Computer reconstruction of the spread of excitation in nerve terminals with inhomogeneous channel distribution

A direct numerical integration method, as modified by Du Fort and Frankel (1953), has been used to solve the partial differential equation system which describes the spread of action potential in a mammalian nerve terminal. Branching of the terminal as well as inhomogeneous distributions of Na+ and K+ voltage-dependent channels (Brigant and Mallart 1982) have been incorporated in the model. Using the channel densities and the kinetic parameters measured in the node of Ranvier, the depolarization in the terminal branches has an amplitude of only 60% of the action potential in the node. Furthermore, the time courses of the calculated membrane currents differ considerably from the ones measured by Brigant and Mallart (1982) and by Konishi and Sears (1984). Increasing the Na+ and K+ channel densities may considerably increase the terminal depolarization and also reproduce qualitatively the current wave-forms observed experimentally. The model can also reproduce some of the effects of pharmacological channel blocks. The simulation allows a new interpretation of the different components of membrane current along the terminal.