voice & speech: journey of the voice: phonation: larynx part II

Larynx part II: Mucosa and Muscles

So far we have seen the structure of the "skeleton" of the larynx, its one bone and several cartilages. Now we must try to understand how those move with reference to one another, via the muscles. But before we do that let's look at the Muscosa, the tissue that covers the muscles of the larynx.

Intrinsic Muscles of the Larynx
This simplified image will help you visualize the placement of each of the muscles described in the table at right. Please note that some of the muscles are labeled using the older more "latinate" terms for them.

Cricothyroid Joint
The action of the cricothyroid tilts the tyroid cartilage down, increasing the front to back distance of the larynx, and thus placing the vocal folds under increased tension.

Movement of the arytenoid cartilages
These four views show how the vocal folds are approximated by the action of the intrinsic muscles on the arytenoid cartilages.

This image is a good starting point for any discussion of the muscles and mucosa because one can see the placent of the vocalis muscle (in the vocal fold) and the various layers that cover it. [Take a look at it (another window will pop up, so you won't loose your place), and then come back to continue.] Notice how narrow the opening is between the vocal folds, compared to the width of the trachea: this space is called the "glottis".

Let's now look at an image of the other spaces created by the various structures of the larynx. In the image at left, we see the vestibule, which is the space above the "vestibular fold". These "false vocal folds" are created by the mucosa passing over the vestibular ligament. They vibrate somewhat during phonation and especially during vibrato singing. The false vocal folds also work with the epiglottis to make a seal so that nothing goes down the windpipe during swallowing. Tucked below the vestibular fold is the ventricle. The space below the glottis is the infraglottic cavity.

Mucosa
The soft tissues of the larynx, particularly the vocal folds.

The "true" vocal folds - are made up of five layers:

• epithelium - the surface "skin" of the the larynx, which is continuous with the lining of the mouth, pharynx and with the trachea below the larynx.
• lamina propria - three distinct layers, each with a different consistency
  • superficial layer: a jelly-like substance, close to the surface
  • intermediate layer: an elastic, fibrous substance, like rubber bands
  • deep layer: a thread-like collagenous fiber layer
• vocalis muscle: the main body of the vocal fold, and very stiff

This image (in black and white) shows the nature of the tissue a little more clearly.

The epithelium and superficial layer of the lamina propria are often considered the cover of the fold, the intermediate and deep layers of the lamina propria are thought to be a transition while the vocalis muscle is the body of the vocal fold. The soft, gel-like composition of the cover is important for creating the "mucosal wave", barely visible in this animated gif image.

Intrinsic Muscles
These smaller muscles, within the structure of the larynx, move the vocal folds in reference to each other: they abduct, adduct and alter vocal fold shape; they also change the longitudinal tension.
[abduct = move apart, adduct = bring together. How to remember this? aDDuct: the 2 D's go together, aB-Duct: the B and D are separated]

This image (not expandable) shows the vocal folds abducted (yes, separated!), for inhalation. You can see the two bumps of the of the arytenoid cartilages near the bottom corners of the picture, the vocal folds are making a V pointing at the thyroid cartilage, and you can see the shiny epiglottis at the top of the image, like a crescent moon. Looking inside the glottis, you can make out the rings of the trachea.

In this image (not expandable), the vocal folds are adducted for phonation on the exhaled breath. You can see that the arytenoid cartilages have swung forwards and together to bring the edges of the folds into contact. The folds are photographed in mid vibration; you can see the degree to which the folds separate during each vibratory cycle (not much!). The folds appear white in colour because there is very little blood flow to the fold tissue, and the arteries are microscopic. However, if a blood vessel were to break, the results would be very dramatic, as the tissue would quickly fill with blood, turning a deep red colour.

To figure out where each of the muscles named below goes, remember that their names are made up of the two cartilages that the muscles join. For instance, the cricoarytenoid muscle moves the cricoid cartilage and the arytenoid cartilage.

Extrinsic Muscles
These larger muscles, outside the larynx, position and support the larynx; they may move the cartilages, which in turn, stretch or compress the vocal folds.

Laryngeal Elevators
These supplemental muscles raise the larynx in the neck or support it. AKA Suprahyoid muscles. Some of these muscles are visible in this image of the tongue, as they are also extrinsic muscles of the tongue.