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

TLDR

The cat hindlimb contains long biarticular strap muscles composed of parallel fascicles that attach to short tendons. The authors dissected gold‑stained fibers from the sartorius, tenuissimus, and semitendinosus to map acetylcholinesterase‑stained end‑plate zones on the muscle surface. The muscles contain fascicles of 2–3 cm fibers that taper into fine interdigitating strands, are crossed by regularly spaced short end‑plate bands forming multiple longitudinal strips, and each strip is selectively innervated by a subset of motor axons that distribute along the fascicle, enabling synchronous activation.

Abstract

The cat hindlimb contains several long, biarticular strap muscles 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 distribution 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 tapering 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 throughout the entire length of the fascicles, providing for synchronous activation of their in-series fibers.