MECHANORECEPTOR ENDINGS IN HUMAN CERVICAL FACET JOINTS

Twenty-one cervical facet capsules, taken from three normal human subj ects, were examined to determine the type, density, and distribution o f mechanoreceptive nerve endings in these tissues. Clearly identifiabl e mechanoreceptors were found in 17 of 21 specimens and were classifie d according to the scheme for encapsulated nerve endings established b y Freeman and Wyke, Eleven Type I, 20 Type II, and 5 Type III receptor s were identified, as well as a number of small, unencapsulated nerve endings. Type I receptors were small globular structures measuring 25- 50 mum in diameter. Type II receptors varied in size and contour, but were characterized by their oblong shape and broad, lamellated capsule . Type III receptors were relatively large oblong structures with an a morphous capsule, within which a reticular meshwork of fine neurites w as embedded. Free (nociceptive) nerve endings were found in subsynovia l loose areolar and dense capsular tissues. The presence of mechanorec eptive and nociceptive nerve endings in cervical facet capsules proves that these tissues are monitored by the central nervous system and im plies that neural input from the facets is important to proprioception and pain sensation in the cervical spine. Previous studies have sugge sted that protective muscular reflexes modulated by these types of mec hanoreceptors are important in preventing joint instability and degene ration. It is suggested that the surgeon take steps to avoid inadverte ntly damaging these tissues when exposing the cervical spine.