The innervation of the human lumbar facet joint capsule was studied
by light and electron microscopy.
Small numbers of encapsulated corpuscular endings were identified in the
dense fibrous layer. Clusters of 2
types of endings were found: small cylindrical corpuscles (type 1) and
fusiform corpuscles (type 2). The
corpuscles were classified structurally as Ruffini-type endings. The 1st
type was predominant and
characterised by a compartmentalised receptor complex, a thin perineurial
capsule and a narrow subcapsular
space. The 2nd type was characterised by a thicker perineurial capsule,
a ‘spindle-like’ receptive complex,
and an extensive subcapsular space with capillaries and concentrically
oriented fibroblast-like cells. Both
types of endings were innervated mainly by thinly myelinated group III
delta) and unmyelinated group
IV (C) nerve fibres that branched and terminated in the receptor complex.
sensory endings were
intimately related to the collagen fibre bundles as multiple enlarged
axonal segments (‘beads’) with
ultrastructural features which were characteristic of receptive sites:
an accumulation of mitochondria and
vesicles, and ‘bare’ areas of axolemma lacking a
Schwann cell investment but covered by a thin basal
lamina. Some beads in the 2nd type of ending contained granular vesicles,
30–60 mm in diameter,
resembling sympathetic nerve endings. Small diameter collagen fibrils
situated within multilayered basal
laminae were found among the multiple receptive sites in the receptive
in both types of ending.
Their possible functional significance in mechanoreception is discussed.
Particular attention has been given
to their apparent variable orientation to the mechanoreceptive site.