Stretching the Face: Mimetic Muscles Have Muscle Spindles.

Most skeletal muscles contain muscle spindles (MS) - sensory receptors which inform the CNS about changes in muscle length and rate of change in lengths. Muscle spindles are the most frequently found sense organ in skeletal muscles. While the face is the primary communication tool for mammals, it remains unclear if MS are present in the facial mimetic muscles. In order to investigate the potential presence of MS in mimetic musculature, we used hematoxylin and eosin and sirius red collagen staining to examine select mimetic muscles among a wide phylogenetic range of mammals (N=23 individuals). Bright light microscopy was used to identify and quantify MS. Contrary to previous studies, we found MS throughout mimetic muscles, including in humans. Preliminary results identified 2 MS in human (Homo sapiens) sections, while only a single muscle spindle was identified across chimpanzee (Pan troglodytes) samples - fewer than other primate and non-primates investigated. Muscle spindles have been shown to atrophy with increasing age, which may explain this discrepancy. Focusing on canids, we were able to identify 23 MS across various dog breeds. There was a significantly greater number of MS in wolf (Canis lupus) and husky breed of dog samples (an "ancient" dog breed) when compared with more derived domestic dog breeds (beagle/chihuahua) and with other mammals. An increased presence of muscle spindles identified in dog and wolf species in both the zygomatic major and orbicularis oris muscles suggests that the mobility of facial muscles in canids may be influenced by proprioceptive mechanisms that are required for finer motor movements corresponding to vocal communication.