Development of fibrillation potentials in denervated fast and slow skeletal muscle

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Abstract
The anterior tibial and soleus muscles of the rat were denervated independently at 2 sites, approximately 0.5 and 3.0 cm from the entrance of the nerve into muscle. Fibrillation potentials were detected with bipolar electrodes in several areas of the anterior tibial muscle and from the midportion of the soleus muscle up to 96 hr. postdenervation. The potentials were analyzed with respect to amplitude and spectrum of amplitude by the use of a histogramming unit together with a CAT computer. Fibrillation potentials in the anterior tibial were present after 42 hr. near the end-plate area and 6 hr. later in the distal portion of the muscle. Greater length of distal nerve stumps did not substantially delay the appearance of these potentials. Fibrillation appeared in the soleus at approximately the same time that it appeared in the anterior tibial, but the subsequent development of activity in the soleus was more dependent on the level at which the denervation had been carried out than it was in anterior tibial. In both muscles the increase in amount of fibrillation observed at greater postdenervation periods was primarily due to the appearance of fibrillation potentials having greater amplitude. Histochemical correlates suggest that the different patterns of development of fibrillation may result also from the difference in muscle fiber types in the 2 muscles.