Mammalian facial muscles contain muscle spindles

Sensorimotor regulation of facial expression - An untouched frontier

Facial expression is a critical form of nonverbal social communication which promotes emotional exchange and affiliation among humans. Facial expressions are generated via precise contraction of the facial muscles, guided by sensory feedback. While the neural pathways underlying facial motor control are well characterized in humans and primates, it remains unknown how tactile and proprioceptive information reaches these pathways to guide facial muscle contraction. Thus, despite the importance of facial expressions for social functioning, little is known about how they are generated as a unique sensorimotor behavior. In this review, we highlight current knowledge about sensory feedback from the face and how it is distinct from other body regions. We describe connectivity between the facial sensory and motor brain systems, and call attention to the other brain systems which influence facial expression behavior, including vision, gustation, emotion, and interoception. Finally, we petition for more research on the sensory basis of facial expressions, asserting that incomplete understanding of sensorimotor mechanisms is a barrier to addressing atypical facial expressivity in clinical populations.

Proprioceptive innervation of the human lips

The objective of this study was to analyze the proprioceptive innervation of human lips, especially of the orbicularis oris muscle, since it is classically accepted that facial muscles lack typical proprioceptors, that is, muscle spindles, but recently this has been doubted. Upper and lower human lips (n = 5) from non-embalmed frozen cadavers were immunostained for detection of S100 protein (to identify nerves and sensory nerve formations), myosin heavy chain (to label muscle fibers within muscle spindles), and the mechano-gated ion channel PIEZO2. No muscle spindles were found, but there was a high density of sensory nerve formations, which were morphologically heterogeneous, and in some cases resemble Ruffini-like and Pacinian sensory corpuscles. The axons of these sensory formations displayed immunoreactivity for PIEZO2. Human lip muscles lack typical proprioceptors but possess a dense sensory innervation which can serve the lip proprioception.

Palpation and Ultrasonography Reveal an Ignored Function of the Inferior Belly of Omohyoid: A Case Series and a Proof-of-Concept Study

BACKGROUND

Palpation, a traditional haptic ability, is used daily by practitioners of all medical and surgical specialties to assess patients. In the current study, one of the authors, in a routine clinical setting, was able to deduce the dynamic features of the putative inferior belly of omohyoid. This led to a proof-of-concept study that yielded results consistent with the clinical findings.

METHODS

The first part of the study involved a survey of 300 rheumatic disease patients in whom the greater supraclavicular fossa was explored by palpation. While the patient kept the head straight, the clinician placed his middle three fingers 2.5-3 cm dorsal to the clavicle in the window between the sternocleidomastoid and trapezius clavicular insertions, explored the supraclavicular fossa, and palpated the paired contractile inferior belly of the assumed omohyoid during flexion in the three orthogonal planes. In the second part of the study, five normal subjects were examined in a similar manner by the same clinician and had independent ultrasonography performed on the dominant side. Descriptive statistics were used, and Yates' corrected chi-squared test was applied to certain nominal variables. Additionally, a comparative anterolateral bilateral neck dissection was performed in a cadaveric specimen.

RESULTS

Both studies showed that the contractile structure was the inferior belly of omohyoid and that its contraction occurred during anterior neck flexion and was opposite to the side of neck rotation, resembling the sternocleidomastoid.

CONCLUSIONS

Palpation uncovered a previously unknown function of the inferior belly of omohyoid, suggesting that physical examination of the musculoskeletal system based on palpation may lead to hypotheses worthy of exploration.

"Untying the knot": The primitive orofacial muscle architecture in the gorilla (Gorilla gorilla) as a key to the evolution of hominin facial movement

The anatomical underpinnings of primate facial expressions are essential to exploring their evolution. Traditionally, it has been accepted that the primate face exhibits a "scala natura" morphocline, ranging from primitive to derived characteristics. At the primitive end, the face consists of undifferentiated muscular sheets, while at the derived end there is greater complexity with more muscles and insertion points. Among these, the role of the human modiolus ("knoten" in German) has been emphasized. Recent studies have challenged this view by revealing significant complexity in the faces of several non-human primates, thereby rejecting the linear notion of facial evolution. However, our knowledge of the facial architecture in gorillas, the second closest living relatives to modern humans, remains a significant gap in the literature. Here, we present new findings based on dissection and histological analysis of one gorilla craniofacial specimen, alongside 30 human hemifaces. Our results indicate that while the number and overall arrangement of facial muscles in the gorilla are comparable to those of chimpanzees and modern humans, several orofacial features distinguish the gorilla's anatomy from that of hominins. Among these are the absence of a modiolus, the continuity of muscular fibers over the region of the mouth corner, the flat (uncurving) sheet of the orbicularis oris muscle, and the insertion of direct labial tractors both anterior and posterior to it. Collectively, the anatomical characteristics observed in the gorilla suggest that the complex anatomy of the hominin face should be considered synapomorphic (shared-derived) within the Pan-Homo clade.