The Structure of the Vocal Folds
When we talk about the vocal folds we are really describing a complex, layered structure, consisting of different types of tissue. The layers of the vocal folds may be categorised differently according to the context in which they are being considered. To the reader, this is sometimes confusing. This article will help you negotiate your way through different ways of describing the tissues that make up the vocal folds.
The vocal folds are made up of five discrete layers.
- Epithelium: a very thin layer of cells that give the vocal folds their distinctive white appearance when healthy. The layer is 0.1 mm thick and is constructed from squamous epithelium. These cells bind firmly together and allow air to pass with little friction.
- Superficial layer of the Lamina propria: this is a thin layer of elastin fibres bound together in a random arrangement. The elastin fibres give this layer an elastic-like quality. It can be stretched and move into many positions but it will always maintain is structural integrity. These fibres allow the vocal fold to thin along its edge. This layer is about 0.5 mm thick.
- Intermediate layer of the Lamina propria: this layer also consists of elastin fibres, along with a few collagen fibres. It is more densely packed. Unlike the superficial layer, these fibres are lined up from front to back along the length of the vocal fold. This layer therefore can only stretch in an anterior-posterior direction. This layer is 1-2 mm thick.
- Deep layer of the Lamina propria: this layer is formed out of collagen fibres. The properties of collagen mean it cannot be stretched and therefore prohibits over-extension of the vocal fold. This layer is also 1-2 mm thick.
- Thyrovocalis muscle: also called the vocalis or the medial section if the thyroarytenoid muscle, this makes up the bulk of the vocal fold.
The epithelium and superficial layer of Lamina propria make up what is known as the mucosa, whereas the intermediate and deep layers of the Lamina propria form the vocal fold ligament. Another more functional classification groups the epithelium with superficial and intermediate layers of the Lamina propria as the cover of the vocal fold and the deep layer and the muscle as the body of the vocal fold.
In summary we could say that, the vocal folds can be considered to have either five, three or two layers, depending on the context in which they are being discussed: cellular (histochemical) structure, five layers; physical structure, three layers; and functional structure, two layers.
© 2004-2014 Matthew Reeve