Distribution and innervation of short, interdigitated muscle fibers in parallel-fibered muscles of the cat hindlimb

The cat hindlimb contains several long, biarticular strap mus- cles composed of parallel muscle fascicles that attach to short tendons. Three of these muscles - sartorius, tenuissimus, and semitendinosus - were studied by dissecting individual gold-stained fibers and determining the surface dis- tribution of acetylcholinesterase-stained end-plate zones. In each muscle, fascicles were composed of muscle fibers that ran only part of the fascicle length and tapered to end as fine strands that interdigitated with other taper- ing fibers within the muscle mass. Most muscle fibers measured 2-3 cm in length. Fascicles of muscle fibers were crossed by short transverse bands of endplates (1 mm wide by 1-5 mm long) that were spaced at fairly regular intervals from the origin to the insertion of the muscle. The endplate pattern suggested that the fiber fascicles were organized into multiple longitudinal strips. In the sartorius, the temporospatial distribution of electromyographic (EMG) activity evoked by stimulating fine, longitudinal branches of the parent nerve confirmed that each strip was selectively innervated by a small subset of the motor axons. These axons appeared to distribute their endings through- out the entire length of the fascicles, providing for synchronous activation of their in-series fibers. Although mammalian muscles can exhibit a remarkable variety of forms (Warwick and Williams, '731, most can be classified as pin- nate or parallel according to their fiber ori- entations (e.g. McMahon, '84; Pierrynowksi and Morrison, '85). Pinnate muscles are usu- ally composed of fibers that are much shorter than the muscle as a whole and run obliquely between two (or more) closely approximated planes of attachment. In contrast, parallel- fibered muscles commonly consist of fiber fascicles running most of the length of the muscle and arranged in sheet-like or strap- like arrays that are parallel to the line-of- pull of the muscle. The functional signifi- cance of fascicle arrangements in pinnate versus parallel muscles has been examined theoretically and experimentally in some de- tail (Gans and Bock, '65; Gans, '82), and it is now well recognized that the architectural arrangement has a significant effect on the length-tension relationship, shortening ve- locities, and force-developing capabilities of the muscle.