Muscle fiber type compartmentalization and expression of an immature myosin isoform in the sternocleidomastoid muscle of rabbits and primates

The sternocleidomastoid muscle is located in the neck and is both a neck rotator and flexor. Cervical dystonia, a focal dystonia disorder, is characterized by forceful involuntary contraction of a group of neck muscles, usually including the sternocleidomastoid. Little is known about the fiber type composition, fiber type compartmentalization and innervation patterns in this muscle in rabbit and primates. Sternocleidomastoid muscles from rabbit and monkey were analyzed for muscle fiber type composition and number, muscle fiber cross-sectional area and patterns of innervation. The sternocleidomastoid muscle was composed of two distinct regions, or compartments, with different fiber type compositions: an outer or superficial region composed of mostly type 2 myofibers and an inner deep region composed of both type 2 and type 1 myofibers. Neonatal myosin heavy chain isoform was detected in approximately 25% of the myofibers in both regions of the muscle. Neuromuscular junctions were located in seven endplate bands approximately 1- 3 cm apart throughout the length of the muscle. There is clear evidence of anatomical subdivisions within this muscle. Not only is there variation in fiber type composition between superficial and deep regions of the muscle, but unlike most other mature skeletal muscles, it continues to express neonatal myosin heavy chain isoform in the adult. The motor program for neck movements is extremely complex, and the histological complexity plays a role in allowing for a continuum of movements of the head and neck, from maintenance of posture to rapid head movements.