Fig. 8.

The distribution of intramuscular axons and relative absence of fast muscle fibers in a typical “all-slow” IFIB muscle in a rotated hindlimb. A, St. 36 whole-mount preparation of intramuscular nerves in an “all-slow” experimental IFIB muscle shows the typical slow muscle innervation pattern with axons growing longitudinal to the muscle fibers and extending intermittent collateral sprouts. Note the complete absence of a fast, transversely oriented pattern of axon branching characteristic of the normal fast, posterior region of the muscle. Myotubes run fromleft (proximal) to right (distal).B, C, Cross section through a similar IFIB with double immunostaining for slow muscle myosin (B) and fast muscle Ca²⁺ ATPase (C) shows that the “all-slow” IFIB muscle is composed almost entirely of slow myotubes. Scale bars:A, 500 μm; B, C, 200 μm.