Quantitative morphology and histochemistry of human intrinsic lingual muscles

R. DePaul, P. Stal, J.H. Abbs, and L.E. Thornell

Data on muscle architecture and fiber contraction properties are important in understanding neuromuscular control systems and equally critical to interpret electromyographic, kinematic and measured force data. Remarkably, for what most consider the most important speech muscles, those of the tongue, such data are almost wholly lacking. To date, the only such detailed information on primate tongue muscles is from Old World monkeys, which many speech researchers consider not relevant. Tongues from five healthy human subjects were studied. Fiber type and size data were obtained from three intrinsic muscles -- superior longitudinalis, verticalis and transversus-- using standard histological, histochemical and quantitative morphological techniques. Four sites (anterior-to-posterior) were analyzed in each muscle. Over 9,400 fibers were typed, counted and measured. Analyses revealed: (1) striking anterior within-muscle predominance of small type IIA fibers, and in parallel a posterior predominance of larger type I fibers, (2) smaller fibers than in limb muscles and (3) significant fiber type and size inter-individual variability. These data indicate functional within-muscle anterior-posterior segmentation and hence highly localized contraction capabilities consistent with some views of lingual biomechanics. Additionally, human tongue muscle fiber type patterns are very similar to those in Old World monkeys, providing implications for speech motor system evolution.