The mechanism of selective reinnervation of fish eye muscles. IV. Identification of repressed synapses

Abstract

(1) Competitive reinnervation of the superior oblique muscle of carp eye was studied in 6 fish after obtaining cross-innervation from the inferior oblique nerve. (2) In 3 cases the original superior oblique nerve took over and muscle contraction attributable to the inferior oblique nerve ceased. (2) In two of these the original nerve was cut, and the fish survived for 5 and 7 days with no return of cross-innervation. (4) Electron microscopic study of these muscles showed many normal appearing synapses dependent on the inferior oblique nerve and some degenerating terminals from the superior oblique. In the fish in which the superior oblique was not cut before histology of the muscle all nerve terminals looked similar. (5) It is concluded that functional repression of foreign innervation does not change synaptic ultrastructure. (6) Those muscles in which cross-innervation had persisted were studied in the same way. They were densely innervated by foreign terminals and the correct innervation was represented by very few degenerating terminals but many myelinated axons. (7) It is concluded that a heavy innervation by foreign nerve fibres can prevent the formation of neuromuscular junctions by the correct nerve.