ENCAPSULATED RUFFINI-LIKE ENDINGS IN HUMAN LUMBAR FACET JOINTS

The innervation of the human lumbar facet joint capsule was studied by light and electron microscopy. Small numbers of encapsulated corpuscu lar endings were identified in the dense fibrous layer. Clusters of 2 types of endings were found: small cylindrical corpuscles (type 1) and large fusiform corpuscles (type 2). The corpuscles were classified st ructurally as Ruffini-type endings. The 1st type was predominant and c haracterised by a compartmentalised receptor complex, a thin perineuri al capsule and a narrow subcapsular space. The 2nd type was characteri sed by a thicker perineurial capsule, a 'spindle-like' receptive compl ex, and an extensive subcapsular space with capillaries and concentric ally oriented fibroblast-like cells. Both types of endings were innerv ated mainly by thinly myelinated group III (A delta) and unmyelinated group IV (C) nerve fibres that branched and terminated in the receptor complex. Their sensory endings were intimately related to the collage n fibre bundles as multiple enlarged axonal segments ('beads') with ul trastructural features which were characteristic of receptive sites: a n accumulation of mitochondria and vesicles, and 'bare' areas of axole mma lacking a Schwann cell investment but covered by a thin basal lami na. Some beads in the 2nd type of ending contained granular vesicles, 30-60 mm in diameter, resembling sympathetic nerve endings. Small diam eter collagen fibrils situated within multilayered basal laminae were found among the multiple receptive sites in the receptive complex in b oth types of ending. Their possible functional significance in mechano reception is discussed. Particular attention has been given to their a pparent variable orientation to the mechanoreceptive site.