The pathology of the human muscle spindle: Effect of denervation

Abstract

The effect of denervation on the morphology and innervation of human muscle spindles has been studied in mixed sensorimotor neuropathies and in sensory and motor denervation in 25 cases examined at necropsy. In sensorimotor denervation the changes found consisted of degeneration and atrophy of the intrafusal muscle fibres, which were often clumped together in the centre of the dilated spindle capsules, degeneration of the equatorial aggregations of sarcolemmal nuclei and, in longstanding denervation, of collagenous thickening of the spindle capsule and deposition of collagen around the intrafusal muscle fibres. There was a reduction in number of sensory and motor nerve fibres in the affected spindles and changes were found in both the sensory and the motor nerve endings. In sensory denervation there was atrophy of the equatorial regions of the intrafusal muscle fibres, with degeneration and loss of the equatorial nuclei and simplification and degeneration of the primary and secondary sensory endings. Excess intraluminal collagen was sometimes found in the equatorial regions. The polar parts of the spindle capsules and of the intrafusal muscle fibres, and the fusimotor innervation, were normal. In tabes dorsalis attempts at regeneration of the sensory endings were sometimes recognisable and there was swelling and attenuation of the terminal part of the primary and secondary sensory axons. In motor denervation degenerative changes were found in both gamma and alpha-collateral fusimotor nerve fibres and motor endings, and in some spindles no motor nerve fibres could be identified. The polar regions of the intrafusal muscle fibres were thin and atrophic, and, as in sensorimotor denervation, degeneration of nuclear chain fibres was more prominent than, and preceded, degeneration of nuclear bag fibres. This could not easily be related to the differing patterns of innervation of these two types of muscle fibre, but seemed to be due to structural differences in the fibres themselves. The equatorial regions of these intrafusal fibres and their sensory innervation were normal.