Figure 1.

(A) Cargos being hauled by motors along MTs inside an axon. The spherical cargo has a radius a in a cylindrical axon of radius R. The center of the cargo is displaced a distance b from the axon’s axis. The distance h is the closest approach of the cargo to the axon wall. Two scenarios are shown. The top shows a single motor hauling a cargo along an MT. The key point of this paper is that the enhanced viscosity encountered by the cargo near the wall of the axon can be overcome by having multiple motors hauling the cargo along closely spaced parallel microtubules as shown at the bottom. (B) Correction factor K from Eq. 2 for a sphere of radius 250 nm for two axon diameters (k = 0.1 and k = 0.5) as a function of the cargo-wall distance h. As a comparison, we have also shown the correction factor obtained from the Faxén formula (blue dotted line) for the same parameters. Far away from the wall in a large axon (magenta dashed curve) our theory agrees well with Faxén’s law. Note that for a relatively large cargo (250 nm radius) in a relatively small axon (k = 0.5, corresponding to a 1 μm diameter axon, solid line), the “edge” effect, represented by the correction factor, extends over the width of the axon, even when the cargo is far away from the wall. (C) Comparison of the theoretical correction factor K given by Eq. 2 to experimental data (82) for a sphere moving through a fluid-filled cylinder for various values of k = a/R. To see this figure in color, go online.