Figure 2.

The energetic cost of making myelin and the ATP saved on axonal action potentials in the guinea pig optic nerve. A, The ATP cost, per length of internode, for assembling myelin from local lipid and protein resources increases quadratically with axon diameter (Eq. 2 of the Materials and Methods). Data points are labeled with the ganglion cell type(s) of each diameter [from the study by Perge et al. (2009)]. B, The ATP saving per action potential, per length of internode, conferred by myelin increases approximately linearly with axon diameter (dashed straight line has slope predicted from Eq. 3 of the Materials and Methods when C_L is set to zero). C, The number of action potentials required to pay back the cost of myelination increases approximately linearly with axon diameter (dashed straight line has slope predicted from Eq. 4 of the Materials and Methods when C_L is set to zero). D, Taking into account mean firing rates [inset: experimental data from which a linear axon diameter-firing rate relationship was derived in the study by Perge et al. (2009)], the time taken to pay back myelination decreases with axon diameter.