Modified Mainen and Sejnowski Model also Exhibits The "kink" and Variability in Somatic Spike InitiationFigure 18. Our model of a full cortical neuron is based upon that of Mainen and Sejnowski, except that we modified the maximal Na+ and K+ conductances in the dendrite, soma, and axon to yield action potentials that exhibited maximal dV/dt and spike widths that were similar to our recordings from layer 5 pyramidal cells in vitro (compare with the first figure of this seminar). This was done in order to make the model as physiologically close to actual pyramidal neurons as is possible.
This modified model also exhibits the same basic features as the real neurons, including the "kink" at spike onset in the soma and the variability in membrane potential at spike onset in the soma. The axon initial segment, on the other hand, exhibits features similar to real axon intial segments including a smooth spike onset and a lower level of variability.
The original Mainen and Sejnowski model is available here. We used the following conductances (in nS/micron2): soma (gNa 0.75, gK 0.15), axon hillock and initial segment (gNa 7.5, gK 1.5); dendrite (gNa 0.1, gK 0.002, gM 0.0003). The length of the axon was increased to 50 microns and the size of the cell body was decreased to 20X30 microns. These parameters were changed to match the maximal dV/dt rates, durations, and initiation site of spikes in our neurons.
The parameter files to run our model in NEURON are available here.