The vocal cords are composed of twin infoldings of 3 distinct tissues: The outer layer is squamous, non-keratinizing epithelium. Below this is the superficial layer of the lamina propria, a gel like layer, which allows the vocal fold to vibrate and produce sound. The vocalis and thyroarytenoid muscles make up the deepest portion. These vocal folds are covered with a mucous membrane and are stretched horizontally, from back to front, across the larynx.
The vocal folds are located within the larynx at the top of the trachea. They are attached posteriorly to the arytenoid cartilages, and anteriorly to the thyroid cartilage. They are part of the glottis which includes the rima glottidis. Their outer edges are attached to muscle in the larynx while their inner edges, or margins, are free forming the opening called the rima glottidis. They are constructed from epithelium, but they have a few muscle fibres in them, namely the vocalis muscle which tightens the front part of the ligament near to the thyroid cartilage. They are flat triangular bands and are pearly white in color. Above both sides of the glottis are the two vestibular folds or false vocal folds which have a small sac between them.
Situated above the larynx, the epiglottis acts as a flap which closes off the trachea during the act of swallowing to direct food into the esophagus. If food or liquid does enter the trachea and contacts the vocal folds it causes a cough reflex to expel the matter in order to prevent pulmonary aspiration.
Males and females have different vocal fold sizes. Adult male voices are usually lower pitched due to longer and thicker folds. The male vocal folds are between 1.75 cm and 2.5 cm (approx 0.75" to 1.0") in length, while female vocal folds are between 1.25 cm and 1.75 cm (approx 0.5" to 0.75") in length. The vocal folds of children are much shorter than those of adult males and females. The difference in vocal fold length and thickness between males and females causes a difference in vocal pitch. Additionally, genetic factors cause variations between members of the same sex, with males' and females' voices being categorized into voice types.
False vocal folds
The vocal folds are sometimes called 'true vocal folds' to distinguish them from the 'false vocal folds' known as vestibular folds or ventricular folds. These are a pair of thick folds of mucous membrane that protect and sit slightly superior to the more delicate true folds. They have a minimal role in normal phonation, but are often used to produce deep sonorous tones in Tibetan chant and Tuvan throat singing, as well as in musical screaming and the death growl vocal style.
Mature human vocal folds are composed of layered structures which are quite different at the histological level. The topmost layer comprises stratified squamous epithelium which is bordered by ciliated pseudostratified epithelium. The inner lining surface of this squamous epithelium is covered by a layer of mucus (acting as a mucociliary clearance), which is composed of two layers: a mucinous layer and serous layer. Both mucus layers provide viscous and watery environment for cilia beating posteriorally and superiorly. The mucociliary clearance keeps the vocal folds essentially moist and lubricated.
The epidermis layer is secured to the deeper connective tissue by basement membrane. Due to the primarily amorphous fibrous and nonfibrous proteins in the lamina propria, the basement membrane applies strong anchoring filaments like collagen Ⅳ and Ⅶ to secure the hemidesmosome of basal cell to the lamina propria. These attachments are strong enough to sustain beating and stretch, to which VFs are subjected. The population density of some of the anchoring fibers in the basement membrane, such as collagen Ⅶ, is genetically determined, and these genetics may influence the health and pathogenesis of the vocal folds.
Larynx showing vocal ligaments
The next three layers comprise lamina of lipopolysaccharides (LPs), which are stratified by their histological composition of elastin and collagen fibers, with fibroblast, myofibroblast and macrophages interspersed sparsely. The superficial layer LPs (SLLPs), also known as Reinke's space, is composed of amorphous substance and microfibrils which allows this cover layer to “slide” over the deep layer easily. The vibratory and viscoelastic characteristics of human VFs are mainly attributed to the molecular composition of SLLPs. In normal vocal fold, the jelly-like “Reinke's space” is very loose and abundant with interstitial proteins such as hyaluronic acid, fibronectin, proteoglycan like fibromodulin, decorin and versican. All these components together regulate the water content of vocal fold and render the viscous shear property for it. The squamous epithelium and superficial lamina propria form the vocal mucosa which serves as vibratory component in phonation. The mucosa layer vibrates at a frequency range of 100–1000 Hz and displacement at 1mm approximately. The intermediate layer of LPs consists primarily of elastic fiber while the deep layer LP consists of fewer elastin and more collagen fibers. These two layers have poor differentiated boundary but are increasingly stiffer than SLLPs. The intermediate and deep layers of LPs compose the vocal ligaments which are enclosed within the vocal folds and are responsible for strain in phonation. Within the ECM community of vocal ligament, fibrous proteins such as elastin and collagen are pivotal in maintaining the proper elastic biomechanical property of vocal fold. Elastin fibers impart the flexibility and elasticity of the vocal folds and, collagen is responsible for the resistance and resiliece to tensile strength. The normal strain level of vocal ligament ranges from 0–15% during phonation These fibrous proteins exhibit distribution variations spatially and temporally due to fibroblast turnover during tissue maturation and aging. Each vocal ligament is a band of yellow elastic tissue attached in front to the angle of the thyroid cartilage, and behind to the vocal process of the arytenoid cartilage.