Speech, Motor Control
Speech is produced by means of the coordinated activity of three subsystems: (1) The respiratory system consists of the lungs, and surrounding muscular and nonmuscular tissue of the thorax and abdomen, which can actively or passively change lung volume. During speech, air expelled from the lungs under relatively constant pressure by means of controlled decreases in lung volumes is modulated by the action of the other two subsystems to produce sound. (2) The phonatory system consists of the cartilages and muscles of the larynx, including, in particular, the vocal folds or cords. This system provides the primary sound source for speech. It allows an alternation between a voiced state during which the vocal folds vibrate, producing acoustic energy, and a voiceless state. Variations in the rate at which the folds vibrate give rise to variations in the perceived pitch of the voice. (3) The articulatory system consists of a number of movable structures—the tongue, mandible, lips, and soft palate—which are brought into juxtaposition with immovable structures—upper teeth, hard palate, rear wall of the pharynx—and with each other, to form various speech sounds. During the voiced state of the larynx, this system acts to produce a set of variable resonating cavities in the airway from the larynx to the mouth (vocal tract), selectively amplifying portions of the acoustic energy of the voice source at frequencies that depend on cavity sizes and shapes. In addition the articulatory system produces a second frictional sound source by forming narrow constrictions through which the egressive air from the lungs flows in turbulent fashion (e.g., for most sounds written as “f,” “v,” “th,” “s,” “z,” and “sh” in English).
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