Muscle length affects the architecture and pattern of innervation differently in leg muscles of mouse, guinea pig, and rabbit compared to those of human and monkey muscles.

The innervation pattern and fascicular anatomy of muscles of different lengths in mouse, guinea pig, rabbit, macaque monkey and human legs were analyzed. Neuromuscular junctions, muscle tendon junctions and ends of intrafascicularly terminating fibers were stained for acetylcholinesterase, and fascicle lengths measured. A high correlation between increasing fascicle length and increasing number of neuromuscular junctions was found, with non-primate (mouse, guinea pig, rabbit) and primate (macaque monkey, human) muscles forming two discrete groups. In non-primates, muscles with a single endplate band, fascicles were always shorter than 35 mm, fixing the limit of fiber length served by one neuromuscular junction. Muscles with fascicles longer than this had multiple discrete bands of motor endplates crossing their width at regular intervals. An increase in muscle length across or within species corresponded to an equivalent, standard increase of 10-12 mm fascicle length per motor endplate band. All human and monkey leg muscles, with the exception of gracilis and sartorius, were singly innervated and all muscle fibers ran the full distance from tendon to tendon. Singly innervated primate muscle fibers were up to 140 mm long whereas the mean distance between endplate bands in the two multiply innervated muscles was also considerably greater than in non-primates. These data indicate that allometric effects of increasing fascicle length, are distinct in common laboratory animals and two primates, when architecture and pattern of innervation are compared. Copyright 2001 Wiley-Liss, Inc.