Slow Muscle Fibers in Adult Tongue Facilitate Human Speech

Tongue movements during human speech are distinctly different from those of other mammals. The tongue changes its shape with any movement such as licking. However, these movements aim to exert external force, and the shape changes are secondary. In speech, the tongue's goal is to form the shape, and usually there is little external force. Specifically, the dorsal surface of the tongue changes into different shapes for the articulation of every vowel and consonant. The way the tongue shape changes between positions is important as every change in shape is directly reflected in the sounds of speech. Therefore, even the tongue's intermediate shapes during speech articulation must be temporally and spatially precise, as these transitions are an important part of speech. The ability to form a large variety of shapes in rapid succession seems to be a skill that is uniquely human.

Despite the importance of tongue's ability to form shapes, little is known about how tongue muscles perform this action or whether the human tongue muscles are in some way specialized for this function. It has long been known that the human tongue has an unique curved shape with differences in the size and position of some muscles, although the significance of these differences is unknown. The complex interweaving of human tongue muscles discourages a reliable isolation and identification of intrinsic tongue muscles for physiological or histological studies. As a result, there has never been a valid demonstration of the activity of any human intrinsic tongue muscles by electromyographic recording or by electrical stimulation.

Another reason why studying tongue function is difficult is that the human tongue, as well as the tongues of all vertebrates, is a muscular hydrostat (MH), or a device for regulating water level (such as an octopus' tentacles or an elephant's trunk). MH biomechanics are poorly understood in contrast to the mechanical lever system of the bony skeleton. As the volume of a MH is kept constant by surrounding connective tissue, contraction in one axis causes expansion in other axes. Therefore, if a muscle contracts to shorten a MH, it becomes wider. Similarly, contraction of muscles oriented in the cross-sectional plane of the MH will simultaneously narrow and lengthen it.

Surprisingly, the basic anatomy of all MHs is quite similar, including the internal muscular anatomy of the tongue. There are longitudinal muscles arranged lengthwise all around the perimeter of the tongue; and there are muscles in the center of the tongue called the vertical and transverse that are oriented in the tongue's cross-sectional plane. Contraction of the longitudinal muscles shortens and widens the tongue while contraction of the vertical and transverse narrows and lengthens it. In addition to shortening and lengthening, the tongue can bend in various directions depending on which muscles are active.

Tongue muscles appear to be compartmentalized; composed of smaller components that can be independently controlled. Therefore, the tongue is capable of activating localized areas along its length independently, and this ability seems to be important in sculpting the tongue shapes seen during speech. Although little is known about the specific contractile abilities of any MH, studies in invertebrates suggest that the functional purpose of slow and fast muscle fibers (MFs) in MH muscles is similar to that of mammalian skeletal muscle. Specifically, MFs can be categorized into either slow (type I) or fast (type II) contracting types by their reaction to the myofibrillar adenosine triphosphatase stain (ATPase). These two types of MFs often have different motor unit sizes, recruitment patterns, and are more active in different types of motor activities. Fast MFs are active in movements requiring great amounts of force. In contrast, slow MFs are generally involved in activities requiring precise control of low forces.

To date, the tongue muscles of some mammals have been studied with ATPase. The intrinsic tongue muscles of the rat and cat have no slow MFs, while the intrinsic tongue muscles of the macaque monkey have been reported to contain an average of almost 25 percent slow MFs. This suggests that the presence of slow MFs in tongue muscles is associated with more precise control of their tongues. Humans have a large amount of volitional control over their tongue muscles and it would be of interest to know the amount and distribution of slow MFs in different human tongue muscles.

To date, intrinsic human tongue muscles have not been studied with ATPase staining. In this study the tongues of neurologically normal adult humans were examined by ATPase staining. In addition, other specimens were studied that allow a comparative, developmental, and pathological comparison to the normals. This study assumed amount and distribution of slow MFs reflects the activity of the tongue. Differences in slow MF distribution would provide insight on tongue structure-function relationships. The author of "The Human Tongue Slows Down to Speak," is Ira Sanders MD, Associate Professor of Otolaryngology at the Mount Sinai School of Medicine, New York, NY. His findings will be presented at the meeting of the American Bronchoesophagological Association http://www.abea.net/ meeting May 2-6, 2003, at the Gaylord Opryland Hotel, Nashville, TN.

Methodology: This study used mATPase histochemical staining to study the percentage of slow twitch muscle fibers (slow MFs) within the tongue muscles of four neurologically normal human adults (deceased) and specimens from a newborn human (neonate), a human two-year-old (infant), an adult with idiopathic Parkinson's disease (IPD), and a macaque monkey.

Results: The data revealed suggested that the ability to precisely control small amounts of force is highly developed in adult human tongue muscles. This is reflected by much greater percentages of slow MFs in human tongue muscles in comparison to other mammals. Additional support is provided by prior studies that show relatively high numbers of MS in human tongues.

The most notable finding in this study was that 56 percent of human tongue muscle fibers are slow MFs, which is more then double that reported for the macaque monkey. There are significant differences in the proportion of slow MFs in different muscles: three of the intrinsic muscles had the highest: the inferior longiture/IL (63 percent), superior longitude/SL (58 percent) and transverse (58 percent) and the fourth intrinsic muscle, the vertical (41 percent). The extrinsic muscles SG (61 percent), hyoglossus (55 percent) and genioglossus (59 percent) were not significantly different from the intrinsic muscles. The tongue specimen from the human neonatal tongue had significantly fewer slow MFs and their distribution differed from the adult normal tongue and was similar to that reported for the monkey. Specifically, in the adult human the vertical muscle has significantly less slow MFs then the other extrinsic muscles. In the neonate and the monkey this relationship is reversed, the vertical muscle has either the same or more slow MFs then the other intrinsic muscles.

The histology of the IPD specimen appeared to be more similar to that of a limb muscle then the normal adult tongues. Specifically, MFs of both types were compactly arranged with little connective tissue between MFs or fascicles. In addition, the MFs were of relatively uniform size and formed a random mosaic pattern rather then the normal specimen's variable sized MFs that fiber type grouping. The significance of these findings is unclear but the overall impression is that the PD specimen had lost much of the specialized features that made normal adult tongues distinctive. As IPD is a central nervous system disorder it appears that much of the specialization of the normal adult tongue depends on the activity patterns and/or trophic factors that originate centrally.

Conclusions: In summary, the data from this study have shown that the adult human tongue has distinctive specializations. Firm conclusions cannot be drawn from a single study, and much more work needs to be done. It appears the adult human tongue has been specialized for speech. These specializations are at many levels; gross anatomy, muscle fascicular structure, muscle fiber size and connections, and muscle fiber contractile characteristics. The common theme for these specializations appears to be the function of causing localized deformations in the tongue, the shape changes that are known as speech articulation.