Fiber differentiation of the human laryngeal muscles using the inhibition reactivation myofibrillar ATPase technique

Summary The aim of the present study was to further subdivide the type II fibers of the human thyroarytenoid and posterior cricoarytenoid muscles by means of a modified myosin ATPase reaction. In order to understand the functioning of these highly strained muscles better, it is important to know the respective percentage of fatigue-resistant type IIA fibers and fatigable type IIB fibers. The material comprised the larynges of seven laryngectomized males aged between 45 and 70 years and four laryngectomized females aged between 39 and 72 years. After having been frozen in nitrogen, 10-m-thick sections were cut from the laryngeal muscles in a cryostat. The pH-lability of the enzyme that can be utilized in a classical myosin ATPase reaction permits a differentiation between fiber types I, IIA and IIB. Evidently, this is not possible with every human muscle. The fiber types IIA and IIB of the thyroarytenoid and the posterior cricoarytenoid muscles could be clearly distinguished by means of the inhibition reactivation myofibrillar ATPase technique. Using this method, the myosin ATPase enzyme was initially inhibited by hydroxymer curibenzoate and subsequently reactivated by cysteine. Regarding the incidence of type I and IIA fibers, there was a statistically significant difference between the thyroarytenoid and the posterior cricoarytenoid muscles. The type IIA fiber content was statistically significantly higher in the arytenoid muscle than in the posterior cricoarytenoid muscle. The percentage of type IIB fibers was low, not only in the thyroarytenoid muscle and the posterior cricoarytenoid muscle but also in the other laryngeal muscles. The share of fiber types I, IIA and IIB in the thyroarytenoid muscles varied greatly from one patient to another. This was also true for the other laryngeal muscles. This aspect may be especially significant with regard to an individual's vocal character and vocal fatigability under stress.