Distribution of A-δ and C-Fiber Receptors in the Cervical Facet Joint Capsule and Their Response to Stretch

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Background: It has been proposed that cervical facet joint capsules are a major source of whiplash pain. However, there is a paucity of neurophysiologic data to support this hypothesis. The purposes of this study were to determine the distribution of A-δ and C-fiber sensory receptors in the facet joint capsule and to test their patterns of response to stretch and related sensory function.

Methods: Laminectomy from C4 to C7 was performed in seventeen goats, while they were under general anesthesia, to expose the C6 nerve roots. Customized dual bipolar electrodes were used to record neural activity from one of the C6 branches. An 8 or 15-V electrical stimulus was used to provoke receptor activity in nine designated areas on the dorsal part of the C5-C6 facet joint capsule. Receptors were classified on the basis of conduction velocities. The waveform of an identified receptor was set up as a template to determine its neural activity in response to capsular stretch. The characteristics of each single receptor's response to capsular stretch were analyzed to determine its sensory function as a mechanoreceptor or nociceptor.

Results: Two hundred and forty-eight receptors on the dorsal part of the C5-C6 facet joint capsule were evoked by electrical stimulation in the seventeen goats. More C-fiber receptors were found on the dorsolateral aspect of the facet joint capsule, where tendons and muscles were attached. The response to stretch of 120 receptors, from twelve goats, were analyzed to classify them into one of four categories (high-threshold mechanoreceptors, non-saturated low-threshold mechanoreceptors, saturated low-threshold mechanoreceptors, and silent receptors) or as unclassified receptors.

Conclusions: The existence of receptors in the facet joint capsule indicates that the capsule has pain and proprioceptive sensory functions.

Clinical Relevance: The cervical facet joint capsule may be a pain generator in whiplash-related disorders and chronic neck pain. On the basis of the assumption that human and goat facet-joint-capsule innervations are similar, it appears that treatment focusing on the dorsolateral aspect of the cervical facet joint may produce more effective outcomes by ablation of neural activity from nociceptors and the mechanoreceptors that are involved in pain.

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