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The Role of Strap Muscles in Phonation Laryngeal Model in Vivo

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Journal of Voice Vol. 11, No. 1, pp. 23-32 © 1997 Lippincott-Raven Publishers, Philadelphia The Role of Strap Muscles in Phonation In Vivo Canine Laryngeal Model Ki Hwan Hong, *Ming Ye, *Young Mo Kim, *Kevin F. Kevorkian, and *Gerald S. Berke Department of Otolaryngology, Chonbuk National University, Medical School, Chonbuk, Korea; and *Division of Head and Neck Surgery, UCLA School of Medicine, Los Angeles, California, U.S.A. Summary: In spite of the presumed importance of the strap muscles on laryn- geal valving and speech production, there is little research concerning the physiological role and the functional differences among the strap muscles. Generally, the strap muscles have been shown to cause a decrease in the fundamental frequency (Fo) of phonation during contraction. In this study, an in vivo canine laryngeal model was used to show the effects of strap muscles on the laryngeal function by measuring the F o, subglottic pressure, vocal in- tensity, vocal fold length, cricothyroid distance, and vertical laryngeal move- ment. Results demonstrated that the contraction of sternohyoid and sternothy- roid muscles corresponded to a rise in subglottic pressure, shortened cricothy- roid distance, lengthened vocal fold, and raised F o and vocal intensity. The thyrohyoid muscle corresponded to lowered subglottic pressure, widened cricothyroid distance, shortened vocal fold, and lowered F 0 and vocal inten- sity. We postulate that the mechanism of altering F o and other variables after stimulation of the strap muscles is due to the effects of laryngotracheal pulling, upward or downward, and laryngotracheal forward bending, by the external forces during strap muscle contraction. Key Words: Strap muscles--Fo- Intensity--Subglottic pressure--Cricothyroid distancewVocal fold length. It is generally agreed that the extrinsic laryngeal The literature has described several human studies muscles are important for laryngeal function in relating the functional role of the strap muscles to deglutition to lift and tilt the larynx as part of bio- the larynx, attributing changes in the length of the logical valving and phonation (1,2), but their pho- vocal folds to external forces such as contraction of natory function is less well defined. A large number the strap muscles (3-7). However, there still re- of muscles participate indirectly or directly in the mains some controversy on the function of these functioning of the larynx, including infrahyoid muscle acts. (strap), suprahyoid, pharyngeal constrictor, and ex- Sonninen (8) stimulated the sternothyroid muscle trinsic tongue muscles. These can be divided mainly of a patient under local anesthesia with the neck into two groups: the strap muscles and the supra- extended and found lowering of the pitch. Faaborg- hyoid muscles (1). Anderson and Sonninen (9) studied laryngeal posi- The location of the origin and insertion of the tion and the electrical activity in the extrinsic laryn- strap muscles are unique for each muscle, implying geal muscles during phonation at different pitches. functional differences among the strap muscles. They reported that the electrical activity of ster- nothyroid muscle showed pronounced activity at Accepted May 24, 1996. low pitch and decreased activity at high pitch. In Address correspondence and reprint requests to Dr. Ki Hwan Hong, Department of Otolaryngology, Chonbuk National Uni- the thyrohyoid and mylohyoid muscles, electrical versity, Medical School, Chonju, Chonbuk, 560-180, KOREA. activity increased at high pitch. Hirano et al. (5) 23 24 K. H. HONG ET AL. reported that greater electrical activity of the ster- glottic pressure (Psub), and vocal intensity were nohyoid muscle at lower pitches appears to be re- calculated from waveform signals. Vertical laryn- lated chiefly to the lower position of the larynx, but geal movement, cricothyroid distance, and vocal also the muscular activity increased at higher fold length were measured using monitored video pitches. Simada and Hirose (10) also reported that a images (19,20). The study allowed us to evaluate the consistent increase in the activity of the sternothy- role of each individual strap muscle on stimulation roid muscle was observed in association with pitch and then postulate the mechanism involved in alter- lowering with decreased activity during pitch rise. ing pitch. However, the activity of the sternohyoid muscle showed no consistent correlation with pitch lower- ing. Ohala and Hirose (l l) reported that the sterno- MATERIALS AND METHODS hyoid muscle is active in both lowering the pitch In vivo canine model and in achieving extremely high pitch, but in speech The in vivo canine model of this experiment (Fig. the involvement in