Spike threshold and corresponding biophysical basis produced by varying γ_<i>w</i> and g¯K.

<p>(A)-(C) summarize the results in the case of varying γ_<i>w</i>. The minimal model exhibit Type I excitability with γ_<i>w</i> = 10mV and Type II excitability with γ_<i>w</i> = 20mV. (D)-(F) summarize the results in the case of varying </p><p></p><p></p><p></p><p></p><p></p><p>g<mo stretchy="true">¯</mo></p><p></p>K<p></p><p></p><p></p><p></p>. The neuron exhibits Type I excitability with <p></p><p></p><p></p><p></p><p></p><p>g<mo stretchy="true">¯</mo></p><p></p>K<p></p><mo>=</mo><mn>7</mn><p></p><p></p><p></p><p>mS</p><mo>/</mo><p>cm</p><p></p><p></p><mn>2</mn><p></p><p></p><p></p><p></p> and Type II excitability with <p></p><p></p><p></p><p></p><p></p><p>g<mo stretchy="true">¯</mo></p><p></p>K<p></p><mo>=</mo><mn>20</mn><p></p><p></p><p></p><p>mS</p><mo>/</mo><p>cm</p><p></p><p></p><mn>2</mn><p></p><p></p><p></p><p></p>. β_<i>w</i> = 0mV for (A)-(C), and β_<i>w</i> = -10mV, γ_<i>w</i> = 13mV for (D)-(F). Other parameters are as indicated in Materials and Methods. (A) and (D) show the activation curves and kinetics of ionic currents for two cell types. Increasing γ_<i>w</i> has no effects on Na⁺ activation <i>m</i>_∞, while it reduces the slope of K⁺ activation <i>w</i>_∞ and increases K⁺ time constant <i>τ</i>_<i>w</i>. Unlike γ_<i>w</i>, increasing <p></p><p></p><p></p><p></p><p></p><p>g<mo stretchy="true">¯</mo></p><p></p>K<p></p><p></p><p></p><p></p> has no impacts on the voltage-dependency and kinetics of <i>I</i>_K, while it affects the magnitude of K⁺ conductance (data is not shown). (B) and (E) give the membrane currents at the subthresholds for two types of neuron model. Increasing γ_<i>w</i> has no effects on <i>I</i>_Na and <i>I</i>_Leak, whereas it leads <i>I</i>_K,SS to activate prior to spike initiation. Increasing <p></p><p></p><p></p><p></p><p></p><p>g<mo stretchy="true">¯</mo></p><p></p>K<p></p><p></p><p></p><p></p> causes the outward <i>I</i>_K,SS to be much stronger without altering its voltage-dependency and kinetics. Both of them result in a much larger outward <i>I</i>_SS at the threshold. (C) and (F) give the spike threshold within a range of d<i>V</i>/d<i>t</i> for each cell. The d<i>V</i>/d<i>t</i> is from 0.8mV/ms to 4.5mV/ms. This is because Type II neuron generates the delayed loss of stability for d<i>V</i>/d<i>t</i><0.8mV/ms, which leads to a big error when computing d<i>V</i>/d<i>t</i>. The specific reason is shown in the Discussion.<p>