Fig. 4.

Numerical simulation of SHG generated by a microtubule array.(A) Schematic drawing of the simulation model. A focused laser beam impinges on a neurite with diameter d_N that contains a hexagonal packed array of plus- or minus-end distal microtubules with intermicrotubule spacing d_MT. SHG intensities were calculated in the forward and backward directions for four classes of neurites: apical dendrites (d_MT = 64 nm, d_N = 3 μm), basal dendrites (d_MT = 64 nm, d_N = 1 μm), distal dendrites (d_MT = 64 nm, d_N = 0.5 μm), and axons (d_MT = 22 nm, d_N = 0.17 μm). (B) Simulated forward-directed SHG intensities generated by each type of neurite (circles). Polarity is defined as the number of plus-end distal microtubules divided by the total number of microtubules in the simulated neurite. The simulated intensities were compared with a simple model (solid line) where SHG intensity ∼[N₊ − N₋]², where N₊ and N₋ are the numbers of plus- and minus-end distal microtubules. Backward-directed signals are too small to be shown in the same vertical scale. (C) Simulated fb SHG intensity ratio. The error bars are ±SD. Uncertainty is caused by the possible variety of microtubule distributions that can be arranged within a neurite.