Figure 4.
Adding an artificial persistent sodium current with dynamic clamp creates a primary firing range (PR) and decreases the subprimary range (SPR) in a real motoneuron. A1, Response of the motoneuron to a slow ramp of current (9 nA amplitude, 1 nA/s, control condition without dynamic clamp). The recruitment current was 3.5 nA. The discharge remained in the SPR during the whole ramp. Note the great variability in the frequencygram. A2, A3, Magnifications of the voltage trace (between the two pairs of dashed vertical lines in A1, truncated spikes) showing the fast oscillations (arrowheads), which account for the variability of the interspike intervals. B1, Response of the motoneuron in dynamic clamp (7 nA amplitude, 1 nA/s, artificial GNap = 0.025 μS). The recruitment current was reduced to 1.6 nA, indicating an increased excitability when the artificial persistent current was added. A firing range with little variability appeared in the frequencygram. B2, Magnification of the voltage trace at the beginning of the ramp (between the two dashed vertical lines in B1) showing the fast oscillations (arrowheads). B3, Magnification of the voltage trace for a higher current intensity (between the second pair of dashed lines in B1) showed no oscillations, as expected in the PR. A PR was thus created at the expense of the SPR, whose width decreased. This PR was observed despite the smaller ramp amplitude compared with control. It was entirely due to the addition of the artificial persistent sodium current. The input resistance of this motoneuron was 1.4 MΩ.