Muscle spindles of the rat sternomastoid muscle

Abstract

The sternomastoid (SM) muscle in rodents presents a peculiar distribution of fiber types with a steep gradient from the ventral, superficial, white portion to the dorsal, deep, red region, where muscle spindles are restricted. Cross section of the medial longitudinal third of the rat SM contains around 10,000 muscle fibers with a mean diameter of 51.28±12.62 (μm +/- SD). Transverse sections stained by Succinate Dehydrogenase (SDH) reaction clearly presents two distinct regions: the dorsal deep red portion encompassing a 40% cross section area contains a high percentage of packed SDH-positive muscle fibers, and the ventral superficial region which contains mainly SDH-negative muscle fibers. Indeed, the ventral superficial region of the rat SM muscle contains mainly fast 2B muscle fibers. These acidic ATPase pH 4.3-negative and SDH-negative 2B muscle fibers are the largest of the SM muscle, while the acidic ATPase pH 4.3-positive and SDH-positive Type 1 muscle fibers are the smallest. Here we show that in thin transverse cryosections only 2 or 3 muscle spindle are observed in the central part of the dorsal deep red portion of the SM muscle. Azan Mallory stained sections allow at the same time to count the spindles and to evaluate aging fibrosis of the skeletal muscle tissue. Though restricted in the muscle red region, SM spindles are embedded in perimysium, whose changes may influence their reflex activity. Our findings confirm that any comparisons of changes in number and percentage of muscle spindles and muscle fibers of the rat SM muscle will require morphometry of the whole muscle cross-section. Muscle biopsies of SM muscle from large mammals will only provide partial data on the size of the different types of muscle fibers biased by sampling. Nonetheless, histology of muscle tissue continue to provide practical and low-cost quantitative data to follow-up translational studies in rodents and beyond.

Fascia Is Able to Actively Contract and May Thereby Influence Musculoskeletal Dynamics: A Histochemical and Mechanographic Investigation

“…In a similar manner, our finding regarding the perimysium could add an interesting perspective to the hypothesis of Stecco et al (2013)—that muscle spindle sensitivity is influenced by the stiffness of the perimysial connective tissue in which the spindle capsules are mostly embedded (Maier, 1999; Boyd-Clark et al, 2002). Based on the influence of spindle derived mechanosensation on alpha motorneuron activation this interaction may contribute to active muscle tonicity (Giuriati et al, 2018). According to this concept, several myofascial pain syndromes could be influenced by changes in perimysial stiffness (Stecco et al, 2016).…”

AChRs Degeneration at NMJ in Aging-Associated Sarcopenia–A Systematic Review

“…Besides, there was no change of junctional AChRs number in mouse soleus that is a well-known slow-twitch muscle (Banker et al, 1983;Anis and Robbins, 1988). Although diaphragm had a larger proportion of fast myosin isoforms, MHC IIa accounted for a larger proportion in diaphragm (Louboutin et al, 1993;Greising et al, 2015), which was different from EDL and sternomastoid muscle with more MHC IIb (Augusto et al, 2004;Giuriati et al, 2018). Banker et al and Herscovich et al reported no changes of junctional AChRs number in mouse diaphragm during aging (Banker et al, 1983;Herscovich and Gershon, 1987).…”

“…Junctional AChRs Number EDL and sternomastoid muscle were typical fast-twitch muscles (Augusto et al, 2004;Giuriati et al, 2018), while junctional AChRs number of the two muscles decreased with aging in both rats and mice (Courtney and Steinbach, 1981;Banker et al, 1983). But this reduction was not observed in mouse gastrocnemius which was also a fast-twitch muscle (Banker et al, 1983;Augusto et al, 2004).…”

Evidence of a new hidden neural network into deep fasciae

“…These nerve corpuscles are specialized to feel the state of contraction of the various muscular bundles and to regulate them. It is well known that their capsules are totally in continuity with the perimysium and endomysium of the muscle 27 , and consequently any alteration of the intramuscular connective tissue can alter the sensitivity of these receptors, as demonstrated for dystrophy 28 . We also know that the intrafusal spindle muscular fibers can stretch the surrounding connective tissue, and consequently through fascial continuity among endomysium, perimysium and epimysium affect the tension of the gluteal fascia.…”