Embedded in the surface membrane are several types of ion channels that are responsible for maintaining the electrical potential and propagating depolarizing currents across the muscle membrane. Diseases of these channels are discussed in Chap. 46. Also constituting a large part of the membrane is a series of anchoring structural proteins, the nature of which have been thoroughly elucidated in the past few decades. These are described in detail in relation to the muscular dystrophies.
In addition to motor nerve endings, muscle contains several types of sensory endings, all of them mechanoreceptors: Free nerve endings subserve the sensation of deep pressure-pain; Ruffini and pacinian corpuscles are pressure sensors; and the Golgi tendon organs and muscle spindles are tension receptors and participate in the maintenance of muscle tone and reflex activity. The Golgi receptors are located mainly at the myotendon junctions; pacinian corpuscles are localized in the tendon but are also found sparsely in muscle itself. Muscle spindles are specialized groups of small muscle fibers that regulate muscle contraction and relaxation, as described in
Chap. 2. All of these receptors are present in the highest density in muscles that are involved in fine movements.
Muscles are not equally susceptible to disease, despite the apparent similarity of their structure. In fact, practically no disease affects all muscles in the body and each pathologic entity has a characteristic topography within the musculature. The topographic differences between diseases provide incontrovertible evidence of structural or physiologic differences between muscles that are not presently disclosed by the light or electron microscope. The factors responsible for the selective vulnerability of certain muscles are not known but several hypothetical explanations come to mind. One may relate simply to fiber size; consider, for example, the large diameter and length of the fibers of the glutei and paravertebral muscles in comparison with the smallness of the ocular muscle fibers. The number of fibers composing a motor unit may also be of significance; in the ocular muscles, a motor unit contains only 6 to 10 muscle fibers (some even fewer), but a motor unit of the gastrocnemius contains as many as 1,800 fibers. Also, the eye muscles have a much higher metabolic rate and a richer content of mitochondria than the large trunk muscles. Differences in patterns of vascular supply may permit some muscles to withstand the effects of vascular occlusion better than others. Histochemical studies of skeletal muscles have disclosed that within any 1 muscle, there are subtle metabolic differences between fibers, certain ones (type 1 fibers) being richer in oxidative and poorer in glycolytic enzymes and others (type 2 fibers) having the opposite distribution. The distribution of certain structural proteins may alter the topography of disease expression; for example, the eye muscles do not contain dystrophin, a submembrane protein that is deficient in Duchenne muscular dystrophy, which explains the muscles’ lack of involvement in this disease. The endomysial fibroblasts of eye muscles contain an abundance of glycosaminoglycans, which renders them susceptible to thyroid diseases. Diseases of the neuromuscular junction show a distribution of weakness in relation to the density of these junctions in different muscles. Doubtless other differences will be discovered.