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.