Innervation distribution pattern, nerve ending structure, and fiber types in pigeon skeletal muscle.

Four fiber types have been characterized in different pigeon skeletal muscles according to their innervation pattern (nerve ending structure and innervation distribution) and histochemical properties (SDH and m-ATPase activities). All fast fibers, types IIA and IIB, present aggregated distribution of their nerve endings with "en plaque" structures and very low innervation frequencies. The two kinds of slow fibers recognized are multiple innervated and present higher innervation frequencies. However, type I fibers have nerve terminals in small knobs with uniform localization, whereas type III fibers present "en grappe" nerve endings, which tend to be randomly distributed. Fiber type composition of skeletal muscles has been found closely related to their biomechanical function. Fast fibers are predominant in muscles with an active role in locomotive movements, whereas slow fibers are mainly or exclusively located in postural muscles.