Structure and Function in Vertebrate Skeletal Muscle

SYNOPSIS. The functional diversity of vertebrate skeletal muscle largely depends upon its structure. An important aspect of this is its hierarchical design. At the cellular level, muscle fibers form three categories whose functional properties grade into each other: slow-oxidative fibers with high endurance to fatigue, fast-oxidative fibers also endurant but with greater metabolic diversity, and fast-glycolytic fibers with limited endurance but quick response. This partitioning of functional properties found among single muscle fibers is conserved at a second level of the structural hierarchy, since the group of myofibers innervated by a single motor neuron (together called a motor unit) is composed of the same fiber type. Different motor units may be recruited in an orderly pattern depending upon the functional demands of a particular behavior. Finally, groups of motor units innervated by axons travelling together in the primary nerve branches may form discrete neuromuscular compartments at a third level of structural hierarchy. Different motor units may be found in regional arrays in these compartments so that slow or fast units tend to be clumped together and may be recruited together as larger functional units. This hierarchical organization of skeletal muscle may be a fundamental vertebrate plan that allows the diversity of functions so evident in vertebrate behavior.