Orbicularis Oris

Orbicularis oris is a complex circular muscle that surrounds the orifice of the mouth and forms the majority of the lips. It belongs to a large group of muscles of facial expression called the buccolabial group.

This muscle may be regarded as a single muscle anatomically, but functionally it appears to consist of different parts that act either independently or together with other facial muscles.
It has neither a bony or tendinous origin.

EMBRYOLOGY

The muscle is a derivative of the mesoderm of the second branchial arch.
Between the 3rd and 8th week, mesoderm of the second arch starts thickening.
Between 6 and 8 weeks, premyloblasts and myeloblasts which appear as sheet-like collection begin to extend to form five different laminae.
The orbicularis oris muscle develops from two embryonic laminae, the mandibular lamina and the infraorbital lamina.

STRUCTURE

Consists of two parts;

A larger peripheral part,
Smaller marginal part.

ORIGIN AND INSERTION

Origin	Medial aspects of maxilla and mandible, perioral skin and muscles, modiolus
Insertion	Skin and mucous membrane of lips

MUSCLES

The orbicularis muscle consists of two well-defined parts, namely a deep and a superficial part. These two parts have different names from different authors, such as

Deep part and superficial part
Pars marginalis and pars superficialis
Marginal part and peripheral part
Intrinsic and extrinsic bundle

FUNCTION

The cheek muscles to create contact between the teeth and lips by pressurizing the dental arches.
Mainly account for facial expression.
Collaborating in the production of speech sounds.
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.
It is also crucial for swallowing, mastication, and sucking.
This muscle also helps in puckering the lips due to which it is sometimes known as the “kissing muscle.”
The musicians also use this muscle when they play brass or woodwind instruments.

RELATIONS

MEDIAL	levator anguli oris, depressor anguli oris, buccinator, risorius and zygomaticus major muscles
LATERAL	buccinator
SUPERFICIAL	risorius muscle

NOTE

Relation with superior and inferior peripheral portion

SUPERIOR PERIPHERAL PORTION	Blends with levator labii superioris, zygomaticus major, zygomaticus minor and levator anguli oris
INFERIOR PERIPHERAL PORTION	Blends with depressor labii inferioris, depressor anguli oris, mentalis, and platysma

INNERVATION

Buccal branch of facial nerve (CN VII)

BLOOD SUPPLY

Facial artery - Superior and inferior labial arteries

Maxillary artery - Mental and infraorbital arteries

Superficial temporal artery - Transverse facial artery

APPLIED CLINICAL ASPECT

Facial palsy : As the muscle receives nerve supply from the seventh cranial nerve, thus, injury to this nerve can result in paralysis of the muscle. In Bell palsy, sagging of the orbicularis oris results in drooling.
Loss of tone : Loss of tone of this muscle to the muscles leads to drooping of the commissure as it controls the oral sphincter, which can cause drooling and difficulty in eating.
Clinical implication in prosthodontics : The orbicularis oris muscle due to its fiber orientation has a positive action in maintaining the stability of the denture.