Pitch Control Mechanism of Two Bellies of Cricothyroid Muscle

Hong, Ki Hwan; Kim, In; Moon, Seung; Kim, Hyun Ki

Abstract

BACKGROUND AND OBJECTIVES: The CT muscle is composed of three distinct bellies, pars recta, pars oblique and pars horizontalis. The function of each subunit is not clearly understood although it is believed that they act differently since their fibers run in different directions. This study is to clarify the pitch control mechanism of two bellies during production of speech using thyroidectomized patients. MATERIALS AND METHODS: Subjects are eight patients undergoing thyroidectomy. During surgery, pairs of 38 gauge hooked wire electrodes were inserted into the bellies of the pars recta and pars oblique. Several days after the surgery, the electrical activities of these muscles were evaluated by running speaking tasks. The EMG signals were rectified, integrated and smoothened using a Viking II EMG system. The EMG activities were analysed according tt the patterns of intonation during the running speech. RESULTS: The maximum EMG activity range of the pars recta and pars oblique are nearly equal. During the running speech, the pars oblique was activated initially and more predominantly than pars recta in the initial task of sentence, and graduxlly activated to the maximum during the Fo increase. However, pars recta was more predominantly activated to the maximum during the Fo increase than pars oblique in the interrogative sentence. This results demonstrated that the patterns of electrical activities of two bellies during speech are different From each other. CONCLUSION: The pars recta and pars oblique may have an individual anatomical function. The combined activity of the pars recta and pars oblique is important in adjustment of the vocal fold length during speech.