What is special about the human tongue that allows it to perform the movements that are unique to human speech and swallowing? The biomechanics of the tongue are dependent on its anatomy, and some of the most basic facts of human tongue anatomy are unknown. It is hypothesized that the human tongue contains specialized anatomy related to the movements of human speech and swallowing. Studying this anatomy will increase our understanding of tongue movements; provide a normative baseline from which to compare pathological conditions, and provide the detail required for progress in surgical procedures on the tongue, including transplantation. The human tongue presents formidable challenges for the anatomist: the small muscle groups that interweave in complex ways are technically difficult to trace; it is often difficult to identify specific muscles in histological sections; and many techniques routinely used in animal studies cannot be used on human post mortem tissue. However, based on experience studying the human larynx, a systematic approach is proposed with a variety of techniques that have all been successfully tested in the preliminary work. Tongue anatomy will simultaneously be studied on the gross anatomical, microscopic and molecular level using the following methods: 1) high-resolution magnetic resonance microscopy of tongue tissue to study 3-D structural detail; 2) Sihler's stain, a process that renders whole tongue specimens translucent while counterstalning the nerve supply and outlines of muscle groups; 3) serial sectioning of whole tongues followed by staining to show details of muscle structure and insertion into connective tissue; 4) micro dissection of muscle fibers followed by silver and acetylcholinesterase staining to study details of muscle fiber size and shape, motor endplate types, and terminal axon branching; 5) myofibrillar ATPase, to type the muscle fibers of each muscle; 6) immunohistochemistry, to identify the myosin heavy chain (MHC) within tongue muscles; and 7) immunoelectrophoresis and immunoblotting, to confirm the immunohistochemistry. Preliminary work has supported the presence of specialized anatomy in the human tongue. Certaln muscles are significantly different in size and position when compared to other mammalian tongues. The genioglossus muscle, for example, is greatly enlarged while the inferior longitudinal is comparatively smaller. In addition, human tongue muscles have unusual internal structure: some appear to be compartmentalized into smaller groups of muscle fibers arranged in series. In the superior longitudinal muscle preliminary work suggests that these muscle compartments are surprisingly short and that the muscle fibers are interconnected in complex webs. Overall, the human tongue has the highest proportion of slow twitch muscle fibers yet reported in any mammalian tongue, and these are arranged in a gradient with the higher proportions found medially and in the tongue base. Among these slow muscle fibers are large numbers of slow tonic muscle fibers, an extremely rare type of muscle fiber with unique contractile properties. In summary, the dearth of information about the human tongue appears to offer an opportunity to increase our understanding of the special nature of speech and swallowing as well as the pathophysiology of dysphagia and dysarthria.
