Computation of action potential propagation and presynaptic bouton activation in terminal arborizations of different geometries.

Action potential propagation in axons with bifurcations involving short collaterals with synaptic boutons has been simulated using SPICE, a general purpose electrical circuit simulation program. The large electrical load of the boutons may lead to propagation failure at otherwise uncritical geometric ratios. Because the action potential gradually fails while approaching the branch point, the electrotonic spread of the failing action potential cannot depolarize the terminal boutons above an assumed threshold of 20 mV (Vrest = 0 mV) for the presynaptic calcium inflow, and therefore fails to evoke transmitter release even for boutons attached at short collaterals. For even shorter collaterals the terminal boutons can again be activated by the spread of passive current reflected at the sealed end of the bouton which increases the membrane potential above firing threshold. The action potential is then propagated in anterograde fashion into the main axon and may activate the terminal bouton on the other collateral. Differential activation of the synaptic boutons can be observed without repetitive activation of the main axon and with the assumption of uniform membrane properties. Axon enlargements above a critical size at branch points can increase the safety factor for propagation significantly and may serve a double function: they can act both as presynaptic boutons and as boosters, facilitating invasion of the action potential into the terminal arborizations. The architecture of the terminal arborizations has a profound effect on the activation pattern of synapses, suggesting that terminal arborizations not only distribute neural information to postsynaptic cells but may also be able to process neural information presynaptically.