The Pars Interna/Media Anatomy and Histology in the Human Larynx
The pars interna/media (PIM) is a small muscle found in the human larynx that has not been successfully described in contemporary literature on laryngeal structure. The objective of this study was to describe the PIM’s anatomy in detail. Thirteen human larynges obtained from postmortem examination were cleaned and preserved. Exposure of the PIM was through a lateral disarticulation of the cricothyroid joint and reflection of the cricothyroid muscle and the thyroid lamina. In the human, the PIM was found to be strap-like in form and to have two bellies with attachments to the medial surface of the thyroid cartilage at the root of the inferior horn and anteriosuperior cricoid arch. It appears to be innervated by a middle division, vestibular branch, of the internal superior laryngeal nerve. The average fiber diameter is 40 µm. Its type 1-to-type 2 fiber ratio places it within the range of other intrinsic laryngeal muscles. A muscle spindle was identified in medial bundle at the PIM’s thyroid attachment. Thyroid medial surface attachment is within few millimeters of the muscular process of the arytenoid cartilage. These data show that the PIM is a robust muscle and deserves attention anatomically. Its orientation within the thyroid and nonrecurrent laryngeal nerve innervations of the human PIM may place it in the vocal fold tensor group rather than the laryngeal sphincter group. It is possible the PIM reports on cricothyroid distance and right versus left cricothyroid joint stresses. Electromyographic examination of the PIM in the Rhesus larynx may help elucidate its physiology to elaborate its human physiology.