Chapter 6 - Excitability and Impulse Propagation

Abstract

Cardiac excitability is a complex process that results from complex, highly controlled, electrophysiologic events through channels in the cardiac membrane, the myoplasm, and the extracellular space. These processes result from ionic flow through paths involving the cardiac cell membrane, the myoplasm, gap junctions between cells, and the extracellular space. The matrical concept of cardiac excitability is discussed in detail in terms of an electrophysiologic matrix with many active and passive electrophysiologic properties that form an electrophysiologic universe. This concept takes into account the essential nonlinear character of cardiac excitability and impulse propagation. Impulse propagation depends on cardiac excitability and the manner in which fibers are connected by gap junctions and separated by insulators. The velocity of impulse propagation most commonly is faster parallel to fiber orientation then perpendicular to orientation, and these velocities are termed longitudinal and transverse, respectively. Passive resistivity within and between cells depends on gap junctional connections and on geometry, including tapering shape and bifurcations, and several of these influences have been elegantly modeled.