Vocal Fold Muscle Research Articles

Fatigue Resistance of Canine Vocal Fold Muscle

The objective of the experiment was to provide data from dogs, the best nonprimate model for the contractile properties of laryngeal muscles, on the fatigue characteristics of vocal fold muscle. Isometric muscle force was recorded from a transducer attached to the section of the thyroid cartilage on which the muscle inserted. Twitch contractions were recorded during supramaximal indirect electrical stimulation by DC pulses for at least 30 minutes. At 5 pulses per second (p/s), potentiation was observed in all records, peaking at an average of 7.5 minutes after stimulation onset. At 10 p/s, the peak of the potentiated average trace was earlier. One third of these records did not show potentiation. Where moderate or large potentiation was observed, the peak twitch tension declined and reached an asymptote above the initial tension, while when potentiation was small or absent, the final force was smaller than initially. Canine vocal fold muscle was observed to be fatigue-resistant, but in phonation other factors may produce fatigue.

Tetanic Contraction in Vocal Fold Muscle

Active properties of canine vocalis muscle tissue were investigated through a series of experiments conducted in vitro. Samples of the vocalis muscle were dissected from dog larynges excised a few minutes before death and kept in Krebs solution at a temperature of 37 +/- 1 degree C and a pH of 7.4 +/- 0.05. Isometric and isotonic tetanic responses of the vocalis muscle were obtained electronically with a Dual Servo System (ergometer). Isometric tension was recorded at various levels of elongation and stimulation rate. Isotonic shortening was recorded at various levels of force, and shortening velocity was obtained by numerical analysis of recorded data. It was found that fused tetanus occurred at stimulation rates of about 90 Hz, where the isometric tetanic force saturates. Repeated stimulation of the muscle in vitro not only caused nonrecoverable fatigue in the tissue, but also decreased its passive tension. The combined active and passive isometric tension increased with elongation of the muscle. Results of isometric active responses were normalized with respect to average passive response. This normalization allowed for better comparison between tetanic contraction and twitch contraction. It was found that maximum tetanic contraction was 6.4 times greater than maximum twitch contraction obtained in a previous study. A tetanic contraction period was defined and investigated for eight samples of vocalis muscle tissue from different dogs. The tetanic contraction period showed a linear increasing trend with strain.

Contractile properties of canine thyroarytenoid muscle reinnervated from the ansa cervicalis.

In an attempt to obtain data on the contractile properties of vocal fold muscle reinnervated from the ansa cervicalis, we severed the recurrent nerve and connected its distal stump to the ansa cervicalis by an end-to-end anastomosis in a series of dogs. Each dog was allowed to heal for 5 months. Then the section of the thyroid cartilage on which the vocal fold muscle inserted was detached, connected to an isometric force transducer, and activated by indirect stimulation successively on both the operated and unoperated sides. A series of twitch contractions was recorded from each side. In two dogs no contractile response was obtained. In the three others, the twitch contraction time was increased significantly, by 23% to 60%. In two of three dogs, the operated muscle was significantly weaker than the unoperated muscle. The reinnervated thyroarytenoid muscle changed its speed in the direction of the donor sternothyroid muscle.

Measurement of Young's modulus in live tissue excised from dog larynges

Stress-strain relationships and Young's modulus were obtained from live excised canine vocal fold tissue under two conditions: immediately following stepwise elongation and after 20 min of stress relaxation. The preparations were oxygenated and maintained in a balanced electrolyte solution at a pH of 7.4 and a temperature of 37°C. Tissue viability was tested via electrical field stimulation. Previously reported studies of the viscoelastic properties of vocal fold muscle have used dead tissue, thus limiting the validity of this information for use in the development of biomechanical models of voice production. Young's modulus is compared with values previously obtained on dead vocal fold tissue [Kakita, Vocal Fold Physiology, edited by Stevens and Hirano (1981)].

Mechanical Properties of the Vocal Fold Muscle

Mechanical Properties of the Vocal Fold Muscle

Physiological mechanisms of phonation: tension of the vocal fold muscle.

A method was devised to determine, in situ, in the dog, the mechanism by which tension is developed in the thyroarytenoid muscle (vocal fold). The origin of the muscle was dissected from the thyroid cartilage and tied to a force transducer; the arytenoid cartilage was fixed. The recurrent laryngeal nerve was electrically stimulated with square wave pulses. Development of isometric tension was measured as a function of in situ resting length (Lo) and of the strength and frequency of the stimulus. In addition, a passive tension length curve was constructed from Lo to 1.3 Lo. Tension increased sigmoidally with changes in initial muscle length, stimulus voltage, and frequency, reaching a maximum at 1.3 Lo and 120 pulses/sec. Vocal fold tension, a factor in determining fundamental frequency of phonation, was found to be a complex function of the above variables. Clinical implications of findings were discussed.