Unusual intrafusal fibres in human muscle spindles

We have studied the morphology and pattern of expression of myosin heavy ch ain (MHC) isoforms of intrafusal fibres in a human first lumbrical muscle. Each intrafusal fibre type, namely nuclear bag(1), nuclear bag(2) and nucle ar chain fibres, had a distinct MHC composition and distribution of differe nt MHC isoforms along the whole length of intrafusal fibres. However, most muscle spindles analyzed also contained one or several intrafusal fibres ex hibiting an extrafusal or mixed pattern of immunoreactivity which did not c orrespond to any of the described intrafusal fibre types. We conclude that the latter fibres do not represent new intrafusal fibre types, but their mo rphology and expression of MHC merely reflects the differences in their inn ervation owing to their unusual localization at the edge or outside the axi al bundle of intrafusal fibres.