Functional Morphology of Mimetic Musculature in Primates: How Social Variables and Body Size Stack up to Phylogeny

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Muscle spindles are sensory receptors in skeletal muscle that provide information on muscle length and velocity of contraction. Previous studies noted that facial muscles lack muscle spindles, but recent reports indicate that the human platysma muscle and “buccal” muscles contain spindles. Mammalian facial muscles are active in social communication, vibrissa movement, and vocalizations, including human speech. Given these functions, we hypothesized that facial muscles contain muscle spindles, and we predicted that humans would have the greatest number, given the role our lips play in speech. We examined previously sectioned and stained (with H&E and trichrome stains) orbicularis oris (upper fibers) and zygomaticus (major) muscles across a broad phylogenetic range of mammalian species, spanning a wide distribution of body size and ecological niche, to assess the presence of muscle spindles. We also stained several sections with Sirius red to highlight the muscle spindle capsule. Our results indicate that mammalian facial muscles contain muscle spindles, supporting our hypothesis. Contrary to our prediction, though, humans (and other primates) had the lowest number of muscle spindles. We instead found that the carnivoran sample and the horse sample had the greatest number of spindles. Larger body size and nocturnality were also associated with a greater number of spindles. These results must be viewed with caution, though, as our sample size was small and there are critical mammalian taxa missing. Future work should use an expanded phylogenetic range of mammalian species to ascertain the role that phylogeny plays in muscle spindle presence and count.

Section: Samplementioning

confidence: 99%

Section: Samplementioning

confidence: 99%

Mammalian facial muscles contain muscle spindles

Omstead

Williams

Weinberg

et al. 2023

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“…The notable exception to this trend is the first paper, by Anne Burrows (Burrows, 2018), which focuses on mimetic musculature. For many years, Burrows has led explorations of the behavioral correlates of the muscles of facial expression in primates.…”

mentioning

confidence: 99%

Behavioral Correlates of Cranial Muscle Functional Morphology

Hartstone‐Rose

Santana

2018

This issue of the Anatomical Record is the first of a two-volume set that focuses on new investigations into behavioral correlates of muscle functional morphology. Much of the research on functional morphology and adaptation to specific functional niches focuses on the shapes of hard-tissues-bones and teeth. Investigations into soft-tissue anatomy tend to be predominantly descriptive with only brief allusion to ontogenetic or evolutionary origins of structures. When muscles are included in analyses of functional systems, their function tends to be oversimplifiedusually considered a simple force vector connecting two osteological points, with the force treated as a constant derived from some simple calculation of muscle size. The goal of these special issues is to present a series of studies that take a more elaborate look at how muscles can be viewed from a functional perspective in studies searching for morphological correlates of behavior. This first volume focuses on the behavioral correlates of cranial muscles-starting with a paper about the mimetic musculature of primates and ending with a series of papers on the masticatory muscles of many lineages of vertebrates. The next issue of the Anatomical Record (March 2018) includes our papers on the behavioral correlates of postcranial muscles. Taken together, we hope you agree that this series presents valuable insights into these form/function relationships using both traditional approaches1 and cutting-edge techniques. Anat Rec, 301:197-201, 2018. V C 2018 Wiley Periodicals, Inc.Key words: masticatory musculature; masseter; temporalis; mimetic musculature; muscle fiber architecture; fossil reconstruction; bite forceThis special issue of the Anatomical Record is the first of a two-volume set focused on new investigations into the behavioral correlates of muscle functional morphology. Most of the anatomical research published to date has primarily focused on describing specific muscles or muscle complexes. Often these papers are briefly contextualized within an evolutionary framework (particularly in comparative studies) or with reference to the ontogenetic development of the muscles. Additionally, most of these descriptive papers make reference to the function of the muscle. These past contributions highlight that

Muscle Functional Morphology in Paleobiology: The Past, Present, and Future of “Paleomyology”

Perry

Prufrock

2018

Our knowledge of muscle anatomy and physiology in vertebrates has increased dramatically over the last two-hundred years. Today, much is understood about how muscles contract and about the functional meaning of muscular variation at multiple scales. Progress in muscle anatomy has profited from the availability of broad comparative samples, advances in microscopy have permitted comparisons at increasingly finer scales, and progress in muscle physiology has profited from many carefully designed and executed experiments. Several avenues of future work are promising. In particular, muscle ontogeny (growth and development) is poorly understood for many vertebrate groups. We consider which types of advances in muscle functional morphology are of use to paleobiologists. These are only a modest subset for muscle anatomy and a very small subset for muscle physiology. The relationship between muscle and bone - spatially and mechanically-is critical to any future advances in "paleomyology". Anat Rec, 301:538-555, 2018. © 2018 Wiley Periodicals, Inc.