Competition between segmental nerves at end-plates in rat gastrocnemius muscle during loss of polyneuronal innervation.

1. The segmental innervation of the rat lateral gastrocnemius (l.g.) muscle is from lumbar nerves L4 and L5. A study has been made of changes in the innervation of the l.g. muscle by nerves L4 and L5 before and after removal of L4 during the loss of polyneuronal innervation which occurs over the first 2 weeks postnatal. 2. The rat l.g. muscle is a complex of three unipennate muscles; the lateral, intermediate and medial heads. Each of these has a characteristic distribution of type I and type II muscle fibres of different diameter by 12 days postnatal. Following the removal of nerve L4 at birth there is no change in the distribution of fibre types in the different heads of the l.g. muscle at 12 days postnatal. 3. The number of fibres in the l.g. muscle doubled between birth and 12 days postnatal and the maximum tetanic force generated by the muscle increased by fourfold. Removal of nerve L4 at birth did not alter the increase in fibre number and there was no difference between the tension generated by the l.g. muscle in the ipsilateral and contralateral limbs of operated animals. 4. The number of detectable motor units in a extracellular calcium concentration [( Ca]o) of 2 mM remained constant in L4 at about fifteen between birth and 12 days postnatal; the number of detectable motor units in L5 declined over this period from about fifteen to six in a [Ca]o of 2 mM. Following the removal of L4 at birth, the number of motor units in L5 increased to twenty by 3 days postnatal and remained there for at least 12 days in a [Ca]o of 2 mM. 5. The mean size of motor units and the range of motor unit sizes declined for both L4 and L5 during the postnatal period. Following the removal of L4, the mean size of motor units and the range of motor unit sizes in L5 failed to decline. 6. The number of motor units detected at birth in a raised [Ca]o of 3-5 mM increased in L4 and L5 to about twenty; this estimate in high [Ca]o remained constant during the postnatal period. Estimates of the size of the large, low threshold motor units declined whether these were made in low or high [Ca]o.(ABSTRACT TRUNCATED AT 400 WORDS)