Figure 1. Responses of biophysical, modeled neurons to monophasic and biphasic stimuli.

(A) Wiring diagram illustrates auditory nerve fibers modeled as segmented wires via a core-conductor approach. Each segment is characterized by its membrane resistance (R_leak), membrane capacitance (C_m), and axial resistance (R_a). Internodes are divided into 9 adjacent segments which have dynamics governed solely by passive. Node of Ranvier segments have additional voltage dependent elements for sodium channels (Na), fast potassium channels (K_f), and slow potassium channels (K_s); each of which are defined by channel density, channel conductance, ionic reversal potential, and the number of channels within the open state. Electrodynamics within each segment are expressed by relating axial, membrane leak, membrane capacitive, and ionic currents using Kirchoff’s law (see Eq. 1). A stimulating electrode placed 3 mm from fiber over 10 node of Ranvier segment provides a field potential (E_field) that varies with distance from the electrode. Fibers are 36 nodes long, the approximate number of nodes between the feline auditory periphery and cochlear nucleus, and most recordings are taken from node 32. Reprinted with permission from Imennov and Rubinstein, 2009. (B–E) Responses recorded from node 32 of 36 of normally myelinated fibers (Rel. Myelination = 1) to 50 presentations of the 4 stimuli waveform types described. For each waveform type, current amplitude was selected to produce an action potential response probability of 0.50. Cathodic (B) and anodic (C) monophasic stimuli and responses are depicted in the top panels while cathode-first (D) and anode-first (E) biphasic stimuli and responses are depicted in the bottom panels.