Muscle spindles and their role in maintaining robust locomotion.

Muscle spindles, one of the two main classes of proprioceptors together with Golgi tendon organs, are sensory structures that keep the central nervous system updated about the position and movement of body parts. Although they were discovered more than one century and a half ago, their function during movement has not been fully understood yet. Here, we summarized the morphology and known functions of muscle spindles, with a particular focus on locomotion. While certain properties such as the sensitivity to dynamic and static muscle stretch are long-known, recent advances in molecular biology allowed the characterization of the molecular mechanisms for signal transduction in muscle spindles. Building upon classic literature showing that the lack of sensory feedback is deleterious to locomotion, we brought to the discussion more recent findings that support a pivotal role of muscle spindles in maintaining murine and human locomotor robustness, defined as the ability to cope with perturbations. Yet, more research is needed to expand the existing mechanistic understanding of how muscle spindles contribute to the production of robust, functional locomotion in real-world settings. Future investigations should focus on combining different animal models to identify, in health and disease, those peripheral, spinal and brain proprioceptive structures involved in the fine tuning of motor control when locomotion happens in challenging conditions. Graphical abstract legend Vertebrates with intact muscle spindles respond to locomotor perturbations by increased kinematic variability and reduced accuracy of muscle activation patterns. When muscle spindles, but not Golgi tendon organs, are genetically removed in mice, the animals fail to modulate kinematics and muscle activity in response to perturbations and appear to be in a constantly perturbed state. This article is protected by copyright. All rights reserved.