pitch lowering is most notice- I) was similar to that used in previous reports able. Erickson et al. (12,13) reported that the strap (17,18). Four healthy mongrel dogs (-25-30 kg) muscles have a negative relationship to frequency were premedicated with acepromazine maleate in- and a strong negative correlation with the activity of tramuscularly. Intravenous pentobarbital sodium the geniohyoid and cricothyroid muscles. Simada (Nembutal) was administered to a level of corneal and Hirose (I0) also reported that the sternohyoid anesthesia. Additional pentobarbital sodium was muscle helps at least to maintain a low pitch at a low used to maintain this level of anesthesia throughout level during an utterence. the procedure. Each dog was placed supine on the There are a few reports that the strap muscles operating table. Orotracheal intubation was per- have a positive relation to frequency with stimula- formed. tion of strap muscles resulting in pitch elevation. A midline incision was made to expose the strap Sonninen (14) reported that the function of the ex- muscles from the mandible to the sternal notch. The trinsic laryngeal muscles, the so-called "external sternohyoid, sternothyroid, and thyrohyoid mus- frame function," are considered to lengthen or cles were dissected carefully. The recurrent laryn- shorten the vocal folds and regulate pitch by chang- geal nerve (RLN) was isolated about 5 cm inferior ing the relation of the thyroid to the cricoid carti- to the larynx, and bilateral ansa cervicalis nerve lage. He argued that simultaneous contractions of branches to the sternohyoid, sternothyroid, and sternothyroid muscles resulted in a forward force thyrohyoid muscles were dissected leaving l-cm on the thyroid cartilage tending to increase the ten- segments. The most active nerve branch to each sion of the vocal folds. He postulated that section- strap muscle was identified via stimulation. A low ing the various external laryngeal muscles would tracheotomy was performed at the level of the su- result in lowering the voice, loss of range, and fail- prasternai notch and an endotracheal tube was ure of the glottis to close completely. Murakami and placed for ventilation. A second tracheotomy was Kirchner (15,16) suggested that the external laryn- performed in a more superior location and a cuffed geal muscles appear to make two contributions to endotrachei tube was passed superiorly with the tip the tensor mechanism of the larynx, shortening of positioned about l0 cm below the glottis. For direct the cricothyroid distance and contraction of the thy- visualization of the larynx through the oral cavity, a roarytenoid muscle. Niimi et al. (6) supposed that button was used to suspend the epiglottis. the sternothyroid muscle could serve as a pitch raiser. They speculated that the thyroid cartilage Nerve stimulation rotates downward around the cricothyroid joint, or Harvard subminiature electrodes (South Natick, the frontal part of the cricoid ring comes closer to MA) were applied to the isolated RLNs. A constant the thyroid cartilage and the arytenoid cartilage, current nerve stimulation (WR Medical electronics which results in higher tension of the vocal folds. RLN Stimulator, Model S2LH, St. Paul, MN) was The present study used an in vivo canine laryn- used to provide constant amounts of current to the geal model (17,18) to evaluate the effects of strap RLNs equal bilaterally. The frequency of stimula- muscle stimulation on the vocal folds. Stimulation tion was 80 Hz with a pulse duration of 1.5 milli- to the sternohyoid, sternothyroid, and thyrohyoid second for both nerve stimulators. A Grass model muscles was performed. The frequency (Fo), sub- 54H stimulator (Quincy, MA) was used to provide Journal of Voice, Vol. 11, No. 1, 1997 THE STRAP MUSCLES 25 video recorder TV monitor video camera macro lens ---. (~Ysound \ endoscope ~ ~/~¢ level meter ~ oscilloscope printer FIG. 1. Schematic presentation of in vivo laryngeal model. RLN = recurrent laryngeal nerve; SH ........~¢ n erve = sternohyoid muscle; ST -- sternothyroid muscle; TH = thy- / image rohyoid muscle. processing • ° .. pressure transducer A A computer digitization RLN stimulation varying amounts of current to the isolated nerves of OR) before recording. Then the signals were re- the strap muscles. Voltage varied from 0 to 3 V for corded on a personal computer with a Labmaster strap muscle stimulation, and was classified as low analog-to-digital microprocessor. The acoustic or high levels according to the contraction of strap waveforms were recorded with subglottic pressure muscles. Low-level stimulation was the level at simultaneously and these signals were low-pass fil- which the strap muscles contracted mildly on direct tered at 3,000 Hz and digitized at a rate of 20 kHz. visual observation. Maximal stimulation was
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