Histochemical and morphological changes in human muscle spindle in upper and lower motor neuron lesions.

The human intrafusal fibers were found to consist of two morphological and three histochemical (ATPase reaction) types. Two types of nuclear bag fibers were seen. Type A showed alkali stable and acid labile and Type B showed acid and alkali stable ATPase reaction. The nuclear chain fibers showed only alkali stable ATPase reaction. In the lower motor neuron atrophy, muscle spindle showed thickening of the capsule, atrophy of the nuclear chain fibers, and splitting of the nuclear bag fibers. ATPase reaction showed targetoid Type B nuclear bag fibers and 2 types of nuclear chain fibers. Oxidative enzymes also showed targetoid nuclear bag and chain fibers. In upper motor neuron atrophy the spindle showed no changes.