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Motor Innervation of Myofiber Types in Porcine Skeletal Muscle¹

Muscle Biology Laboratory, University of Wisconsin, Madison 53706

Abstract

Motor innervation was studied in developing longissimus and semitendinosus muscles of pigs with a technique which gave simultaneous staining of myofiber types and motor end-plates. Both primary and secondary myofibers were innervated (as judged by acetylcholinesterase localization) as soon as they were indentifiable by morphological characteristics. Therefore, last formed secondary myofibers (Type II) were innervated later than first formed primary myofibers (Type I). The grouping pattern of Type I myofibers in pig muscle is a unique situation which should aid in study of the mechanism that determines myofiber type. Our preliminary studies on structure of the motor unit discounted the hypothesis that the groups of Type I myofibers are motor units, but revealed, as already determined in other mammalian muscle, that the fibers of a given motor unit are distributed individually throughout an area of the muscle encompassed by a number of fasciculi.

¹ Supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and by the Graduate School through a Romnes Faculty Fellowship to R.G.C. Muscle Biology Laboratory manuscript number 133.

² Visiting scientist on leave from the Tierarztlichen Hochschule of Hanover, Germany.