Differential activity of the pars recta and pars oblique in fundamental frequency control

The human cricothyroid muscle: three muscle bellies and their innervation patterns

We hypothesized that the phonatory and respiratory functions of the human cricothyroid (CT) muscle are subserved by separately controlled muscle bellies. In this work, 30 autopsied adult human hemilarynges were used to determine the neuromuscular organization of the CT muscle using microdissection, histology, and Sihler's stain. The results showed that the human CT was composed of three bellies: rectus, oblique, and horizontal. External superior laryngeal nerve (ESLN) was found to enter into the CT muscle as a single trunk (37.5%) or multiple (two to five) branches (62.5%). Within the CT muscle, the ESLN gave off three to seven branches to innervate the rectus belly and one or two branches to supply the oblique and horizontal bellies, respectively. Notably, ESLN also gave off branches to innervate the ipsilateral thyroarytenoid muscle (46%) and subglottic mucosa (67%) or connect with the recurrent laryngeal nerve (25%). These findings suggest that the CT bellies appear to be functionally designed for different motor tasks. The data are also useful for further clarifying the functions of the CT bellies and the ESLN branches and for developing belly-based reinnervation procedures to treat laryngeal paralysis.

Electronic analysis of intrinsic laryngeal muscles in canine sound production

This study explores the relationship between voice production and intrinsic laryngeal muscle (ILM) activities as expressed by orderly recruitment of their specific motor units. In 5 dogs, both the recurrent laryngeal nerve (RLN) and the vagus nerve (cranial nerve X) were stimulated via tripolar electrodes with stimulating frequencies (Fs) of 10 to 60 Hz and 0 to 7 mA during application of symmetric 600 Hz, 7 to 0 mA blocking currents. The fundamental frequency (Fo) and the intensity (I) of sounds generated by tracheal insufflation of humidified air were recorded while electromyograms of the cricothyroideus (CT), thyroarytenoideus (TA), and posterior cricoarytenoideus (PCA) were obtained via surface electrodes. Contractions of the CT were concurrently induced by stimulating the superior laryngeal nerve (SLN). The recruitment rates were highly specific and were affected by which nerve was stimulated. For the RLN, PCA ramping was lowest for Fs of < or =50 Hz. For Fs of 10 to 30 Hz, the recruitment rate of the TA was significantly steeper than that for the other ILMs, and the CT had the highest rate for Fs of 40 to 50 Hz. Conversely, for the vagus nerve, PCA recruitment was highest for Fs of > or =30 Hz. The average Fo was significantly higher with the RLN than with the vagus nerve. When the TA recruited faster than the CT (ie, via the RLN, but not the vagus nerve), the Fo was higher. While only CT ramping was significantly related to changes in sound intensity, there was a trend toward a decrease when PCA ramping was higher than CT ramping, as occurred when only the vagus nerve was stimulated. Stimulation of the SLN always increased Fo and loudness. We conclude that changes in Fo occur mainly through RLN-mediated CT and TA contraction. Loudness is controlled by the CT. The PCA exerts reciprocal coupling on both functions via the vagus nerve, and they are boosted across the board by the SLN. These findings may allow artificial manipulation of voice.

The role of the pars recta and pars oblique of cricothyroid muscle in speech production

To evaluate the functional difference of the pars recta and pars oblique during speech production, the electromyographic activities of these muscles were measured in thyroidectomized patients. The hooked wire electrodes were inserted into the normal side of the bellies of the pars recta and pars oblique bundles. Two kinds of sentences were used to obtain pitch changes, a simple interrogative sentence and a complex sentence with stress contrasts. The pars recta and pars oblique were simultaneously activated for initial lengthening and tensing of vocal folds to produce speech. The pars oblique might be initially more active than the pars recta at the initial task of speech and the pars recta might be more active at the pitch elevation in the interrogative sentence and the stress contrast of the complex sentence. The maximum electromyographic activity range of the pars recta and pars oblique seemed to be nearly equal. These results demonstrated that the patterns of electrical activities of the two bellies are different during speech and the combined activities of the pars recta and pars oblique are important in the adjustment of the vocal fold length during speech.