The non‐selective innervation of muscle fibres and mixed composition of motor units in a muscle of neonatal rat.

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Abstract
1. Motor-unit size was measured by tension recording in neonatal (3-5 day) rat skeletal muscle (fourth deep lumbrical muscle). Each unit was then depleted of glycogen and its fibers studied in mid-belly frozen sections, by staining for glycogen (periodic acid-Schiff reagent) and antibody labelling for slow myosin. The contralateral muscle acted as control, and further controls for the method are described. 2 All the motor units contained both slow-myosin-containing (S; antibody-positive) and slow-myosin-free (F; antibody-negative) fibers. 3. The proportion of each unit that was made up of S fibers was compared with the whole muscle. Of the twelve units studied seven were not selectively innervated, four may have been selectively innervated in favor of F fibers, and one was selectively innervated in favor of S fibers. The last unit was much smaller than the others. 4. Fiber cross-sectional areas were measured in units and in the whole muscles. Mean cross-sectional areas for individual F fibers in all the motor units were smaller than in the corresponding whole muscles (ratio 0.71), implying that small fibers have higher levels of polyneuronal innervation than larger ones (each small fiber occurring in more overlapping units than each larger fiber). There was no such difference in S fibers (ratio 0.96). 5. Motor-unit sizes (as a percentage of whole muscle) were smaller when obtained from summed fiber cross-sectional areas than from fiber counts (this follows from 4, above). Comparisons with unit sizes from tension recording are discussed. 6. Controls show that there is little, if any, non-specific fatigue of muscle fibers that are not part of the unit subjected to glycogen depletion. 7. Evidence is given that muscle fiber conduction block occurs during the depletion regime, leading to less glycogen depletion towards the ends of the muscle fibers than in the end-plate zone.