The clinical suspicion of neuromuscular disease, disclosed by any of the symptoms or syndromes in the succeeding chapters, finds ready confirmation in the laboratory. The intelligent selection of ancillary examinations requires knowledge of the biochemistry and physiology of nerve action potentials, neuromuscular transmission, and muscle fiber contraction. These basic subjects, with the relevant anatomy, serve as an introduction to the descriptions of the laboratory methods and the subject matter of the chapters on the diseases of muscle and nerve that follow.
Since the early studies of Hodgkin (1951) and of Hodgkin and Huxley (1952), tomes have been written on the subject of the conduction of electrical impulses in neural tissues. Such conduction in nerve and muscle depends first on the maintenance of a fluid internal environment that is distinctly different from the external or interstitial medium. The main intracellular constituents are potassium (K), magnesium (Mg), and phosphorus (P), whereas those outside the cell are sodium (Na), calcium (Ca), and chloride (Cl). In both nerve and muscle the intracellular concentrations of these ions are held within a narrow range by both passive electrical and active chemical forces, which maintain the membranes in an electrochemical equilibrium termed the resting membrane potential. These forces are the result of selective permeability of the membranes to various ions and to the continuous expulsion of intracellular Na through specific channels by a pump mechanism (the sodium pump). The function of the sodium pump is dependent on the enzyme Na-K-ATPase (adenosine triphosphatase), which is localized in the membranes.
This resting membrane potential is the result of the differential concentrations of K and Na. The interior of the cell is some 30 times richer in K than the extracellular fluid, and the concentration of Na is 10 to 12 times greater in the extracellular fluid. In the resting state, the chemical forces that promote diffusion of K ions out of the cell (down their concentration gradient) are counterbalanced by electrical forces, the internal negativity, that opposes further diffusion of K to the exterior of the cell. The situation of Na ions in the equilibrium state is the opposite; they tend to diffuse into the cell, both because of their concentration gradient and because of the relative negativity inside the cell. Because the membrane at rest is less permeable to Na than to K, the amount of K leaving the cell exceeds the amount of Na entering the cell, thus creating the difference in electrical charge across the membrane. This discrepancy creates an electrical potential across the membrane such that, in the resting state, the intracellular compartment is 70 to 90 mV negative relative to the extracellular space (the resting membrane potential). The actions of the Na:K pump and the presence of impermeant, negatively charged intracellular proteins, contribute to maintaining this negative potential.
From this resting potential, any electrical discharge of neural and muscular tissue is predicated on the special property of excitable membranes; namely, ...