Novel fascial mapping of muscle spindles distribution: insights from a murine model study

Abstract

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

Keywords: distribution; fascia; motor control; muscle spindle; perimysium; proprioception.

FIGURE 1

Thickness and surface area values of MSs. (A) Maximum and minimum thicknesses of each of the MSs. (B) Surface areas of the MSs. Sections within the equatorial region of the MS with intact structure and no splitting were selected to measure the thickness and area.

FIGURE 2

Distribution of MSs in the muscles. (A) Quantity of MSs in different regions in the longitudinal section. (B) MS quantity in the transverse section.

FIGURE 3

Relationships between the MSs and vessels/nerves. (A) Number of MSs associated with nerves or vessels; vessels: 17 MSs consistently, 4 MSs partially; nerves: 2 MSs consistently, 5 MSs partially. (B) The percentage of MSs present in each structure was calculated by dividing the number of MSs in the structure by 23. (C) MSs with only vessels. (D) MSs with only nerves. (E) MSs with both nerves and vessels. (F, F1) Nerves are not associated with the MS in all segments, and these pictures are from a single MS. (G–G2) Vessels are not associated with MSs in all segments, and these pictures are from a single MS. (H–H3) MSs identified from the four rat gastrocnemius muscles, where vessels are observed in (H, H3) and no nerve or vessel are observed in (H1, H2).

FIGURE 4

Series sections of a single MS. (A, B) Different slices from MS no. 9; this MS receives tension from (A) six or (B) five directions in different segments, where the directions are marked with red dots. (C, D) Different slices from MS no. 3. (E, F) Different slices from MS no. 8.

FIGURE 5

Results of the MSs identified in the rat quadriceps femoris muscle. (A) Number of fascia (perimysium) connections with the MS capsule; there are differences in this number between MSs and also different segments of each MS. (B) The comparison of thickness (maximal part of the capsule versus minimal) shows no statistical significance. (C) Schematic illustration of the anatomical relationship between the MS and surrounding fascia as well as the tension that it is subjected to.

FIGURE 6

MSs in human muscles. (A–A2) Three MSs were found in one human TA muscle sample. (B) A single MS was found in the human QF muscle sample. (C) A single MS was found in one of the deltoid muscle samples. (D, D1) Two MSs in another deltoid muscle sample based on H&E staining. (E, F) MSs found in one rectus femoris and one fibularis brevis, where (E) is based on Sirius Red staining and (F) is based on Van Gieson staining. DEL, deltoid; FB, fibularis brevis; QF, quadratus femoris; RF, rectus femoris; TA, tibialis anterior.