School of Biology, University of St Andrews, St Andrews, Fife KY16 9TS, Scotland, United Kingdom.
Proc Natl Acad Sci U S A. 2011 Jul 12;108(28):11674-9. doi: 10.1073/pnas.1018512108. Epub 2011 Jun 27.
Locomotion in immature animals is often inflexible, but gradually acquires versatility to enable animals to maneuver efficiently through their environment. Locomotor activity in adults is produced by complex spinal cord networks that develop from simpler precursors. How does complexity and plasticity emerge during development to bestow flexibility upon motor behavior? And how does this complexity map onto the peripheral innervation fields of motorneurons during development? We show in postembryonic Xenopus laevis frog tadpoles that swim motorneurons initially form a homogenous pool discharging single action potential per swim cycle and innervating most of the dorsoventral extent of the swimming muscles. However, during early larval life, in the prelude to a free-swimming existence, the innervation fields of motorneurons become restricted to a more limited sector of each muscle block, with individual motorneurons reaching predominantly ventral, medial, or dorsal regions. Larval motorneurons then can also discharge multiple action potentials in each cycle of swimming and differentiate in terms of their firing reliability during swimming into relatively high-, medium-, or low-probability members. Many motorneurons fall silent during swimming but can be recruited with increasing locomotor frequency and intensity. Each region of the myotome is served by motorneurons spanning the full range of firing probabilities. This unfolding developmental plan, which occurs in the absence of movement, probably equips the organism with the neuronal substrate to bend, pitch, roll, and accelerate during swimming in ways that will be important for survival during the period of free-swimming larval life that ensues.
在不成熟的动物中，运动通常是不灵活的，但逐渐获得了多样性，使动物能够有效地在环境中运动。成年动物的运动活动是由复杂的脊髓网络产生的，这些网络是从更简单的前体发育而来的。在发育过程中，复杂性和可塑性是如何产生的，从而赋予运动行为灵活性？这种复杂性在发育过程中是如何映射到运动神经元的周围神经支配区域的？我们在胚胎后期的非洲爪蟾蝌蚪中表明，游泳运动神经元最初形成一个同质的池，每个游泳周期释放一个动作电位，并支配游泳肌肉的大部分背腹范围。然而，在早期的幼虫生活中，在自由游泳的前夕，运动神经元的神经支配区域变得局限于每个肌肉块的更有限的区域，每个运动神经元主要到达腹侧、内侧或背侧区域。然后，幼虫运动神经元也可以在每个游泳周期中释放多个动作电位，并根据它们在游泳过程中的发射可靠性分化为相对高、中或低概率的成员。许多运动神经元在游泳时沉默，但可以随着运动频率和强度的增加而被招募。肌节的每个区域都由跨越整个发射概率范围的运动神经元提供服务。这种展开的发育计划发生在没有运动的情况下，可能为生物体提供了在游泳过程中弯曲、俯仰、滚动和加速的神经元基础，这对于随后的自由游泳幼虫生活期间的生存至关重要。
