The CT muscle is located on the exterior of the larynx, just under the skin. The thyroid cartilage pivots or rocks forward on top of the cricoid cartilage when the CT muscle contracts.
Inferior to the thyroid cartilage and superior to the cricoid cartilage is a groove. Before the Heimlich maneuver became popular, many people knew of this groove as the location for placing a hole in the neck in the event someone was choking on food at dinner time. There is not much tissue between the skin and the airway here and it is actually a relatively safe place to make a hole for breathing in an emergency.
If you place your finger in the groove and attempt a high pitch sound, you will feel this groove pinch closed as the cartilages are pulled together. The CT muscle pulls these cartilages toward each other. It is located to the side of the middle groove.
The effect of this rocking is to stretch the vocal cords. For a singer, the additional tension provided by stretching the vocal cords provides the falsetto (upper or head) register. The tension of the CT muscle also allows us to yell with less effort.
Some common complaints when this muscle is not working are:
- inability to sing as high as in the past,
- additional effort to project as loud,
- discomfort with projected speaking.
This muscle is not visible directly on endoscopy since it is attached on the external surface of the thyroid and cricoid cartilages. The effect of the muscle though is easily visualized as a lengthening of the vocal cord as the pitch is increased during phonation.
Both photos are taken during the most open phase of vibration. Left: the vocal cords vibrating at A3#. Right: At C5#, over one octave higher, the CT muscle has stretched the vocal cords longer. Their amplitude of oscillation is reduced.
She has copious thin secretions which are not interfering with the vibrations, but ar visible on the superior surface of the true cords.
The fact that we have two muscles to change pitch, one primarily low in the range (TA) and the other primarily high in the range (CT) means that there is a midrange location where the tensioning of the two muscles blend together. For singers, this is the area of the vocal break. Classical singers spend a great deal of time smoothing the transition between the use of these two muscles.