The vocal cords, or vocal folds, are dual folding membranes. These folds are stretched horizontally over the larynx in order to create the vibrations required to make sound. These folds are controlled by the vagus nerve and are located at the top of the trachea. Comprising these complex folds are many different structures. The classification of dual folds, however, is a purely functional one. The classification of the folds depends on the level on which they are examined. There can be different classifications: five layers, cellular (histological) structure; three layers, physical structure; or two layers, functional structure.
Looking at the physical structure, the epithelium and superficial layer of the Lamina propria make up the mucosa, considered the cover, while the intermediate and deep layers form the vocal fold ligament. Combined with the thyrovocalis, the intermediate and deep layers form the second layer. From a purely functional standpoint, however, there are only two layers. The epithelium, superficial, and intermediate layers make up the mucosa, or cover of the vocal cords, while the deep layer of the Lamina propria and thyrovocalis make up the body of the vocal fold.
There are five distinct, cellular layers of the vocal folds: the epithelium, the superficial, intermediate, and deep layers of the Lamina propria, and the thyrovocalis muscle. With the breakdown of the cellular structure of the folds, one must look at each level individually.
First, is the epithelium, a very thin layer of cells that make vocal folds appear white when they are healthy. This layer is about .01 millimeters thick and its cells link together tightly to allow air to pass through smoothly.
Next, is the superficial layer of the Lamina propria, a thin layer of elastin fibers. It is joined together in seemingly random structures. This layer is extremely elastic and can stretch and move into many different positions without structural compromise. It is around .05 mm thick.
After this dexterous layer of the Lamina propria, is the intermediate layer. It is made up of both elastin and collagen fibers. It is 1-2 mm thick and can only stretch in an anterior-posterior fashion. The deep layer of the Lamina propria, unlike the intermediate, is formed only of collagen fibers. Like the intermediate layer, it is approximately 1-2 mm thick but, unlike the previous layers, it is rigid.
The nature of collagen fibers, being the main ingredient comprising the connective tissue in mammals, make it sturdy and this sturdiness keeps the vocal folds from over-extending, resulting in damaged cords. Finally, the thyrovocalis muscle makes up the bulk of the vocal fold. It is also called the vocalis or the medial section of the thyroarytenoid muscle.