24 - Cable Properties and Propagation of Action Potentials

Abstract

This chapter examines the mechanism and propagation of action potentials (APs) and excitability from one part of a neuron or muscle to a distal part. Although the biological cable (nerve fiber or skeletal muscle fiber) is the best possible, it is a relatively poor cable with a short length constant and relatively long time constant. Thus, for faithful and rapid signal transmission over long distances, energy must be put into the system at each point along the way. The chapter discusses frequency-modulated signals, cable properties, conduction of action potentials, and external recording of action potentials. The discussion on cable properties includes biological fiber as a cable, length constant, time constant, input resistance, and local potentials. The discussion on conduction of action potentials includes local-circuit currents, propagation velocity determinants, saltatory conduction, and wavelength of the impulse. External recording of action potentials include monophasic, diphasic and triphasic recording, and compound action potential. The systems evolved are that of AP generation, which are all-or-none signals of constant amplitude and constant propagation velocity, in addition to having refractory periods and sharp thresholds. This is a frequency-modulated system (FM), in which increasing strength of sensation or motor response follows from an increase in frequency of the AP signals. AP propagation occurs by means of local-circuit currents. The transmembrane current has three phases—outward, inward, and outward. The internal and external longitudinal currents have two phases—forward and backward (for internal) or backward and forward (for external). The external currents use the path of least resistance, enabling electrograms (for example, ECG, EMG) to be recorded from the body surface. The compound AP is graded in amplitude, reflecting the summation of the external currents generated from each fiber that is activated; that is, the more fibers simultaneously activated, the greater the amplitude of the electrogram signal. The myelin sheath evolved by vertebrates enables much faster propagation velocity and at a lower energy cost. Myelination raises the effective membrane resistance and lowers the effective capacitance, and excitability occurs only at the short nodes of Ranvier that periodically interrupt the myelin sheath.