Fig. 3.

Analysis of velocity gain using a linear paranodal taper. The conduction velocity is plotted vs. nodal diameter for values of fiber diameter (internode plus twice the myelin thickness) ranging from 4 μm to 20 μm (indicated below each line) and 5 × 10³ nodal sodium channels (A), 1.5 × 10⁴ nodal sodium channels (B), and 2.5 × 10⁴ nodal sodium channels (C). The numbers below the curves indicate the fiber diameters. The dashed lines indicate velocities for unconstricted axons, i.e., where the nodal and internodal axon diameters are identical. The thick solid lines indicate the maximum conduction velocity for each fiber diameter. The insets in A–C indicate the percentage gain of conduction velocity compared with an unconstricted axon. In D, we show the conduction velocity as a function of the nodal diameter for three values of the fiber diameter with a constant specific sodium conductance of g_Na = 3.0 S/cm², i.e., when the numbers of nodal sodium channels increase linearly with the nodal diameter. The inset displays the linear tapering of the axon and myelin sheath in the PARA region.