Histological study of motor innervation of nuclear bag1 intrafusal muscle fibers in the cat.

The nerve supply to spindles of the cat tenuissimus muscle was reconstructed with light and electron microscopy of serial transverse sections. Fifty-two poles of the nuclear bag1 intrafusal muscle fiber were examined for motor innervation. The fiber poles were supplied by 71 myelinated motor axons that either terminated on bag1 fibers exclusively (93%) or coinnervated a chain fiber of the same intrafusal bundle (7%). No axons coinnervated both the bag1 and bag2 fibers. The unmyelinated preterminal segments of the axons were frequently short. Lengths and pre- and postsynaptic features of motor endings on bag1 fibers were variable. These features did not permit reliable classification of the endings into more than one morphological category. Moreover, the terminals of fusimotor (gamma) and skeletofusimotor (beta) axons on bag1 fibers appeared similar in cross-section. The degree of indentation of axon terminals into the surface of bag1 fibers increased with increasing distance from the spindle equator. However, cross-sectional areas of sole plates and axon terminals were relatively constant regardless of distance from the equator. The subjunctional membranes of both gamma and beta bag1 endings were typically smooth in contour. Bag1 endings differed from those on bag2 and typical chain fibers in having a thicker sole plate, frequently indented axon terminals, and unfolded subjunctional membranes. None of the bag1 endings resembled an extrafusal end plate. These observations indicated that (1) the dynamic (bag1) and static (bag2 and chain) intrafusal systems of the cat spindle are under separate motor control, and (2) the type of intrafusal fiber and the distance of the motor ending from the equator have a greater influence on the form and structure of bag1 endings than do supplying axons.