Attachments of human intrafusal fibers

The human intrafusal fibers have two major types of attachments: (1) extracapsular, involving the bag and long chain fibers which tapered into the extrafusal endomysium; and (2) intracapsular, involving only the short chain fibers which attached to the outer or inner capsule as well as to the bag, and other chain fibers. At all the attachment sites except the interfiber attachments, the intrafusal fibers showed alterations in the basement and plasma membrane, streaming of the Z band, and rod bodies. This profile was similar to the myotendinous junctions. The terminal sarcomere segment consisted of the actin filaments, thickening of the plasma membrane, and folded basal lamina. The interfiber attachments were of two types--the gap and tight junctions. There were thus two major functional types of attachment, the myotendinous type contacts which were mechanical and the interfiber attachments which represent electrical continuity between the two chain fibers.