Location and Structure
The orbicularis oris muscle forms part of the muscular layer of the lips. The lips form the anterior wall of the oral cavity, while their free margins outline the oral fissure and form the angles of the mouth (anguli oris).
Topography of the lips (layered structure from superficial to deep):
- Skin — thin, contains numerous sebaceous and sweat glands. The skin is fused with the orbicularis oris muscle, making it relatively immobile.
- Vermilion border — transitional zone.
- Mucous membrane — continuous with the buccal mucosa laterally and the gingiva superiorly and inferiorly. In the midline, it forms the frenula of the upper and lower lips.
- Submucosa — loose connective tissue containing mucous glands, superior and inferior labial arteries, veins, and nerves.
- Muscular layer — orbicularis oris muscle (m. orbicularis oris).
Functions
The orbicularis oris muscle performs the following functions:
- Closes the lips.
- Protrudes the lips forward, forming a "pucker".
- Participates in facial expression: the muscle tightens during smiling.
- Helps maintain the oral seal; compromise of this seal can lead to drooling.
- Lip damage impairs the articulation of labial speech sounds.
Blood Supply and Innervation
- Blood supply: Provided by branches of the facial artery (a. facialis), specifically the superior and inferior labial arteries (aa. labiales superiores et inferiores) and the angular artery (a. angularis). Additional blood supply comes from the infraorbital (a. infraorbitalis), mental (a. mentalis), and buccal (a. buccalis) arteries.
- Innervation: Innervated by the facial nerve (n. facialis), specifically via the buccal branches (rr. buccales), zygomatic branches (rr. zygomatici), and marginal mandibular branch (r. marginalis mandibulae).
- Central regulation: Upper motor neurons responsible for voluntary movements of facial muscles are located in the lower part of the precentral gyrus.
Clinical Significance in Surgery and Orthodontics
- Traumatology: A deep wound of the lower lip involving the muscular layer exhibits marked gaping caused by the transection and retraction of m. orbicularis oris fibers.
- Reconstructive surgery: When reconstructing the lower lip using a tubed pedicle flap (Filatov flap), a lip formed solely from the flap tends to sag and fails to provide an oral seal. To restore mobility, the Mikhelson technique is used: the orbicularis oris muscle of the upper lip is split longitudinally into two parts, and the inferior part is embedded into the adipose tissue of the reconstructed lower lip.
- Orthodontics: Preventive appliances, including preorthodontic trainers and Hinz plates, normalize lip closure and strengthen the orbicularis oris muscle through myofunctional training.
- Congenital anomalies: In double lip, characterized by hypertrophy of the mucous membrane, submucosa, and mucous glands, the pathology becomes apparent during speech and especially smiling: as the orbicularis oris muscle stretches, the excess submucosal tissue protrudes outward.
Neurological Diagnostics
Contraction of the orbicularis oris muscle is the basis of the snout reflex, a type of primitive oral reflex.
Lightly tapping the upper lip with a neurological hammer causes the lips to pucker due to the contraction of m. orbicularis oris. These reflexes indicate disinhibition of subcortical structures and are characteristic of pseudobulbar syndrome.
Pseudobulbar palsy results from bilateral lesions of the corticonuclear pathways. Common lesion locations include the pons, bilateral subcortical basal ganglia, and bilateral frontal lobes.