The long-range aim of this project is to improve our understanding of the role the intrinsic laryngeal muscles in phonation. More specifically, we plan to activate each laryngeal muscle in a graded fashion to predict the following: (1) the pre-phonatory posture (shape) of the vocal folds including (a) vocal fold length, (b) glottal width, (c) glottal convergence, and (d) medial surface bulging, (2) the resultant vibration pattern of the vocal folds using high-speed photography, and (3) the resultant phonation type (aphonic, falsetto-like, chest-like, fry-like, subharmonic, biphonic, aperiodic, etc.) which results from each pre-phonatory condition. In order to accomplish this objective, a full range of neuromuscular inputs will be studied systematically with respect to all five intrinsic laryngeal muscles on either side of the larynx (cricothyroid [CT], thyroarytenoid [TA], lateral cricoarytenoid [LCA], interarytenoid [IA], and posterior cricoarytenoid [PCA]), both individually and in combination. For conditions related to unilateral laryngeal paralysis and paresis of the recurrent laryngeal nerve (RLN), the same measurements will be repeated following variations of common surgical interventions for these vocal pathologies, including arytenoid adduction and medialization thyroplasty. A systematic study of this type is needed to resolve the complex interactions between both antagonistic and synergistic muscles over the intricate three-dimensional laryngeal structure, which ultimately enables the laryngeal tissues to couple with the glottal airflow, resulting in both vocal fold vibration and phonatory output. A better understanding of the phonatory impact of neuromuscular coordination of these intrinsic laryngeal muscles is important to both speech scientists and clinicians. For example, neuromuscular control of the vocal folds is known to influence vocal intensity, phonation onset pressure, fundamental frequency, and phonation type. Furthermore, hyper-stimulation of these laryngeal muscles is known to result in vocal pathologies, such as spasmodic dysphonia and muscle tension dysphonia. At the other extreme, hypo-stimulation of these muscles results in well-known vocal pathologies, such as laryngeal paralysis or paresis (both bilateral and unilateral) associated with either the recurrent laryngeal nerve (RLN) or the superior laryngeal nerve (SLN). Because no systematic study of neuromuscular input to this complete set of intrinsic laryngeal muscles has yet been conducted, our understanding of the phonatory impact of neuromuscular coordination is rudimentary and deficient. Thus, the proposed study of neuromuscular control of the larynx is extremely timely.
Laryngeal paresis (weakness) and paralysis due to neuromuscular disease of the larynx are the most common illnesses leading to abnormal voice. At present, even for the neurologically intact larynx, speech scientists and clinicians do not understand how the five muscles on each side of the larynx interact to control vocal fold shape, voice production, control of airflow during speech, and vibratory patterns of the vocal folds, and current surgical treatments for these diseases have not yielded consistent results. This proposal will investigate the effect of each laryngeal muscle on vocal fold shape, voice, airflow, and vocal fold vibration, both in normal and pathologic conditions.
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