Muscle spindles in the rhesus monkey platysma

Abstract

The platysma of the rhesus monkey consists of two parts: a platysma myoides located similar to the human platysma, and a platysma cervicale passing the dorsal cervical region and being in contact with the cheek pouch. Our investigation showed that the muscle fiber morphology was comparable in both parts. Muscle spindles were only present in regions connected to the cheek pouch and contained only nuclear chain fibers. It is tempting to speculate that they sense the filling of the cheek pouch rather than mimic activities.

Mammalian facial muscles contain muscle spindles

“…As an aside, May et al (2022) reported the presence of muscle spindles in the platysma muscle of the rhesus macaque, Macaca mulatta , but only in the portion of the platysma muscle that surrounds the cheek pouch. While these authors did not examine other facial muscles in M. mulatta , their results are intriguing considering our findings.…”

“…Previous studies focusing on identifying muscle spindle presence have used traditional histochemistry staining techniques, such as trichromes (Boyd‐Clark et al, 2002; Cobo et al, 2017; Takeuchi et al, 2019) and hematoxylin & eosin staining (Cobo et al, 2017; May et al, 2018, 2022; Mayer et al, 2021). May et al (2018, 2022) and Cobo et al (2017) successfully used H&E staining and trichrome staining with the human and rhesus macaque platysma muscle and zygomaticus major muscle, which are branchiomeric in origin. We do not aim in the present study to undertake more sophisticated analyses such as fiber‐typing the intrafusal fibers; thus, we employ traditional histochemical protocols (see Thornell et al, 2015 and Österlund et al, 2015 for more sophisticated analyses).…”

“…Previous studies focusing on identifying muscle spindle presence have used traditional histochemistry staining techniques, such as trichromes (Boyd-Clark et al, 2002;Cobo et al, 2017;Takeuchi et al, 2019) and hematoxylin & eosin staining (Cobo et al, 2017;May et al, 2018May et al, , 2022Mayer et al, 2021). May et al (2018May et al ( , 2022 and Cobo et al (2017) successfully used H&E staining and trichrome staining with the human and rhesus macaque platysma muscle and zygomaticus major Note that, in species where more than one section was examined, we examined sections that were far removed from one another to ensure that we were not counting the same muscle spindle multiple times.…”

“…Previous studies focusing on identifying muscle spindle presence have used traditional histochemistry staining techniques, such as trichromes (Boyd-Clark et al, 2002;Cobo et al, 2017;Takeuchi et al, 2019) and hematoxylin & eosin staining (Cobo et al, 2017;May et al, 2018May et al, , 2022Mayer et al, 2021). May et al (2018May et al ( , 2022 and Cobo et al (2017) successfully used H&E staining and trichrome staining with the human and rhesus macaque platysma muscle and zygomaticus major Note that, in species where more than one section was examined, we examined sections that were far removed from one another to ensure that we were not counting the same muscle spindle multiple times. Body size categories were organized as follows: small (<5 kg), medium (6-45 kg), and large (>45 kg).…”

“…Recent research has challenged the notion that facial muscles in mammals lack muscle spindles. May et al reported the presence of muscle spindles in the platysma in both the rhesus macaque (May et al, 2022) and humans (May et al, 2018) and these spindles were all robust in appearance. While these studies only concentrated on the platysma muscle in primates, it is reasonable to expect that spindles may be present in other facial muscles, not just in humans but across Mammalia.…”