When many excitable cells are present in a living tissue, their electrical behavior can be detected by macroelectrodes placed on the body at a distance from these cells.
Several clinically important electrophysiological diagnostic procedures are based on this concept.
Underlying these procedures is a theory called volume conduction. This theory describes the spread of ionic currents within the extracellular fluid from a group of neurons or muscle cells to more distant points in the body, such as the skin.
These ionic currents can be measured from the skin. Their waveforms are characteristic of the tissues from which they arise. The best-known electrophysiological recording is the electrocardiogram from heart muscle (Chapter 20). The electromyogram from skeletal muscle (Chapter 6) and electroretinogram (Chapter 14) are other examples.This chapter introduces two additional clinical electrophysiological tools: the electroencephalogram and sensory- evoked potentials, particularly brainstem auditory-evoked responses. These tools represent two general types of clinical electrophysiological recordings. The first is a record of the spontaneous activity of tissue. The second is a record of potentials that are artificially evoked by electrical or magnetic stimulation of tissue or by activation of sensory receptor organs. Before discussing the electroencephalogram and sensory-evoked potentials, it is necessary to understand more about the histology and electrophysiology of the cerebral cortex.