The Orbicularis Oris Muscle is a complex muscle structure that surrounds the lips and is involved in mouth movements. It is not a single sphincter muscle, but rather consists of four interconnected, independent quadrants that allow the lips to close and contract. This muscle plays a particularly important role in playing wind instruments.

Anatomy

Morphology and Connection Points

The Orbicularis Oris Muscle, a multilayered muscle surrounding the mouth and responsible for lip movements, is the primary facial muscle responsible for facial expression. Rather than a simple ring muscle, it is composed of fibers extending in different directions. The deep layer contains predominantly fibers from the buccinator muscle. These fibers cross at the corner of the mouth and extend to the upper and lower lips. A second superficial layer is formed by fibers from muscles such as the caninus and depressor anguli oris (triangularis). Oblique fibers from the quadratus labii superioris, quadratus labii inferioris, and zygomaticus muscles also integrate into the structure. The muscle fibers insert at various points on the upper jaw (maxilla), the nasal septum, and the lower jaw (mandible). The upper lip has two main fiber bands: lateral and medial. The lateral band originates from the alveolar border of the maxilla, while the medial band inserts on the nasal septum. An anatomical groove called the philtrum forms between the two medial bands. In the lower lip, the inferior incisor muscle fibers originating from the mandible pass toward the corner of the mouth and insert into the inferior portion of the orbicularis oris. These connections form the flexible structure of the muscle, enabling both vertical and horizontal lip movement.

Embryological Origin and Classification

The Orbicularis Oris Muscle is one of the facial muscles originating from the second branchial arch and is embryologically classified as the muscles of facial expression. This classification is important both for its structural characteristics and source of innervation. Because the muscle's fibers contain both inward-facing (deep) and outward-facing (superficial) components, it is closely related to facial muscles that develop from different embryonic segments.

Nerve Innervation and Vascular Supply

The Orbicularis Oris Muscle is motor innervated by the buccal and mandibular branches of the facial nerve (CN VII). Sensory fibers are supplied by the maxillary and mandibular branches of the trigeminal nerve (CN V). Its blood supply is generally provided by branches of the facial artery and the superior/labial artery. This vascular network creates extensive capillary networks in both the superficial and deep layers, contributing to the high vascularity of the lips.

Function and Clinical Relationships

The Orbicularis Oris Muscle performs multiple functions, including closing, inward depressing, extending, and shaping the lips. It prevents food from spilling out of the mouth during chewing and plays a critical role in motor actions such as speaking, blowing, and whistling. It is the primary muscle responsible for the sucking reflex in newborns. Clinically, in cases of paralysis (e.g., facial nerve palsy), lip closure inability, drooling, and speech disorders are observed. In cosmetic surgery and Botox applications, the muscle's medial fibers in the upper lip are targeted to alleviate expression lines.