Histochemical evidence for the existence of skeletofusimotor (beta) innervation in the primate

A total of ten alpha motor axons which innervated the peroneus brevis muscle were isolated in two cynomolgus monkeys. In each experiment, the isolated alpha axons were stimulated collectively to deplete glycogen from their muscle units. The muscle was then frozen quickly , cut serially, and stained for glycogen. Of the 52 muscle spindles that were examined, zones of glycogen depletion were found in the intrafusal fibres of 32 spindles. The glycogen-depleted motor units included both fast-twitch and slow-twitch types. Depleted zones were observed in all three types of intrafusal muscle fibres. It was concluded that skeletofusimotor (beta) efferents were among the stimulated motor axons. This finding constitutes the first anatomical evidence for the existence of beta innervation in the primate.