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.