Figure 6.

Transitions between states elicited by current pulses. (A) Quiescence/firing bistability. CP motoneuron (axonal conduction velocity: 90 m/s, input conductance: 1.0 μS). The current pulse (24 nA, 1 s) elicited an accelerating discharge that started with a frequency of 14 Hz and reached 50 Hz at the end of the pulse. After the pulse, the motoneuron kept firing at a lower frequency (5 Hz) instead of going back to rest. A hyperpolarizing current pulse (−24 nA) terminated firing. Artificial L-type current conductance: 350 nS, half-activation voltage: −50 mV, steepness of the activation curve: 2 mV, activation time constant: 400 ms. (B) Firing bistability. CP motoneuron (axonal conduction velocity: 90 m/s, input conductance: 1.1 μS). A 31 nA bias current was applied, which made the neuron steadily discharge at the mean frequency of 19 Hz. A pulse of 10 nA lasting 1.5 s was then superimposed to shift the motoneuron toward its up state (57 Hz, three times more than before the pulse). After the pulse, the motoneuron kept firing at a mean frequency of 30 Hz, about half more than the before the pulse. Two different firing states were thus obtained for the same injected current. The discharge in the up state (during and after the pulse) was more irregular than in the down state (before the pulse). L-type conductance: 820 nS, half-activation: −58 mV, steepness: 1 mV, activation time constant: 400 ms, reversal potential: 20 mV.