Neck muscles in the rhesus monkey. I. Muscle morphometry and histochemistry

Morphometric methods were used to describe the musculotendinous lengths, fa scicle lengths, pennation angles, and cross-sectional areas of neck muscles in adult Macaca mulatta monkeys. Additionally, muscles were frozen, sectio ned, and stained for ATPase activity to determine fiber-type composition. I ndividual rhesus muscles were found to vary widely in their degree of simil arity to feline and human muscles studied previously. Suboccipital muscles and muscles supplied by the spinal accessory nerve were most similar to hum an homologs, whereas most other muscles exhibited architectural specializat ions. Many neck muscles were architecturally complex, with multiple attachm ents and internal aponeuroses or tendinous inscriptions that affected the d etermination of their cross-sectional areas. All muscles were composed of a mixture of type I, IIa, and IIb fiber types the relative proportions of wh ich varied. Typically, head-turning muscles had lower proportions of type I I (fast) fibers than homologous feline muscles, whereas extensor muscles co ntained higher proportions of type II fibers. The physical and histochemica l specializations described here are known to have a direct bearing on func tional properties, such as force-developing capacity and fatigue-resistance . These specializations must be recognized if muscles are to be modeled acc urately or studied electrophysiologically.