The fast-twitch extensor digitorum longus (EDL) and the slow-twitch soleus muscle of the rat consist of heterogeneous fiber populations . EDL muscle fibers differ in size, mitochondrial content, myoglobin concentration, and thickness of the Z line . The sarcoplasmic reticulum, on the other hand, is richly developed in all fibers, with only small variation . Myofibrils are clearly circumscribed at both the A and I band level . The soleus muscle is composed primarily of fibers with moderate mitochondrial content and myoglobin concentration . In most fibers the sarcoplasmic reticulum is poorly developed, with the exception of the portion of reticulum in phase with the Z line . As a consequence the myofibrillar fields are amply fused together. Contacts between sarcoplasmic reticulum and T system are discontinuous and may occur in the form of "dyads" instead of the typical triad structure . In a small proportion of soleus muscle fibers the organization and development of the sarcoplasmic reticulum is similar to that of EDL muscle fibers, with prominent fenestrated collars at the H band level . In these fibers mitochondria are larger and more abundant . The results are correlated with physiological studies on motor units in the same and in similar rat muscles . It is suggested that the variable structural pattern of rat muscle fibers is related to two distinct physiological parameters, speed of contraction and resistance to fatigue .

SUMMARY1. Senile muscle atrophy is characterized by a marked reduction in the frequency of spontaneous transmitter release with no electrophysiological evidence of denervation.2. In spite of the reduced number of muscle fibres, there is no ultrastructural evidence for denervation at the end-plates. There is agglutination of synaptic vesicles, neurotubules and filaments, thickening of the basement membrane, widening of the primary synaptic cleft, and irregular branching of the junctional infoldings, but no axonal degeneration.3. The contractile process in senile muscles is slowed down as is indicated by a prolongation of contraction time, latency period, maximum rate of twitch tension and relaxation time.4. The muscle fibres show proliferation of the T system and increased SR but no fragmentation as is observed in denervation atrophy.5. Senile muscle atrophy thus presents some specific features affecting both pre-and post-synaptic structures, related to a very slow process of deterioration of the neuromuscular contact.

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