The application of engineering principles and technology to medicine and biology has had an increasing influence on the practice of medicine. The most visible of these contributions is in the form of medical devices.

This article, however, describes the engineering introduction of quantitative methods in the field of bioelectricity. When such contributions first be- came evident, in the early 1950s many physiology researchers were already employing modern quantitative methods to develop and utilize governing equations and suitable models of bioelectric phenomena.

Today it appears that systems physiology lives on as biomedical engineering, while physiology has become more concerned with cell and molecular biology. On the other hand, biomedical engineering is also currently involved in efforts to develop and apply quantitative approaches in cellular and molecular levels.

This article, which is concerned with the electric behavior of tissues, reviews what is known about the biophysics of excitable membranes and the volume conductor in which they are embedded. Our approach emphasizes the quantitative nature of physical models.

We formulate an engineering description of sources associated with the propagating action potential and other excitable cellular phenomena. With such sources and a mathematical description of fields generated in a
volume conductor the forward problem, namely a determi-nation of the potential field at the body surface from underlying bioelectric activity, can be formulated.

The cardiac forward problem starts with a quantitative description of the sources in the heart; the resulting body surface potentials are known as the electrocardiogram. In a similar way sources associated with the activation of skeletal muscle lead to the electromyogram.

We will also consider the electroencephalogram and electrogastrogram, where we will discover bases for sources other than propagating action potentials. We consider these applications of basic theory only in an introductory way, because there are separate articles for each. It is the goal of this article to elucidate the underlying principles that apply to each of the aforementioned and other applications.

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