Change in motor neurone activity modifies the differentiation of a slow muscle in chick embryo.

Slow-tonic anterior latissimus dorsi (ALD) muscle properties were studied following chronic spinal cord stimulation in chick embryo. Stimulation at a fast rhythm was applied from day 7, 8 or 10 of development until the end of embryonic life. When stimulation was applied from day 7 up to day 18 of development, ALD muscle exhibited at day 18 a large decrease in half time to peak of tetanic contraction, a large proportion of fast type II fibres and an increase in fast myosin light chain content as compared to control muscle. When stimulation started at day 8 of development, changes in properties of ALD muscle were reduced when compared to the previous experimental series. Indeed, no fast type II fibres were observed within the muscle, even when stimulation was prolongated until the 20th day of embryonic development. In addition, chronic stimulation at a fast rhythm initiated at day 10 of development did not modify ALD muscle differentiation. The present results indicate that a fast pattern of motor neurone activity can induce some slow-to-fast transformations of ALD muscle fibres. However, after the first week of embryonic life, ALD myotubes appeared refractory to these transformations. The possible mechanisms responsible for the transformation of slow myotubes and for their further loss of plasticity are discussed.