Volume conduction: Extracellular waveform generation in theory and practice

The extracellular waveform manifestations of the intracellular action potential are the quintessential diagnostic foundation of electrodiagnostic medicine, and clinical neurophysiology in general. Volume conduction is the extracellular current flow and associated voltage distributions in an ionic conducting media, such as occurs in the human body. Both surface and intramuscular electrodes, in association with contemporary digital electromyographic systems, permit very sensitive detection and visualization of this extracellular spontaneous, voluntary, and evoked nerve/muscle electrical activity. Waveform configuration, with its associated discharge rate/rhythm, permits the identification of normal and abnormal waveforms, thereby assisting in the diagnosis of nerve and muscle pathology. This monograph utilizes a simple model to explain the various waveforms that may be encountered. There are a limited number of waveforms capable of being generated in excitable tissues which conform to well-known volume conductor concepts. Using these principles, such waveforms can be quickly identified in real time during clinical studies.