Lee R H, Heckman C J
Department of Physiology, Northwestern University Medical School, Evanston, Illinois 60201, USA.
J Neurophysiol. 1998 Aug;80(2):572-82. doi: 10.1152/jn.1998.80.2.572.
In the presence of the monoamines serotonin and norepinephrine, spinal motoneurons can exhibit bistable behavior, in which a brief period of excitatory input evokes prolonged self-sustained firing. A brief inhibitory input returns the cell to the quiescent state. To determine whether motoneurons differ in their capacity for bistable behavior, intracellular recordings were obtained in the decerebrate cat preparation. To enhance the likelihood of encountering bistable behavior, the noradrenergic alpha1 agonist methoxamine was applied to the ventral surface of the cord. The capacity of the cells to produce bistable behavior was assessed from the duration of self-sustained firing evoked by a brief (1.5 s) excitatory synaptic input from muscle spindle Ia afferents. About 35% (17 of 49) of the cells produced steady self-sustained firing for >3 s and were considered fully bistable. The other 32 cells ( approximately 65%) were partially bistable, with self-sustained firing lasting <3 s. Fully bistable cells tended to have lower current thresholds for spike initiation and slower axonal conduction velocities than did partially bistable cells. This suggests that fully bistable motoneurons innervate fatigue resistant muscle fibers. The frequency-current (F-I) relations of the motoneurons were characterized with slow triangular current ramps. Fully bistable cells displayed an acceleration in firing rate immediately on initiation of rhythmic firing. The F-I gain after completion of the acceleration was positive. Fully bistable cells also displayed a hysteresis in the current level for firing threshold with the ascending threshold occurring at substantially higher current level than the descending one. Additionally, these current thresholds usually were centered about zero current, so that the ascending current threshold was positive while the descending current threshold was negative. This negative offset meant that fully bistable cells could exhibit tonic firing without depolarizing injected current. Partially bistable cells exhibited very different F-I characteristics. Firing rate acceleration was just as large as in fully bistable cells but did not occur until well above the current level needed to initiate rhythmic firing. F-I gain after acceleration was negative, there was little to no hysteresis between the ascending and descending firing thresholds, and both thresholds were above the zero current level. These properties of partially bistable cells suggest their functional role is in tasks requiring relatively brief, high forces. The low thresholds of fully bistable cells mean they will be readily recruited in low force tasks like posture, where their prolonged self-sustained firing would be advantageous.
在单胺类物质血清素和去甲肾上腺素存在的情况下，脊髓运动神经元可表现出双稳态行为，即短暂的兴奋性输入能引发长时间的自发放电。短暂的抑制性输入会使细胞恢复到静息状态。为了确定运动神经元在双稳态行为能力上是否存在差异，在去大脑猫的标本上进行了细胞内记录。为了增加出现双稳态行为的可能性，将去甲肾上腺素能α1激动剂甲氧明应用于脊髓腹侧表面。根据肌肉梭Ia传入纤维短暂（1.5秒）的兴奋性突触输入所引发的自发放电持续时间，评估细胞产生双稳态行为的能力。约35%（49个中的17个）的细胞产生持续超过3秒的稳定自发放电，被认为是完全双稳态的。另外32个细胞（约65%）是部分双稳态的，自发放电持续时间小于3秒。完全双稳态的细胞相较于部分双稳态的细胞，往往具有更低的动作电位起始电流阈值和更慢的轴突传导速度。这表明完全双稳态的运动神经元支配抗疲劳的肌纤维。运动神经元的频率-电流（F-I）关系通过缓慢的三角形电流斜坡来表征。完全双稳态的细胞在节律性放电开始时，放电频率立即加速。加速完成后的F-I增益为正。完全双稳态的细胞在放电阈值的电流水平上还表现出滞后现象，上升阈值出现在比下降阈值高得多的电流水平。此外，这些电流阈值通常以零电流为中心，因此上升电流阈值为正，而下降电流阈值为负。这种负偏移意味着完全双稳态的细胞在不使注入电流去极化的情况下就能表现出紧张性放电。部分双稳态的细胞表现出非常不同的F-I特性。放电频率的加速与完全双稳态的细胞一样大，但直到远高于引发节律性放电所需的电流水平才会出现。加速后的F-I增益为负，上升和下降放电阈值之间几乎没有滞后现象，且两个阈值都高于零电流水平。部分双稳态细胞的这些特性表明它们在需要相对短暂、高强度力量的任务中发挥功能作用。完全双稳态细胞的低阈值意味着它们在诸如姿势等低强度力量任务中很容易被募集，在这些任务中它们长时间的自发放电将是有利的。
