Biophysical Journal 60: 538-555 (1991)
© 1991 the Biophysical Society

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Analytical theory for extracellular electrical stimulation of nerve with focal electrodes. II. Passive myelinated axon.

Department of Otolaryngology, Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston 02114.

ABSTRACT

The cable model of a passive, myelinated fiber is derived using the theory of electromagnetic propagation in periodic structures. The cable may be excited by an intracellular source or by an arbitrary, time-varying, applied extracellular field. When the cable is stimulated by a distant source, its properties are qualitatively similar to an unmyelinated fiber. Under these conditions relative threshold is proportional to the cube of the source distance and inversely proportional to the square of the fiber diameter. Electrical parameters of the model are chosen where possible, from mammalian peripheral nerve and anatomic parameters from cat auditory nerve. Several anatomic representations of the paranodal region are analyzed for their effects on the length and time constants of the fibers. Sensitivity of the model to parameter changes is studied. The linear model reliably predicts the effects of fiber size and electrode-fiber separation on threshold of cat dorsal column fibers to extracellular electrical stimulation.