Bistable behavior in spinal motoneurons consists of self-sustained firing evoked by a brief period of input. However, not all motoneurons possess an equal capacity for bistable behavior. In the companion paper, we found that self-sustained firing was persistent for long periods only in motoneurons with low rheobases and slow axonal conduction velocities. High rheobase, fast conduction velocity motoneurons tend to be only partially bistable in that self-sustained firing lasts at most 1-2 s. The mechanisms underlying these differences between fully and partially bistable motoneurons were investigated by measuring their current voltage (I-V) relationships in the decerebrate cat preparation after administration of the noradrenergic α1 agonist methoxamine. Slow (8 mV/s) triangular voltage commands were applied using the discontinuous single-electrode voltage-clamp technique. Both fully and partially bistable cells exhibited a region of negative I-V slope due to activation of a strong, persistent inward current. The peak amplitude of the total persistent inward current (I(PIC)) was equally large in fully and partially bistable cells, but there were substantial differences in how I(PIC) was activated and deactivated. In fully bistable cells, the offset of I(PIC) on the descending phase of the triangular voltage command occurred at a substantially more hyperpolarized voltage then its onset on the rising phase. Thus the I-V function of fully bistable cells exhibited marked hysteresis. Partially bistable cells had significantly less hysteresis. The lack of hysteresis in partially bistable cells was due to a greater decay of I(PIC) with time than that seen in fully bistable cells. Furthermore, the range over which activation and deactivation of I(PIC) occurred was more depolarized in partially than in fully bistable cells. The I-V functions were compared with frequency-current (F-I) functions from the same cells, the characteristics of which were presented in the companion paper. The strong onset-offset difference in I(PIC) in fully bistable cells corresponded to a similarly large hysteresis for the thresholds of their F-I functions. The reduced onset-offset difference for I(PIC) in partially bistable cells corresponded to a lack of hysteresis in F-I thresholds. Thus the properties of I(PIC) accounted for the main differences in the F-I behavior seen between fully and partially bistable cells.
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