Regional Heterogeneity in the Electromyography Activity of Extrinsic Tongue Muscles Is Critical to Tongue Function

The tongue, as a muscular hydrostat, performs several dynamic behaviors and functions, including feeding, vocalizing, and respiration. As such, this hydrostat must be capable of performing complex movements, which are powered via a set of muscles typically defined as "extrinsic" (originating outside of the tongue) and "intrinsic" (contained wholly in the tongue). These muscles are typically classified based on their gross anatomical positions and also are often ascribed functions based on these positions, with the extrinsic muscles being assigned the role of positioning the tongue, and the intrinsic muscles thought to function to shape it. For example, genioglossus is typically described as a tongue protruder, whereas hyoglossus is often described as a tongue retractor. However, the neural control of these muscles involves relatively small motor units, and defining the function of tongue muscles based on anatomy, when they occupy overlapping space and exhibit refined control may oversimplify their function. Yet, distinguishing between gross anatomical structures and refined neural control can be challenging due to the complexity of functions the tongue performs. Here, we used an infant animal model (pigs) to evaluate how the neural control of the tongue is modulated in anatomical space given a relatively simplified behavior (suckling). We tested for variation in control along the anteroposterior and dorsoventral axis of the tongue using high speed videofluoroscopy coupled with electromyography (EMG). We found variation in EMG firing timing along both axes, which correspond to differences in behaviors. Furthermore, this variation in activity is likely reflected by regional variation in function within a muscle. These data suggest that defining muscles by their anatomical structure over-simplifies their functional roles and that studies investigating the three-dimensional structure and function of the tongue should evaluate it based on regional variation in control, in the context of the behavior of interest.