Experimental investigations on hypokinesis of skeletal muscles with different functions VII. Ultrastructural observations.

Ultrastructural investigations were carried out parallel with biochemical measurements on the limb muscles of rabbits immobilized by putting in plaster. The cellular structure of normal and immobilized (1--6 weeks) m. soleus and m. gastrocnemius (red tonic and white tetanic, respectively) were compared. During immobilization significant changes occurred in the cellular structure of m. soleus, while little change was observed in the white fibres of m. gastrocnemius. The fibres of m. soleus decreased in diameter and fatty infiltration set in. The myofibrils decreased in amount and their structure was damaged and disorganized but did not disappear completely. Mitochondria showed little damage, like membraneous structures in general, in number. Loss of material seems to be by way of vesicles formed by the heavily invaginated plasma membrane and by the deflation of the numerous vesicles observed. The red fibres of m. gastrocnemius behaved like the fibres of m. soleus; in the white fibres some focal disorganization was observed near the Z line. The cystae of mitochondria and the SR appear to be dilated.