Development and inhibition of the target phenomenon in tenotomized rat muscle.

The sequence of development of the target phenomenon in tenotomized gastrocnemius muscle was studied: the presence of target fibres was preceded by the occurrence of contraction bands and of "moth eaten" appearance of the fibres. This phenomenon was far more pronounced and occurred earlier in type II than in type I fibres. This target phenomenon and the contraction artefacts could be inhibited in the tenotomized muscles by simultaneous neurotomy or immobilization of the muscle with a plaster cast. Delayed denervation inhibited also the target phenomenon, if performed less than 5 days after the tenotomy. These series of experiments seem to indicate that the target phenomenon occurs in more irritable muscle fibres and that muscular activity is needed for its development.