The LCA muscle brings the vocal cords from an open breathing position together to a closed position in preparation for making sound. This muscle is located parallel and just lateral to the TA muscle within the vocal cord. It is not particularly visible on an endoscopic exam although the effect of the contraction of the muscle is easily visualized. The LCA muscle is attached to the outer end of the arytenoid cartilage, which acts like a lever. The other end of the arytenoid lever is the vocal process, a white cartilage visible towards the posterior of the membranous vocal cord. Contraction of the LCA muscle rotates the vocal process to the midline and if necessary beyond the midline.
When relaxed, with the other muscles functioning normally, the vocal process is at the apex of an approximately 160° angled, gently curving corner, formed by the membranous vocal cord and the soft tissue posterior to it (see photo in PCA section).
During normal contraction this corner disappears to form a straight line although most of the time the vocal processes are covered up by the arytenoid itself with the vocal cords closed.
The LCA muscles have brought the vocal cords completely together at the moment of phonatory onset at D3, this man's comfortable speaking pitch. Arrows point to the tips of the vocal processes.
The normal configuration of the vocal process during LCA contraction is best understood by viewing the configuration when the LCA muscle on one side is injured and that vocal process cannot move to the midline. Compensation may occur. The opposite healthy LCA muscle has or can develop enough strength to move the vocal process across the midline and many times can reach or approach the vocal process on the weaker side. This may result in an inversion of the angle such that the vocal process now protrudes into the airway.
Ultra closeup view of the vocal processes at the posterior vocal cords taken while this patient, with a left LCA muscle paralysis, is trying to make a sound. The endoscope was positioned between the arytenoids.Because of a LCA weakness on the left, the right vocal process is compensating by pushing (short arrow) beyond the midline (oblique line), while the left vocal process remains canted laterally, forming an obtuse angle (left arrows). If this patient’s vocal cords were healthy, they would be lined up parallel during phonation, along either side of the central axiss as in the previous photos.