recording methods. The units were classified based on their CVs. The waveform of each classified unit was saved as a template for later single unit discharge search among multiunit discharges during the stretch of the capsule. The C5/6 facet joint with capsule was pulled by a computer-controlled actuator instrumented with a load cell at a rate of 0.5 mm per second. The evoked neural activity and load were recorded, digitized, and analyzed to determine CV, discharge rate, and response to the stretch.
Results: Miniature bipolar electrodes recorded the neural activity in both channels, with single unit CVs being measured. There was no discernible motion between the electrode and dorsal root when the capsule was pulled. Both local compression and stretch on capsule evoked multiunit discharges. A-β, A-δ, and C-fiber units were found among these multiunit discharges. The rate of single unit and multiunit discharges increased during capsule stretch in the physiologic range and afterdischarges occurred beyond the physiologic range.
Conclusions: The novel miniature electrodes not requiring a micromanipulator made it feasible and reliable to record neural activity from short cervical spinal roots. Waveforms of different units could be identified, making it possible to study sensory functions of the facet joint capsule. A-β, A-δ, and C-fiber units were found responding to mechanical stimuli, indicating that facet joint capsule has functional proprioceptors and nociceptors.