Facial Muscles - Lower Group

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Introduction

Located underneath the skin of the face and scalp are a group of 20 flat skeletal muscles. These muscles of facial expression, also named craniofacial muscles, are found in the subcutaneous tissue and emanate from bone or fascia, to attach onto the skin. They are a group of muscles that exclusively attach to skin and by contracting, the muscles pull on the skin and create movements of the face, such as smiling, grinning and frowning. Therefore, these muscles are commonly called muscles of facial expression, or mimetic muscles. All of the facial muscles are innervated by the facial nerve (CN VII) and vascularised by the facial artery.

The facial muscles are located around facial openings (mouth, eye, nose and ear) or extend over the skull and neck. Hence, they are divided into several groups;

• Muscles of the nose (nasal group)
• Muscles of the cranium and neck (epicranial group)
• Muscles of the external ear (auricular group)
• Muscles of the mouth or oral group (buccolabial group)

More specifically the oral group are accountable for movements of the mouth and lips. Such movements are necessary in singing and whistling and give emphasis to vocal communication.

Description

There is a total of eleven facial muscles that create movement at the mouth and their functions include:

• Lifting up and everting the upper lip: levator labii superioris, levator labii superioris alaeque nasi, risorius, levator anguli oris, zygomaticus major and zygomaticus minor muscles.
• Lowering and everting the lower lip: depressor labii inferioris, depressor anguli oris and mentalis muscles.
• Closing the lips: orbicularis oris muscle.
• Compacting the cheek: buccinator muscle.

The bulk of the mouth muscles are joined by a fibromuscular hub where their fibers insert. This structure is called the modiolus, it is founded at the angles of the mouth and it is primarily formed by the buccinator, orbicularis oris, risorius, depressor anguli oris and zygomaticus major muscles.

Each muscle is described individually below;

Orbicularis Oris

Origin

The fibres of the orbicularis oris surround the opening to the oral cavity. It comprises two parts; peripheral and marginal, with the border between them relating to the margin linking the lips and the surrounding skin. Both parts originate from the modiolus.

Insertion

• The peripheral part travels medially into the labial areas to insert on the dermis of the lips. In the midplane, a portion of the fibers intermingle with their respective fellow muscles to form the philtrum of the mouth.
• The marginal part travels from the modiolus on one side to the modiolus on the opposite side of the mouth. Some of the fibers curl upon themselves, forming the vermilion border, which is the boundary between the lips and the adjacent skin.

Nerve

Innervated by the buccal and mandibular branches of the facial nerve (CN VII).

Artery

Supplied from the superior and inferior labial branches of facial artery, with input from the mental and infraorbital branches of maxillary artery and the transverse facial branch of superficial temporal artery.

Function

A bilateral contraction of the whole muscle closes the mouth. An isolated contraction of particular parts of the muscle can produce various movements of the mouth, such as lip pouting, puckering, twisting and others. Through it's contractions the orbicularis oris assists speech and helps create a variety of facial expressions.

Assessment

Wearing a glove, place palpating fingers on the tissue of the lips. Ask the patient to pucker up the lips, and feel the contraction of the muscle. Once felt, palpate the entire muscle as the patient alternately contracts and relaxes.