Appearance in slow muscule sarcolemma of specializations characteristic of fast muscle after reinnervation by a fast muscle nerve.

We utilized quantitative freeze-fracture electron microscopy to study the plasticity of orthogonal clusters of 60-A particles (the “square array”) found in the sarcolemma of fast-twitch muscle. The membrane macromolecular composition of normally slow-twitch rat soleus muscle was examined 1 year after surgical reinnervation by the nerve from fast-twitch extensor digitorum longus muscles. The isometric contraction times and histochemical profiles were monitored and it was confirmed that conversion of fiber types had occurred. The sarcolemma of the switched “fast” soleus developed square arrays of 60-A particles characteristic of fast-twitch muscle whereas the sarcolemma of the contralateral control, the “slow” soleus, contained only random particles. Square array density per square micrometer in cross-reinnervated fast soleus fibers resembled that of normal fast extensor digitorum longus muscles and varied as a function of distance from the neuromuscular junction. This experiment demonstrates that the appearance of these unusual clusters of 60-8, particles is neuronally regulated. We further suggest that these macromolecules are under the influence of the same subtle aspects of innervation that regulate the differentiation of myosin adenosine triphosphatase and thereby the contractile behaviors of fastand slow-twitch muscle. The function of the sarcolemmal square array is unknown ; however, a correlation with a membrane property that is more highly developed in fast-twitch muscle is to be expected.