Laboratory of Neuromotor Physiology, Istituto di Ricovero e Cura a Carattere Scientifico Fondazione Santa Lucia, 00179 Rome, Italy.
Department of Pediatric Neurorehabilitation, Istituto di Ricovero e Cura a Carattere Scientifico Fondazione Santa Lucia, 00179 Rome, Italy.
Proc Natl Acad Sci U S A. 2020 Apr 28;117(17):9604-9612. doi: 10.1073/pnas.1920984117. Epub 2020 Apr 13.
Mature locomotion involves modular spinal drives generating a set of fundamental patterns of motoneuron activation, each timed at a specific phase of locomotor cycles and associated with a stable muscle synergy. How locomotor modules develop and to what extent they depend on prior experience or intrinsic programs remains unclear. To address these issues, we herein leverage the presence at birth of two types of locomotor-like movements, spontaneous kicking and weight-bearing stepping. The former is expressed thousands of times in utero and postnatally, whereas the latter is elicited de novo by placing the newborn on the ground for the first time. We found that the neuromuscular modules of stepping and kicking differ substantially. Neonates kicked with an adult-like number of temporal activation patterns, which lacked a stable association with systematic muscle synergies across movements. However, on the ground neonates stepped with fewer temporal patterns but all structured in stable synergies. Since kicking and ground-stepping coexist at birth, switching between the two behaviors may depend on a dynamic reconfiguration of the underlying neural circuits as a function of sensory feedback from surface contact. We tracked the development of ground-stepping in 4- to 48-mo-old infants and found that, after the age of 6 mo, the number of temporal patterns increased progressively, reaching adult-like conformation only after independent walking was established. We surmise that mature locomotor modules may derive by combining the multiple patterns of repeated kicking, on the one hand, with synergies resulting from fractionation of those revealed by sporadic weight-bearing stepping, on the other hand.
成熟的运动涉及模块化的脊髓驱动，产生一组基本的运动神经元激活模式，每个模式都在运动周期的特定相位计时，并与稳定的肌肉协同作用相关。运动模块如何发展以及它们在多大程度上依赖于先前的经验或内在程序仍然不清楚。为了解决这些问题，我们利用出生时存在的两种类似运动的运动，自发性踢腿和负重踏步。前者在子宫内和出生后表达数千次，而后者是通过将新生儿第一次放在地上而新产生的。我们发现踏步和踢腿的神经肌肉模块有很大的不同。新生儿踢腿的时间激活模式与成人相似，但在整个运动过程中缺乏与系统肌肉协同作用的稳定关联。然而，在地上，新生儿踏步的时间模式较少，但所有模式都以稳定的协同作用组织。由于踢腿和地面踏步在出生时同时存在，两种行为之间的切换可能取决于基础神经回路的动态重新配置，作为来自表面接触的感觉反馈的函数。我们跟踪了 4 至 48 个月大婴儿的地面踏步的发展，发现 6 个月后，时间模式的数量逐渐增加，只有在独立行走建立后才达到成人样的构象。我们推测，成熟的运动模块可能来源于重复踢腿的多种模式的组合，一方面，另一方面来源于由零星负重踏步揭示的协同作用的细分。
