Types of human intrafusal muscle fibers.

The histochemical and fine structural profiles of human intrafusal muscle fibers were studied. Spindles were located in freshly frozen specimens taken from biopsied normal external intercostal muscles, and periodic 10- and 50-mum-thick cross sections were processed alternately for enzyme histochemical and electron microscopic examination. Nuclear bag fibers were of two types, bag1 and bag2, histochemically, and they displayed two distinct types of ultrastructure. Nuclear chain fibers were histochemically and ultrastructurally homogeneous. Regional differences in enzymatic staining and ultrastructure occurred along individual intrafusal fibers. Human bag1 and bag2 fibers appear to be analogous to the two types of nuclear bag fiber identified in animal spindles and are considered to have different roles in spindle function. The presence of three types of intrafusal fibers should be taken into account when studying spindle abnormalities in human neuromuscular disorders.