Table 1.
Assumed values for the calculations, listed in order of appearance in Materials and Methods
| Energy budget for the white matter | |||
| No. axons in optic nerve | 100,000 | Lam et al. (1982) | |
| Percentage axons myelinated at P12 | 10 | % | Foster et al. (1982) |
| Mean diameter of myelinated axon in rat | 0.77 | μm | Foster et al. (1982) |
| Mean diameter of myelinated axon in guinea pig | 0.89 | μm | Koch et al. (2006) |
| Mean diameter of unmyelinated axon | 0.3 | μm | Foster et al. (1982) |
| Optic nerve length at P12 | 5.5 | mm | Matheson (1970) |
| Mean firing rate of axons | 4.34 | Hz | Koch et al. (2006), Perge et al. (2009) |
| Voltage change for action potential | 100 | mV | Attwell and Laughlin (2001) |
| Internode length | 240 | μm | Ransom et al. (1991) |
| Node length | 0.8 | μm | Ishibashi et al. (2002) |
| Membrane capacitance | 1 | μF/cm2 | |
| g ratio | 0.81 | Guy et al. (1989) | |
| Myelin wrap periodicity | 15.6 | nm | Agrawal et al. (2009) |
| Space between axon and myelin | 4 | nm | Halter and Clark (1991) |
| Node capacitance | 0.019 | pF | Calculated |
| Internode capacitance | 0.49 | pF | Calculated |
| Action potential Na+ entry scaling factor | 1.3 | Alle et al. (2009) | |
| Optic nerve cross-sectional area at P12 | 0.13 | mm2 | Matheson (1970) |
| OPC EPSC charge transfer | 366 | fC | Kukley et al. (2007) |
| No. OPCs in P12 rat optic nerve | 45,400 | Barres et al. (1992) | |
| No. axons contacting 1 OPC | 141 | axons | Kukley et al. (2007) |
| Vesicles released onto 1 OPC/axon/action potential | 0.34 | vesicles | Kukley et al. (2007) |
| Release cost/vesicle | 23,400 | ATPs | Attwell and Laughlin (2001) |
| Nernst potential Na | 50 | mV | Assumed |
| Nernst potential K | −100 | mV | Assumed |
| No. oligodendrocytes in P12 optic nerve | 38,100 | Barres et al. (1992) | |
| No. astrocytes in P12 optic nerve | 15,650 | Barres et al. (1992) | |
| Astrocyte resting potential | −80 | mV | Assumed |
| All other cells' resting potential | −70 | mV | Bakiri et al. (2011), or assumed |
| Axon resistance for unit area | 7.35 | Ω.m2 | Alle and Geiger (2006) |
| Axon internal resistivity | 1.5 | Ω.m | D'Angelo et al. (1997) |
| Oligodendrocyte input resistance | 200 | MΩ | Karadottir et al. (2005) |
| OPC input resistance | 800 | MΩ | Clarke and Attwell, unpublished |
| Astrocyte input resistance | 560 | MΩ | Clark and Mobbs (1994) |
| ATP use on nonsignaling processes | 25 | % | Attwell and Laughlin (2001) |
| Optic nerve cross-sectional area in adult | 0.494 | mm2 | Phillips et al. (1991) |
| Cortex: no. synapses/neuron | 8000 | Braitenberg and Schuz (1998) | |
| Cortex: neuron density | 9.20×1013 | /m3 | Braitenberg and Schuz (1998) |
| Energetic costs and benefits of myelination | |||
| Fraction of myelin that is protein | 0.25 | Jahn et al. (2009) | |
| Fraction of myelin that is lipid | 0.75 | Jahn et al. (2009) | |
| Density of myelin | 1.1 | g/cm3 | Inouye and Kirschner (1988) |
| Mean firing rate d = 0.76 μm | 3 | Hz | Perge et al. (2009) |
| Mean firing rate d = 0.86 μm | 4 | Hz | Perge et al. (2009) |
| Mean firing rate d = 1.06 μm | 6 | Hz | Perge et al. (2009) |
| Mean firing rate d = 1.26 μm | 8 | Hz | Perge et al. (2009) |
| O2 use by muscle mitochondria | 5.8 | ml O2/min/ml | Schwerzmann et al. (1989) |
| ATPs generated per glucose molecule | 30 | ATP | Stryer (1995) |
| Energy supply and production in myelinated axons | |||
| O2 molecules consumed per glucose molecule | 6 | O2 | Stryer (1995) |
| Turnover rate of GLUT3 | 6500 | /s | Maher et al. (1991) |
| GLUT3 Km for glucose | 1 | mm | Nualart et al. (2009) |