Sillar K T, Reith C A, McDearmid J R
School of Biomedical Sciences, University of St. Andrews, Gatty Marine Laboratory, Fife, Scotland.
Ann N Y Acad Sci. 1998 Nov 16;860:318-32. doi: 10.1111/j.1749-6632.1998.tb09059.x.
In this article we review our research on the development and intrinsic neuromodulation of a spinal network controlling locomotion in a simple vertebrate. Swimming in hatchling Xenopus embryos is generated by a restricted network of well-characterized spinal neurons. This network produces a stereotyped motor pattern which, like real swimming, involves rhythmic activity that alternates across the body and progresses rostrocaudally with a brief delay between muscle segments. The stereotypy results from motoneurons discharging a single impulse in each cycle; because all motoneurons appear to behave similarly there is little scope for altering the output to the myotomes from one cycle to the next. Just one day later, however, Xenopus larvae generate a more complex and flexible motor pattern in which motoneurons can discharge a variable number of impulses which contribute to ventral root bursts in each cycle. This maturation of swimming is due, in part, to the influence of serotonin released from brain-stem raphespinal interneurons whose axonal projections innervate the cord early in larval life. Larval swimming is differentially modulated by both serotonin and by noradrenaline: serotonin leads to relatively fast, intense swimming whereas noradrenaline favors slower, weaker activity. Thus, these two biogenic amines select opposite extremes from the spectrum of possible output patterns that the swimming network can produce. Our studies on the cellular and synaptic effects of the amines indicate that they can control the strength of reciprocal glycinergic inhibition in the spinal cord. Serotonin and noradrenaline act presynaptically on the terminals of glycinergic commissural interneurons to weaken and strengthen, respectively, crossed glycinergic inhibition during swimming. As a result, serotonin reduces and noradrenaline increases interburst intervals. The membrane properties of spinal neurons are also affected by the amines. In particular, serotonin can induce intrinsic oscillatory membrane properties in the presence of NMDA. These depolarizations are slow compared to the cycle periods during swimming and so may contribute to enhancement of swimming over several consecutive cycles of activity.
在本文中，我们回顾了我们关于控制简单脊椎动物运动的脊髓网络的发育和内在神经调节的研究。非洲爪蟾胚胎幼体的游泳是由一组特征明确的脊髓神经元的受限网络产生的。这个网络产生一种刻板的运动模式，就像真正的游泳一样，涉及有节奏的活动，这种活动在身体两侧交替，并从前向后推进，肌肉节段之间有短暂延迟。这种刻板性源于运动神经元在每个周期发放单个冲动；由于所有运动神经元的行为似乎都相似，因此从一个周期到下一个周期改变对肌节的输出的空间很小。然而，仅仅一天后，非洲爪蟾幼体就产生了一种更复杂、更灵活的运动模式，其中运动神经元可以发放可变数量的冲动，这些冲动在每个周期中促成腹根爆发。游泳的这种成熟部分归因于脑干中缝脊髓中间神经元释放的5-羟色胺的影响，其轴突投射在幼体生命早期就支配脊髓。幼体游泳受到5-羟色胺和去甲肾上腺素的不同调节：5-羟色胺导致相对快速、强烈的游泳，而去甲肾上腺素则有利于较慢、较弱的活动。因此，这两种生物胺从游泳网络可以产生的可能输出模式的范围内选择了相反的极端。我们对胺类的细胞和突触效应的研究表明，它们可以控制脊髓中相互甘氨酸能抑制的强度。5-羟色胺和去甲肾上腺素分别在甘氨酸能连合中间神经元的终末前突触作用，在游泳期间分别减弱和增强交叉甘氨酸能抑制。结果，5-羟色胺减少而 去甲肾上腺素增加爆发间隔。脊髓神经元的膜特性也受到胺类的影响。特别是，5-羟色胺在存在NMDA的情况下可以诱导内在振荡膜特性。与游泳期间的周期相比，这些去极化是缓慢的，因此可能有助于在连续几个活动周期中增强游泳。
