Marked non-uniformity of fiber-type composition in the primate suboccipital muscle obliquus capitis inferior

Obliquus capitis inferior (OCI) is a monoarticular suboccipital muscle linking the transverse process of the atlas (C1) to the spinous process of the axis (C2). Histochemical analysis of fiber-type composition showed that the muscle has a marked gradient of fiber-type distribution in which type I fibers comprise 95-100% of fibers in the deepest region but less than 10% of fibers in the superficial layer. Step-like changes in fiber-type proportions occurred between groups of fascicles. In most instances the boundaries between these fascicles did not exhibit different perimysial features from those fascicles with similar fiber-type proportions. OCI contained large numbers of muscle spindles, which were concentrated in deep regions rich in type I fibers. The degree of nonuniformity in fiber-type distribution seen in OCI is unusually large when compared with patterns described in other primate muscles, and has implications for the way that the muscle is studied anatomically and physiologically.