larynx, voice box

Man is capable of producing an incredibly wide range of sounds. He can talk or sing for hours. Many people have unique voices with which we can identify them.

All these are made possible by a relatively small structure called larynx or voice box. The larynx may not appear too complicated in its structure but to simulate its function is a Herculean task.

It is situated in the midline of neck and is continuous with the windpipe (trachea) below.

It is about 44 millimeters long in adult males and 36 millimeters long in females. The frame of larynx is made of nine cartilages-three paired and three unpaired.

Thyroid cartilage: This is the biggest cartilage. This is responsible for the prominence in the midline of neck (Adam’s apple) which is more prominent in man than in women.

It consists of two roughly quadrilateral plates which converge in front. A notch can be felt in front just above the fusion of the two plates.

From the posterior borders of the plates there are vertical projections both above and below. They are called superior (upper) and inferior (lower) horns.

The epiglottis is a leaf shaped cartilage which guards the opening of the larynx. It is attached to the thyroid cartilage below the notch by a ligament. This cartilage closes the opening of the larynx when we swallow food or drink and prevents choking.

The cricoid cartilage is thick and contributes to the lower portion of the larynx. Its lower border is connected to the trachea.

The arytenoid cartilages are two in number. They are pyramidal in shape. They are situated on the upper border of the cricoid cartilage at the back of the larynx.

The corniculate cartilages are two in number. They are conical structures situated on the apex of the arytenoid cartilages.

The cuneiform cartilages are two elongated pieces placed one on either side of the larynx in the aryepiglottic fold (this is a fold of mucus membrane between the side of the epiglottis and the apex of the arytenoid cartilage.

When viewed from above the laryngeal opening appears to be triangular wide in front and narrow behind. It slopes backwards and downwards.

Its boundaries are formed by the epiglottis in front, by the apices of the arytenoid cartilages and the corniculate cartilages behind, the aryepiglottic fold on the sides.

Two folds of the laryngeal mucus membrane are seen on either side when we look at the larynx from above. The one above is called the ventricular fold or false vocal cord and the other below it is the true vocal cord. There are two false vocal cords and two true vocal cords.

The true vocal cords are concerned with the production of sound. They contain two vocal ligaments which are made of elastic tissue. Each ligament is attached in front to the thyroid cartilage and behind to the arytenoid cartilage.

Muscles of larynx: There are two groups of laryngeal muscles-extrinsic and intrinsic. The extrinsic muscles extend between the larynx and the surrounding structures and control the position of the organ. The intrinsic muscles control the movements of the vocal cords and are concerned with the production of sound.

Posterior cricoarytenoid muscles one on each side extends from the back of cricoid cartilage to the arytenoid cartilage. They move the vocal folds away from each other (abduction).

Each vocal fold contains a muscle called thyroarytenoid which extends from the thyroid cartilage to the arytenoid cartilage. These muscles shorten the vocal cord.

The interarytenoid muscle is a single muscle. It extends from one arytenoids cartilage to the other. It consists of oblique and transverse parts. This muscle approximates the arytenoid cartilages and brings the vocal cords together (adduction of vocal cords).

Lateral cricoarytenoid muscles are situated on the sides. Each muscle extends from the side of the cricoid cartilage to the arytenoid cartilage on that side. These muscles also adduct the vocal cords.

The cricothyroid muscle is triangular in shape. It extends between the antero-lateral aspect of the cricoid cartilage on each side to the lower horn and the inferior border of the thyroid cartilage. When it contracts it tilts the thyroid cartilage forwards tensing the vocal cords. This produces high pitched sounds.

These muscles by varying the length and tautness of the vocal folds help us to produce sounds of different pitch and loudness.

Pitch is a measure of frequency of vibration (higher the frequency higher the pitch) and loudness a measure of amplitude of vibration (greater the amplitude of vibration louder the sound).

Another phenomenon which operates in the vibration of the vocal folds is the Bernoulli Effect which states that as the velocity of fluid or gas increases its pressure decreases. When the vocal cords are adducted (brought together by muscular action) there is resistance to air flow.

The pressure below the vocal folds builds up and the increased pressure forces the vocal folds to separate causing the air to flow at higher velocity just as the flow of water speeds up when a finger is placed at the end of a hosepipe causing obstruction.

When the velocity of airflow increases, the pressure falls and the vocal cords are brought together and the cycle goes on.

The rate at which the vocal cords come together and separate in a unit time determines the pitch of the voice. The extent to which the vocal cords move away from each other determines the loudness.

In small babies the cartilages of the larynx are very soft and pliable. Hence they collapse due to negative pressure generated during inspiration. This produces a noise called stridor when the baby breathes in.

The condition is called laryngomalacia and is harmless. It usually resolves within a few months. If stridor is severe or continuous pathological causes have to be considered.

Infection of the larynx causes swelling of the mucus membrane and may result in stridor. Inflammation of the epiglottis (epiglottitis) is a serious and potentially fatal condition as it can cause complete airway obstruction.

SBI!

Larynx our voice box