Active and passive characteristics of the canine cricothyroid muscles

Active and passive characteristics of the canine cricothyroid muscle were i nvestigated through a series of experiments, conducted in vitro and compare d with their counterparts in the thyroarytenoid muscle. Samples from separa te portions of canine cricothyroid muscle, namely, the pars recta and pars obliqua, were dissected from dog larynges excised a few minutes before deat h and kept in Krebs-Ringer solution at a temperature of 37 degrees C +/- 1 degrees C and a pH of 7.4 +/- 0.05. Active tetanic stress was obtained in i sometric and isotonic conditions by applying field stimulation to the muscl e samples through a pair of parallel-plate platinum electrodes and using a train of square pulses of 0.1-ms duration and 85-V amplitude. Force and elo ngation of the samples were obtained electronically with a dual-servo syste m (ergometer). The results indicate that the dynamic response of the canine cricothyroid muscle is almost twice as slow as that of the thyroarytenoid muscle. The average 50% tetanic contraction times for pars recta and pars o bliqua were 84 ms and 109 ms, respectively, in comparison to 50 ms for thyr oarytenoid. The examination of force-velocity response of this muscle indic ates a maximum shortening velocity of 2 to 3 times its length per second, w hich is about half of the thyroarytenoid shortening speed. The passive prop erties of the pars recta and pars obliqua portions are similar to those of thyroarytenoid muscle.