Loss of polyneuronal innervation and establishment of a topographical map in the glutaeus muscle of Bufo marinus during generation of secondary muscle cells.

The development of synaptic connections to the toad (Bufo marinus) glutaeus magnus from segmental nerves 8 (N8) and 9 (N9) was determined in the postmetamorphic period. Three different-size toads were studied: small (0.3-2.0 g), medium-size (5-15 g) and large (greater than 20 g). The number of cells in the glutaeus increased about 9-fold during development; this involved the appearance and subsequent maturation of secondary fibres throughout the muscle. The glutaeus in small toads, which consisted almost entirely of primary fibres, was innervated to a similar extent by N8 and N9 as assessed by tetanic contraction measurements. During late development there was a progressive increase in the percentage of the muscle innervated by N9 and a decrease in the percentage innervated by N8. This change in the segmental innervation was accompanied by changes in the innervation of the ventral glutaeus as assessed by intracellular recording. In small toads this surface of the muscle was innervated predominantly by N8, with N9 frequently appearing as a low-efficacy terminal on dually innervated fibres. With further development there was a progressive reduction in the percentage of dually innervated fibres and a concomitant decrease in the percentage innervation of the entire ventral glutaeus by N8. These results suggest that the topographical projection is established by the initial distribution of N9 terminals on the primary fibres of the muscle. The multiple innervation of newly generated fibres and the on-going process of terminal elimination results in N9 terminals, many of which were initially weak, preserving their position in the muscle. This occurs at the expense of N8 terminals, whose relative incidence declines during development. The competitive advantage of N9 motoneurones may be due to their greater capacity to lay down axon collaterals and preferentially innervate newly generated fibres; alternatively N9 terminals may displace N8 terminals, which were initially more efficacious, from dually innervated fibres. Secondary muscle fibres generated throughout the muscle are thus incorporated into an increasingly precise topographical map.