Comparative study of the physiological properties of the vocalis and cricothyroid muscles

Viable tissue samples of the vocalis muscle and pars recta portion of the cricothyroid muscle were dissected from the excised larynges of middle-aged, male, mixed breed dogs. The samples were maintained in an aerated Krebs-Ringer solution and curarized. Temperature and pH were controlled. Field stimulation was performed with parallel-plate platinum electrodes. Using a dual mode ergometer, in vitro measurements were made at increased levels of strain. Least-squares regression analyses were performed on contraction times and 50% relaxation times as a function of strain. Results of this investigation indicated important differences in the active and passive properties of the pars recta portion of the cricothyroid and the vocalis muscle. The mean contraction time of the vocalis muscle was 24 ms and that of the cricothyroid muscle, 33.5 ms. The mean half relaxation time was 20 ms for the vocalis muscle and 30 ms for the cricothyroid. The twitch contraction and half relaxation times of the vocalis remained independent of strain, whereas those of the cricothyroid showed strong positive dependence on strain level. Differences in the passive properties of the two muscles were also evidenced; the vocalis muscle was found to be stiffer at all levels of strain.