Muscle-fiber compartmentalization in cat splenius muscles.

Splenius is a structurally complex muscle that is supplied by nerves from four cervical segments. We have examined the architectural organization of extrafusal fibers in splenius by microdissecting muscles stained "en bloc" with gold chloride. We also analyzed the distribution of motor units supplied from different cervical segments using the glycogen-depletion technique. Microdissections show that the fiber architecture of splenius is different in medial and lateral muscle parts. In medial splenius, muscle fascicles run uninterrupted from the muscle origin to insertion, while in lateral splenius, separate compartments of muscle fascicles are linked in series by two tendinous inscriptions. In muscle fascicles longer than 2.0 cm, many extrafusal fibers do not run the whole length of the fascicle. Instead, these fibers end intramuscularly by tapering to a fine strand that is closely adherent to adjacent extrafusal fibers. Less than 1% of extrafusal fibers in splenius exceeds 3.0 cm in length. Experiments using glycogen depletion showed that nerves from different spinal segments innervate different muscle regions. Nerves from C1 supply muscle fibers in a small rostromedial zone. Nerves from C2-C4 have motor domains that extend from the medial to the lateral muscle edge and are arranged in series. In lateral splenius, the motor domains are separated by tendinous inscriptions. In medial splenius, where no inscriptions are present, the motor domains of adjacent segmental nerves have overlapping boundaries due to the extensive interdigitation of motor units supplied from different segmental nerves. A single muscle fascicle in medial splenius is commonly composed of two sets of fibers supplied by adjacent segments that are distributed at opposite ends of the fascicle and interdigitate in midregions of the fascicle. The in-series compartmentalization of splenius has important implications for its motor control. To secure effective muscle shortening or tension development, central mechanisms may be required to coordinate the recruitment of in-series motor units. The presence of many short fibers with intramuscular insertions leads us to question how contractile forces are developed and transmitted in splenius and other mammalian muscles that are composed of short, in-series fibers.