abstract

It is proposed that in comparison to other mammals, humans have added a highly specialized motor system to the upper airway muscles; that this system is capable of performing unique actions with great precision; and that this system has evolved for speech production.

Speech is perhaps the defining characteristic of the human species. At present the nature of the protolanguage that bridged the gap between humans and primates is unknown. Most investigators agree that the most important evolutionary changes occurred in the mechanism for producing speech rather then in perceiving it. It is well known that the human tongue and pharynx have evolved unique morphologies to allow for the articulation of speech. In contrast, the vocal folds, the actual site of sound production, are externally similar to those of other mammals and have been relatively ignored. Almost completely unstudied is the internal specializations within these muscular organs. Based on the study of nearly 100 human larynges and a smaller sample of other upper airway muscles we have found a type of muscle tissue called slow tonic. In humans, slow tonic muscle fibers (STMF) have only been found in the extra ocular muscles, the most precisely controlled of the skeletal muscles. Although extremely rare in mammalian muscles STMF are common in amphibians and birds, and most of what is known about their anatomy and physiology comes from studies in these species. One important aspect of STMF biomechanics is that they do not contract with a twitch like other skeletal muscle fibers, instead they shorten in a graded and precisely controlled manner. Therefore the neural control of STMF is fundamentally different from other muscles in the body. The muscle tissue containing STMF in the human upper airway has so many unusual specializations that it may be considered to be a fourth class of muscle, in addition to smooth, cardiac and skeletal muscle. The exact locations where STMF are concentrated suggest that their function is directly related to speech production. For example, in the vocal folds most STMF concentrate in an area just underneath the vibrating edge of the vocal fold. In this area the STMF appear to be arranged to control the shape and tension within the vibrating vocal fold, and by extension, the quality of the sounds produced. The same area in the vocal folds of other mammals contains only soft tissue, including those of non-human primates. Very recently we have also found STMF to be present in the human tongue and pharynx. In these regions it is concentrated just beneath the mucosa, suggesting that it participates in shaping the pharyngeal walls and dorsal surface of the tongue, critical actions seen during speech articulation.

This presentation will review comparative aspects of vocalization in rats and bats (two species that use both audible and ultrasonic vocalization), carnivores (dogs and cats), and non-human primates. Then the recent anatomical studies of the human larynx, tongue and pharynx will be presented. The significance of the unique human anatomy for the biomechanics of vocal fold vibration, sound production, and speech articulation will be discussed.

Supported by grant DC 01764 from the National Institute on Deafness and Other Communication Disorders

 Conference site: http://www.infres.enst.fr/confs/evolang/