Types of intra‐ and extrafusal muscle fibre innervated by dynamic skeleto‐fusimotor axons in cat peroneus brevis and tenuissimus muscles, as determined by the glycogen‐depletion method.

Abstract

The types of intra- and extrafusal muscle fiber innervated by dynamic skeleto-fusimotor (.beta.) axons were determined by using a modification of the glycogen-depletion method of Edstrom and Kugelberg combined with histochemical tests for various enzyme reactions. A single .beta. axon was prepared in each of the experiments, which were carried out on 6 peroneus brevis and 2 tenuissimus muscles. The intrafusal distribution of dynamic .beta. axons was almost exclusively restricted to bag1 fibers. The bag1 fiber was depleted in each of 24 .beta.-innervated spindle poles; the only fibers of a different type depleted intrafusally were a bag2 fiber in 1 pole and a long chain in another. Depletion in the bag1 fibers was usually restricted to 1 zone in 1 pole, generally in a mid-polar location. The extrafusal muscle fibers depleted by dynamic .beta. axons belong to the slow oxidative type as defined by Ariano, Armstrong and Edgerton. The number of such fibers in each motor unit could not be accurately determined, but is almost certainly small. The slow oxidative muscle fibers innervated by dynamic .beta. axons were not depleted over their entire length. Since there was no reason to assume that they were not twitch fibers, it would seem that the localized depletions result from the conditions required to obtain glycogen depletion, i.e., long periods of motor stimulation applied during the occlusion of the muscle''s blood supply. Under similar experimental conditions depletion of glycogen was also restricted to portions of fibers in fast oxidative-glycolytic motor units, but extended over most of the length of the fibers in fast glycolytic units.