Activity of tongue muscles during respiration: it takes a village?

Almost twenty years have passed since the muscular hydro-stat theory of tongue function was set forth (7). Yet few studies have explored one of its central predictions: that " extrinsic " tongue muscles (muscles with origin outside the tongue body) and " intrinsic " tongue muscles (muscles with both origin and insertion in the tongue body) must be coactive in most tongue movements. An exclusive focus on extrinsic tongue muscles is especially notable in studies of tongue movement during respiration for which a single tongue " protrusor " muscle, the genioglossus, commonly serves as the indicator of respiratory drive to the entire tongue. That the situation is substantially more complex is evident from the accompanying study of Bailey and Fregosi (2). The authors demonstrate, in an in vivo rat model, that at least one intrinsic tongue muscle, the superior longitudinal, can be coactive with the extrinsic tongue muscle hyoglossus during respiration. This study thus serves to caution against approaches that reduce tongue movement to functionally discrete extrinsic and intrinsic muscle components, as well as approaches that investigate the neuromuscular control of the tongue in respiration by isolated study of the genioglossus muscle. The mammal tongue is undeniably complex. Seven or eight tongue muscles, each with extensive terminations in the tongue body, are variously described. In the rat, over 3,000 motor units comprise the muscles of one side of the tongue alone. Tongue muscles are typically categorized according to morphology (i.e., extrinsic versus intrinsic) or to presumed function (i.e., protrusor versus retrusor), and these categorizations form the basis for virtually all studies of the tongue motor system, from cellular to systems levels of investigation. Yet morphologic and kinematic evidence suggests that neither categorization scheme is functionally based or even heuristic. Extrinsic and intrinsic muscles share overlapping courses in the tongue body (1). Thus, with the exception of the extralingual portion of extrinsic muscles, the mechanical effects of many extrinsic and intrinsic muscle fibers are likely similar, not disparate. The same may be said for some protrusor and retrusor muscles (3). Additionally, movements of the mammal tongue are not restricted to the protrusion-retrusion axis but involve complex changes of tongue shape in three dimensions. These changes include the simultaneous lengthening and shortening of different tongue regions (6), a behavior clearly incompatible with a simple protrusion-retrusion model of tongue function. A conceptual basis for approaching the morphological and kinematic complexity of the mammal …