Muscle is not too smart and can only react in a limited number of ways to insult. Thus most primary muscle diseases have non-specific features in common, such as muscle fiber necrosis, evidence for muscle fiber regeneration, structural abnormalities such as centrally located muscle fiber nuclei and an increase in muscle connective tissue (figure 6). Some primary muscle diseases do show diagnostic changes such as nemaline rod formations or central cores. Inflammation in muscle is important as it may indicate a treatable disease
Figure 6 Typical, non-specific pathological findings in a primary myopathy. A necrotic fiber (asterix), and a hypercontracted muscle fiber (star), are shown. The entire muscle is shortened and thus, the hypercontracted fiber is thicker. The connective tissue between the muscle fibers is increased.
Anterior horn cell disease or a peripheral neuropathy result in exactly the same histological findings in the muscle! The poor muscle can only interpret these events as "I am denervated." The pathological hallmarks of denervation are type grouping and group atrophy (figure 7). Because one anterior horn cell/motor axon innervates a number of muscle fibers, it follows that disease of an anterior horn cell or its axon results in denervation of a number of muscle fibers. These muscle fibers that have lost their innervation may now be innervated by healthy axons that normally innervate adjacent muscle fibers. The end result is that now one axon innervates more muscle fibers than normal, (a giant motor unit) and also the normal checkerboard pattern of innervation is lost. That is, a whole group of type 1 or 2 fibers can now be seen adjacent to one another (type grouping). With progression of the disease, the axon that sprouted to innervate previously denervated muscle fibers may now also become diseased, resulting in an entire group of adjacent muscle fibers becoming atrophic (group atrophy).
Denervation: Loss of nerve supply. There are many causes of denervation. Denervation may be due to a disease as, for example, in polio where the death of motor neurons causes the denervation of muscle fibers. Denervation may be due to a chemical (such as botox) or physical injury or interruption of a nerve (as by accident or to relieve pain).
Diseases that affect the lower NERVES at any point cause muscle fiber atrophy. The motor nerve exerts a influence on muscle. This
influence is mediated by electrical impulses and by chemical substances (trophic factors) which, acting through the synapse,
influence protein synthesis in muscle. Myofibers that lose their innervation become angular and shrink. They lose 80% to 90% of their
mass within a few months. At an extreme stage of atrophy, virtually all sarcoplasm is lost and the myofiber is reduced to a cluster of
nuclei. In the process of denervation, there is loss and disarray of myofilaments but no myonecrosis occurs. Myofiber atrophy can be best
appreciated in cross sections
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