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

Obliquus capitis inferior (OCI) is a monoarticular suboccipital muscle Link ing 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 fi bers 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 proportion s occurred between groups of fascicles. In most instances the boundaries be tween these fascicles did not exhibit different perimysial features from th ose fascicles with similar fiber-type proportions. OCI contained large numb ers of muscle spindles, which were concentrated in deep regions rich in typ e I fibers. The degree of nonuniformity in fiber-type distribution seen in OCI is unusually large when compared with patterns described in other prima te muscles, and has implications for the way that the muscle is studied ana tomically and physiologically.