The size of motor units during post‐natal development of rat lumbrical muscle.

Abstract

The number of muscle fibers innervated by individual motor neurons (motor unit size) was measured in lumbrical muscles of rats aged 0-28 days, during the period of elimination of polyneuronal innervation. Motor unit sizes were determined from twitch tension measurements combined with muscle fiber counts made from histological sections of the muscles. The relative tensions contributed by individual motor units declined from about 25% of the total tension at birth, to about 9% at 28 days of age. Part of this decrease reflected the elimination of synapses from polyneuronally innervated muscle fibers. During the same period, new muscle fibers were produced. The total number of muscle fibers present increased from about 500 at birth to about 950 fibers in mature muscles. These two processes were offsetting: some synapses were eliminated (from polyneuronally innervated fibers) while simultaneously others were formed de novo (on newly produced muscle fibers). Quantitative measurements showed that for the first 10 ten days after birth, there was little change in motor unit size. Thereafter production of new muscle fibers ceased and motor unit size decreased to the adult level. During early post-natal development, a lumbrical motor neuron maintains a nearly constant number of synapses, but extensively reorganizes its synaptic field, retracting synapses from some muscle fibers, while forming new synapses with other fibers.