Figure 2.

Myelin Sheath Ultrastructure Is Consistent with Double-Cable Parameters

(A) Left, EM image of a L5 axon labeled with HRP (black precipitation). Right, higher magnification (white dotted box in left) revealing myelin membranes and lamellae (here, n_my = 13), false colored in blue. The cytoplasmic loop is marked with an asterisk. Scale bars, 200 nm (left) and 100 nm (right).

(B) Cross-sectional schematic showing internode parameters with their radial circuit correlates, including those for myelin (blue: R_my, C_my, n_my, and δ_my) and axon core (gray: R_m, C_m, and R_i). Also shown are axon core diameter (d) and total fiber diameter (D).

(C) Linear variation of myelin lamellae with internodal d (linear regression, 10.5 lamella μm^–1, F test p < 0.0001, R² = 0.823; n = 18 internodes from 8 axons).

(D) Linear variation of inner axon d with outer axon D reveals a g ratio of 0.698 (linear regression, F test p < 0.0001, R² = 0.959; n = 18 internodes from 8 axons).

(E) Schematic illustration of R_my and C_my composed by each myelin membrane (R_mm and C_mm).

(F) Left and right, EM estimates for n_my and δ_my, respectively. Data are shown as mean ± SEM and individual internodes (open circles, n = 8). Red dotted line, DC model prediction (n = 6 axons).

(G) Population data for radial resistance values in DC models (Friedman test with Dunn’s correction p < 0.001 R_my versus R_m; p < 0.0001 R_m versus R_mm; n = 6 models). (See Equations 7 and 8; STAR Methods.)

(H) Population data for DC model estimates (Friedman test with Dunn’s correction p < 0.0001 C_my versus C_m and C_mm; p = 0.0429 C_m versus C_mm; n = 6 models). Note, both C_mm and C_m are near 1 μF cm^–2 (Equations 9 and 10; STAR Methods). Data represent mean ± SEM.