Figure 4.

Simulating the effects of extracellular sodium concentration, [Na ⁺]_o, on neuronal firing in a simple neuron soma built in NEURON (equations are in the Methods). (a) The earlier AP (pink) at [Na ⁺]_o of 160 mM, and the faster firing rate when [Na ⁺]_o is increased from 145 to 160 mM. (b) The sodium current peaks earlier in 160mM [Na ⁺]_o (pink) compared to that in 145mM [Na ⁺]_o (blue). (c) The potassium current peaks earlier in 160mM [Na ⁺]_o (pink) compared to that in 145mM [Na ⁺]_o (blue). (d) When [Na ⁺]_o increases from 145 to 160 mM, the change in the firing rate was plotted against Na,K-ATPase density and revealed that the altered AP rate increased more distinctly above an Na,K-ATPase density of 2.1 μA/cm ². An Na,K-ATPase density of 2.2 μA/cm ² (arrow) was used in all of our other simulations. (e) With [Na ⁺]_o between 100 and 200mM and Na,K-ATPase density of 2.2 μA/cm ², the firing rate of the model neuron increased when [Na ⁺]_o reached 130 mM. The lines a and b represent the range between normal and the 17% increase in sodium observed in the ²³Na MRI studies after NTG.