Lee R H, Heckman C J
Department of Physiology, Northwestern University Medical School, Evanston, Illinois 60201, USA.
J Neurophysiol. 1998 Aug;80(2):583-93. doi: 10.1152/jn.1998.80.2.583.
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 alpha1 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 (IPIC) was equally large in fully and partially bistable cells, but there were substantial differences in how IPIC was activated and deactivated. In fully bistable cells, the offset of IPIC 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 IPIC with time than that seen in fully bistable cells. Furthermore, the range over which activation and deactivation of IPIC 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 IPIC in fully bistable cells corresponded to a similarly large hysteresis for the thresholds of their F-I functions. The reduced onset-offset difference for IPIC in partially bistable cells corresponded to a lack of hysteresis in F-I thresholds. Thus the properties of IPIC accounted for the main differences in the F-I behavior seen between fully and partially bistable cells.
脊髓运动神经元的双稳态行为包括由短暂输入诱发的自持放电。然而，并非所有运动神经元都具有同等的双稳态行为能力。在配套论文中，我们发现，自持放电仅在具有低基强度和慢轴突传导速度的运动神经元中持续很长时间。高基强度、快传导速度的运动神经元往往只是部分双稳态，因为自持放电最多持续1 - 2秒。通过在给予去甲肾上腺素能α1激动剂甲氧明后，在去大脑猫标本中测量其电流 - 电压（I - V）关系，研究了完全双稳态和部分双稳态运动神经元之间这些差异的潜在机制。使用不连续单电极电压钳技术施加缓慢（8 mV/s）的三角波电压指令。完全双稳态和部分双稳态细胞均由于一种强大的、持续内向电流的激活而呈现负I - V斜率区域。完全双稳态和部分双稳态细胞中总持续内向电流（IPIC）的峰值幅度同样大，但IPIC的激活和失活方式存在显著差异。在完全双稳态细胞中，三角波电压指令下降阶段IPIC的偏移发生在比其上升阶段起始时明显更超极化的电压处。因此，完全双稳态细胞的I - V函数表现出明显的滞后现象。部分双稳态细胞的滞后现象明显较少。部分双稳态细胞中缺乏滞后现象是由于IPIC随时间的衰减比完全双稳态细胞中更大。此外，IPIC激活和失活发生的电压范围在部分双稳态细胞中比在完全双稳态细胞中更去极化。将I - V函数与来自相同细胞的频率 - 电流（F - I）函数进行比较，其特征在配套论文中给出。完全双稳态细胞中IPIC明显的起始 - 偏移差异对应于其F - I函数阈值的类似大滞后现象。部分双稳态细胞中IPIC减小的起始 - 偏移差异对应于F - I阈值中缺乏滞后现象。因此，IPIC的特性解释了完全双稳态和部分双稳态细胞之间在F - I行为上的主要差异。
