Orbicularis oris muscle

Structure

Orbicularis oris is a composite muscle that consists of two parts; a larger peripheral part, and a smaller marginal part. The border between these two portions corresponds to the margins of the lips that separates them from the surrounding skin. Both parts span between the left and right modiolus, which is a dense fibromuscular nodule at the angle of the mouth onto which most of the buccolabial muscles attach.

Origin and insertion

The peripheral portion, also called the extrinsic portion, originates from the modiolus, by arising from the fibers of the five of the many buccolabial muscles that attach to it. Namely, the modiolar muscles that contribute to orbicularis oris are the levator anguli oris, depressor anguli oris, buccinator, risorius and zygomaticus major muscles. However, most of the fibers that comprise the orbicularis oris originate from the buccinator muscle. 

Upon reaching the modiolus, the uppermost and lowermost fibers of the peripheral portion pass into the superior and inferior labial areas, respectively. The middle fibers decussate, so that the upper half passes into the inferior labial area, and lower into the superior labial area. The fibers in the superior labial area course medially to reach the midline and insert on the the dermis of the superior lip. Some fibers pass the midline and decussate with their contralateral counterparts, forming the philtrum of upper lip. Along the way, some of the most superior fibers insert to the nasal ala, nasal septum and dermis of nasolabial sulcus. The fibers in the inferior labial area course towards the midline and insert on the dermis of the lower lip and inferior labial area. Some fibers also pass the midline and decussate with their contralateral counterparts, forming the philtrum of lower lip. Some of the most inferior fibers also insert to the dermis of the mentolabial sulcus.

The marginal portion, also called the intrinsic portion, consists of a single or occasionally double band of muscle fibers. They stem from the modiolus toward the midline passing through the connective tissue of the lips. The fibers curl upon themselves, forming the vermilion border of the lips at the junction between the skin and mucous membrane of the lips. Upon passing the medial plane, the fibers interlace with the contralateral fibers, similarly to the peripheral portion of the muscle.

Relations

Orbicularis oris lies medial to and is continuous with the levator anguli oris, depressor anguli oris, buccinator, risorius and zygomaticus major muscles, i.e. the buccolabial muscles that provide with fibers for its peripheral portion. Most fibers are derived from the buccinator, which is found directly lateral to the orbicularis oris on both sides. Superficial to the buccinator is the risorius muscle which extends horizontally and blends with the lateral edge of the orbicularis oris.

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The superior peripheral portion of the orbicularis oris blends with the levator labii superioris, zygomaticus major, zygomaticus minor and levator anguli oris. The inferior peripheral portion blends in a similar manner with the depressor labii inferioris, depressor anguli oris, mentalis, and platysma. Found deep to the orbicularis oris are inferior and superior labial arteries, while superficially it is overlined by the skin of the periorbital area.

Innervation

Like other facial muscles, the orbicularis oris is innervated by the buccal and mandibular branches of facial nerve (CN VII).

Blood supply

The arterial blood supply of the orbicularis oris is mostly derived from the superior and inferior labial branches of facial artery, mental and infraorbital branches of maxillary artery and the transverse facial branch of superficial temporal artery.

Function

Bilateral uniform contraction of the orbicularis oris will bring the lips together and close the mouth. This action is important for mastication, as it works in cooperation with the buccinator muscle and the tongue to prevent the food from escaping the teeth and accumulate in the oral vestibule during eating. Isolated contraction of respective parts of the muscle causes the movements such as lip pouting, twisting, pursing and others. These movements greatly contribute to facial expression and speech.

Marginal portions of orbicularis oris muscle are also referred to as the labial cords, as by changing their length and tension similarly to the vocal cords, they produce some consonantal (labial) sounds. These consonants include /p/ of peas, the /b/ of basket that are produced by bringing the lips together; /f/ of feet or /v/ of valley are produced by pressing the lower lip against the upper teeth and others. In addition, the labial cords can act as vibrating strings and enable whistling or playing a wind instrument.

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