Figure 1.

Action potential thresholds are highly flexible in NM neurons. A, Depolarizing current steps (top) evoke single action potentials, followed by a steady-state depolarized potential in a representative P3 NM neuron. B, Steady-state membrane potential, recorded at the end of the current step in A, showed a nonlinear change as a function of current input for neurons (open symbols; mean ± SD; n = 12) and for the multicompartment model (solid line). C, Input resistance, calculated as the slope at each point in B, decreased as a function of membrane potential. D, A two-step current-clamp protocol was used to measure spike thresholds as a function of membrane potential. D1, From resting membrane potential, rheobase is measured as the minimal current that evokes a spike (2.0 nA in this neuron). Voltage threshold is marked with an open arrowhead. D2, When depolarized by ∼10 mV with an initial current step of 0.8 nA, rheobase increased (subthreshold response to control rheobase marked by asterisk) and voltage threshold accommodation was apparent (arrowhead). D3, Additional depolarization led to greater shifts in rheobase and voltage threshold. D4, Each action potential is shown with an expanded timescale, showing upward shifts in the inflection point of the voltage trace that indicates threshold. E, Voltage threshold plotted as a function of membrane potential for neurons (open squares), the multicompartment model (solid line) and the single-compartment model (dashed line). F, The difference between voltage threshold and membrane potential shows an upward shift, indicating that accommodation maintains a large difference between rest and threshold across a broad range of membrane potentials. G, Rheobase increases as a function of membrane potential, indicating decreased excitability during depolarization.