Computer model for action potential propagation through branch point in myelinated nerves

A mathematical model is developed for simulation of action potential propag ation through a single branch point of a myelinated nerve fiber with a pare nt branch bifurcating into two identical daughter branches. This model is b ased on a previously published multi-layer compartmental model for single u nbranched myelinated nerve fibers. Essential modifications were made to cou ple both daughter branches to the parent branch. There are two major featur es in this model. First, the model could incorporate detailed geometrical p arameters for the myelin sheath and the axon, accomplished by dividing both structures into many segments. Second, each segment has two layers, the my elin sheath and the axonal membrane, allowing voltages of intra-axonal spac e and periaxonal space to be calculated separately. In this model, K ion co ncentration in the periaxonal space is dynamically linked to the activity o f axonal fast K channels underneath the myelin in the paranodal region. Our model demonstrates that the branch point acts like a low-pass filter, bloc king high-frequency transmission from the parent to the daughter branches. Theoretical analysis showed that the cutoff frequency for transmission thro ugh the branch point is determined by temperature, local K ion accumulation , width of the periaxonal space, and internodal lengths at the vicinity of the branch point. Our result is consistent with empirical findings of irreg ular spacing of nodes of Ranvier at axon abors, suggesting that branch poin ts of myelinated axons play important roles in signal integration in an axo nal tree.