The IA muscle holds the vocal cords in a closed position after the LCA muscles bring them together. The IA muscle from one side attaches with the IA muscle from the other side. Because the IA muscles connect in the midline, this is one muscle where there seems to be cross innervation or nerve input from the opposite side. So even when there is complete paralysis of one side of the larynx, the IA muscle on the injured side may still function because of this cross innervation. In this situation, on endoscopy we may see a twitch of the arytenoid of an otherwise apparently completely paralyzed vocal cord due to innervation from the opposite IA muscle.
The interarytenoid muscle (yellow bar) is located between the arytenoid cartilages in the midline at the back of the glottis.