Toward a concept of the functional unit of mammalian skeletal muscle.

The current concepts for the functional unit of mammalian skeletal muscle are reviewed and shown to lack components that are required for determining the unit. To secure a definition for the functional unit, requisite criteria were selected, and the manner by which these criteria were used to define the functional unit are discussed. For deriving a definition of the unit the following values were obtained: the unit is associated with the total length of the muscle fiber, which may achieve a maximum length of 60 cm; it exhibits an average diameter of 40 micrometers/fiber; it is associated with a capillary net whose length (arteriole to venule) is about 750-1,000 micrometers max; the net exhibits a capillary-to-fiber ratio for long capillaries of approximately 1-1.5:1, with a transverse capillaries occurring approximately every 150 micrometers; and it has a fiber-to-motor end plate ratio of 1:1. The correlation between anatomic data and functional data indicate that a functional unit of muscle is delimited by about 1 mm2 of the cross-sectional area of a muscle bundle, since this is the maximum area under autonomic control of its particular arteriolar blood supply, the metabolic throttle that determines the power being expended by the muscle bundle.