SUBOCCIPITAL MUSCLES IN THE CAT NECK - MORPHOMETRY AND HISTOCHEMISTRYOF THE RECTUS CAPITIS MUSCLE COMPLEX

The morphometry, histochemistry, and biomechanical relationships of re ctus capitis muscles were examined in adult cats. This family of muscl es contained six members on the dorsal, ventral, and lateral aspects o f the upper cervical vertebral column. Three dorsal muscles (rectus ca pitis posterior major, medius, and minor) formed a layered complex spa nning from C1 and C2 to the skull. Rectus capitis posterior major was composed predominantly of fast fibers, but the other two deeper muscle s contained progressively higher proportions of slow fibers. One ventr al muscle, rectus capitis anterior major, was architecturally complex. It originated from several cervical vertebrae and appeared to be divi ded into two different heads. In contrast, rectus capitis anterior min or and rectus capitis lateralis were short, parallel-fibered muscles s panning between the skull and C1. The ventral muscles all had nonunifo rm distributions of muscle-fiber types in which fast fibers predominat ed. Dorsal and ventral muscle groupings usually had cross-sectional ar eas of 0.5 cm2 or more, reflecting a potential capacity to generate ma ximal tetanic force in excess of 9 N. Biomechanical analyses suggested that one muscle, rectus capitis lateralis, had its largest moment in lateral flexion, whereas the other muscles had large, posturally depen dent moment arms appropriate for actions in flexion-extension. The obs ervation that most rectus muscles have relatively large cross-sectiona l areas and high fast-fiber proportions suggests that the muscles may have important phasic as well as postural roles during head movement.