Banks Robert W, Proske Uwe
Department of Biosciences, and Biophysical Sciences Institute, Durham University, Durham, UK.
School of Biomedical Sciences, Monash University, Clayton, Victoria, Australia.
Exp Physiol. 2024 Nov 4. doi: 10.1113/EP092264.
Muscle spindles are stretch-sensitive mechanoreceptors found in the skeletal muscles of most four-limbed vertebrates. They are unique amongst sensory receptors in the ability to regulate their sensitivity by contraction of the intrafusal muscle fibres on which the sensory endings lie. Muscle spindles have revealed a remarkable diversity of functions, including reflex action in posture and locomotion, contributing to bodily self awareness, and influencing wound healing. What were the circumstances which gave rise to the evolution of such complex end-organs? We argue that spindles first appeared in early amniotes and only later in frogs and toads. This was considered an example of convergent evolution. Spindles in amphibians and reptiles are characterised by their simple structure, pointing to key features essential for spindle function. Spindle sensitivity in amphibians and reptiles is controlled by intrafusal fibre contractions evoked by branches of motor axons supplying extrafusal muscle. Modern phylogenetic evidence has revised our views on the origin of birds, placing them closer to the dinosaurs than had previously been thought. Birds are the only group, other than mammals, which has a dedicated fusimotor innervation of spindles, another example of convergent evolution, given the widely different origins of the two groups. One factor that may have played a role here was that both groups are endotherms, allowing motor control to develop further in an optimal internal environment. This, as well as other changes within the spindle, has led to the astonishing sophistication of motor control observed especially in many modern mammals.
肌梭是大多数四足脊椎动物骨骼肌中对拉伸敏感的机械感受器。它们在感觉感受器中独一无二，能够通过感觉末梢所在的梭内肌纤维的收缩来调节自身的敏感性。肌梭展现出了显著多样的功能，包括在姿势和运动中的反射作用、有助于身体自我感知以及影响伤口愈合。是什么样的情况导致了这种复杂终末器官的进化呢？我们认为肌梭最早出现在早期羊膜动物中，后来才出现在青蛙和蟾蜍中。这被认为是趋同进化的一个例子。两栖动物和爬行动物的肌梭以其简单的结构为特征，这指向了肌梭功能所必需的关键特征。两栖动物和爬行动物的肌梭敏感性由供应梭外肌的运动轴突分支诱发的梭内纤维收缩所控制。现代系统发育学证据修正了我们对鸟类起源的看法，认为它们比之前认为的更接近恐龙。鸟类是除哺乳动物之外唯一拥有专门的肌梭传出神经支配的群体，鉴于这两类动物起源差异巨大，这又是趋同进化的一个例子。这里可能起作用的一个因素是，这两类动物都是恒温动物，使得运动控制能在最佳的内部环境中进一步发展。这一点以及肌梭内部的其他变化，导致了尤其是在许多现代哺乳动物中观察到的运动控制的惊人复杂性。
