The Structure of the Vocal Folds
The vocal folds are complex, layered structures. The layers can be categorised and described in different ways depending on the context in which they are being considered.
Analyses of the cellular and chemical structure of the vocal folds (the histochemical structure) reveals five discrete layers.
- Epithelium: a very thin layer of cells that give healthy vocal folds their distinctive white appearance. 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. The layer can be stretched and moved into many positions but will always maintain structural integrity. These fibres allow the vocal folds to thin along their edge. This layer is about 0.5 mm thick.
- Intermediate layer of the Lamina propria: this layer consists of elastin fibres and collagen fibres. Unlike the superficial layer, the fibres are densely packed and 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 from collagen fibres. The properties of collagen mean it cannot be stretched and therefore the presence of this layer 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 of the thyroarytenoid muscle, this layer makes up the bulk of the vocal fold.
The epithelium and superficial layer of Lamina propria comprise what is known as the mucosa, whereas the intermediate and deep layers of the Lamina propria form the vocal fold ligament. These two layers along with the thyrovocalis muscle describe a three-layered classification of the vocal fold structure that relates to its physical structure.
A functional classification of the vocal fold groups the epithelium with superficial and intermediate layers of the Lamina propria as the cover of the vocal fold. The deep layer of the Lamina propria and the thyrovocalis muscle are described as the body of the vocal fold.
In summary, 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.
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