Mechanoreceptor Endings in Human Cervical Facet Joints
Twenty-one cervical facet capsules, taken from three normal human subjects, were examined to determine the type, density, and distribution of mechanoreceptive nerve endings in these tissues. Clearly identifiable mechanoreceptors were found in 17 of 21 specimens and were classified according to the scheme for encapsulated nerve endings established by Freeman and Wyke. Eleven Type I, 20 Type II, and 5 Type III receptors were identified, as well as a number of small, unencapsulated nerve endings. Type I receptors were small globular structures measuring 25–50 μm 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 amorphous capsule, within which a reticular meshwork of fine neurites was embedded. Free (nociceptive) nerve endings were found in subsynovial loose areolar and dense capsular tissues. The presence of mechanoreceptive and nociceptive nerve endings in cervical facet capsules proves that these tissues are monitored by the central nervous system and implies that neural input from the facets is important to proprioception and pain sensation in the cervical spine. Previous studies have suggested that protective muscular reflexes modulated by these types of mechanoreceptors are important in preventing joint instability and degeneration. It is suggested that the surgeon take steps to avoid inadvertently damaging these tissues when exposing the cervical spine.