Figure 45-1. Schematic of the major subcellular components of a myofibril. The transverse (T) system, which is an invagination of the plasma membrane of the cell, surrounds the myofibril midway between the Z lines and the center of the A bands; the T system is approximated to, but apparently not continuous with, dilated elements (terminal cisternae) of the sarcoplasmic reticulum on either side. Thus, each sarcomere (the repeating Z-line-to-Z-line unit) contains two “triads,” each composed of a pair of terminal cisternae on each side of the T tubule. (From Peter, by permission.)
Figure 45-2. The molecular organization of the dystrophin-glycoprotein complex in the membrane and sarcolemma and endoplasmic reticulum-Golgi apparatus. These proteins are related to Duchenne, limb-girdle, Miyoshi, and certain congenital dystrophies. Details in text.
Figure 45-3. Expanded schematic of the nuclear and contractile proteins of the muscle. These proteins are referable to Emery-Dreifuss dystrophy and a number of the distal and the congenital dystrophies, as well as several of the limb-girdle dystrophies. Details in text.
Chapter  45    Diseases of Muscle
Disorders of the Neuromuscular Junction,
Myotonias, and Persistent Muscle Fiber Activity
These disorders are discussed together because they cause weakness or changes in salience of the musculature but are not strictly speaking disease of muscle, instead having their origins in problems with the terminal motor axons or neuromuscular junction, the ends of the line in motor transmission, so to speak. Their pathophysiology and clinical features sometimes overlap with immune mechanisms that underlie a number of them. In most ways, however, they stand apart from each other but are most efficiently described together here.
Included under this category is a group of diseases, the most important of which is myasthenia gravis. Most of these disorders exhibit the characteristic and striking features of fluctuating weakness and fatigability of muscle. Some degree of weakness is usually present at all times but it is made worse by activity. The weakness and fatigability reflect physiologic abnormalities of the neuromuscular junction that are demonstrated by clinical signs and by special electrophysiologic testing. As an aid to understanding the diseases discussed in this chapter, the reader should consult the discussion of the structure and function of the neuromuscular synapse given in Chap. 2.
The cardinal feature of myasthenia gravis, usually referred to simply as myasthenia, is fluctuating weakness of voluntary (skeletal) muscles, particularly those innervated by motor nuclei of the brainstem, that is, ocular, masticatory, facial, deglutitional, and lingual. Manifest weakening during continued activity, quick restoration of power with rest, and dramatic improvement in strength following the administration of anticholinesterase drugs such as neostigmine are the other notable characteristics. Myasthenia is an immune disease in which circulating antibodies against components of the motor postsynaptic membrane and subsequent structural changes in that membrane explain virtually all the features of the disease.