Role of electrotonic current in excitable cells

Abstract

The aim of the present chapter is to review basic properties of electrotonic current flow in excitable cells, such as neuronal axons and cardiac tissue, during subthreshold stimulation, excitation threshold and impulse conduction. Electrotonic current is proportional to the spatial gradient of the transmembrane potential and consists of a current flow across the membrane with the effect to depolarize it. There is a close interrelationship between electrotonic current that originates from local source-sink interactions and excitation threshold. Successful impulse conduction requires that the amount of active current supplied by the membrane at the source location must be equal to or exceed the amount of electrotonic current required to excite the membrane at the sink location. Such condition is determined by the state of membrane excitability at the source, at the sink and by the degree of electrical coupling between source and sink. Conversely, conduction slowing induced by source-sink mismatch in cardiac tissue may be responsible for unidirectional conduction block and reentry, a condition leading to increased arrhythmia vulnerability, both in normal and pathological tissue. In addition to affecting impulse conduction, electrotonic current flow originating from an activation sequence locally modulates action potential repolarization, determining its duration and spatial dispersion across the tissue. Ultimately, experimental evidence is presented in support of the hypothesis of electrotonic current modulation of ventricular repolarization by two different activation sequences, sinus beat and ventricular test site drive, in normal rat heart.