A Novel Fascial Mapping of Muscle Spindles Distribution: Insights from Murine Model Study

Yunfeng Sun ^1,2,3 Lucia Petrelli ² Caterina Fede ² Damiana Incendi ² Xiaoxiao Zhao ^1,2* Andrea Porzionato ² Carmelo Pirri ² Carla Stecco ² Carlo Biz ³

• ¹ Padua Neuroscience Center, University of Padua, Padua, Veneto, Italy
• ² Institute of Human Anatomy, Department of Neuroscience, University of Padova, padova, Italy
• ³ Department of Orthopedics and Orthopedic Oncology, University of Padova, padova, Italy

Muscle spindles (MSs) are pivotal in proprioception and motor control. The precise distribution and localization of MSs have been a major focus of research, providing a foundation for understanding their role in various diseases and motor dysfunctions. However, there are disagreements in the distribution pattern of MSs at present, the discrepancies hinder the advancement of novel physical therapy techniques based on MS functionality. In this study, we presented an innovative fascia-based distribution pattern for MSs. Using the rat quadriceps femoris muscle, serial sections of the muscle were meticulously prepared following tissue sampling, fixation, and embedding. Another four rat gastrocnemius and 8 human muscles were processed and cut into non-successive sections by following the above method. The MSs were identified and characterized using Sirius Red staining, with their locations, quantities, associated structures, and basic parameters documented under microscopy. Our findings demonstrated that MSs are primarily located within the fascial layers, predominantly within the perimysium, the MS capsule is structurally continuous with the perimysium, forming multiple connections in different orientations. This study demonstrated that MSs are influenced not only by changes in muscle length but also by alterations in fascia tension or state, which may have a more significant impact. Furthermore, both nerves or vessels were observed near or within the muscle spindles' capsule but were not always presented. In some sections, no microscopically distinguishable vessels or nerve fibers were observed around the MSs. This study proposed a novel fascial-based distribution model for MSs, highlighting that MSs are embedded within the fascial matrix and that fascia may serve as a key structural marker for locating MSs.Additionally, the fascia's structural continuity with the MS capsule suggests its role as a potential mediator in MS function. This research challenges traditional concepts of MS distribution and introduces a more refined and efficient approach to studying MSs through the fascial perspective, representing a significant advancement in the field.