The depressor labii inferioris (or quadratus labii inferioris) is a
facial muscle
The facial muscles are a group of striated skeletal muscles supplied by the facial nerve (cranial nerve VII) that, among other things, control facial expression. These muscles are also called mimetic muscles. They are only found in mammals, alth ...
. It helps to lower the
bottom lip
The lips are the visible body part at the mouth of many animals, including humans. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be ...
.
Structure
The depressor labii inferioris muscle arises from the lateral surface of the
mandible
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
.
This is below the
mental foramen
The mental foramen is one of two foramina (openings) located on the anterior surface of the mandible. It is part of the mandibular canal. It transmits the terminal branches of the inferior alveolar nerve and the mental vessels.
Structure
The m ...
, and the origin may be around 3 cm wide.
It inserts on the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...
of the
lower lip
The lips are the visible body part at the mouth of many animals, including humans. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be ...
, blending in with the
orbicularis oris muscle
In human anatomy, the orbicularis oris muscle is a complex of muscles in the lips that encircles the mouth.
It is a sphincter, or circular muscle, but it is actually composed of four independent quadrants that interlace and give only an appearance ...
around 2 cm wide.
At its origin, depressor labii is continuous with the fibers of the
platysma muscle
The platysma muscle is a superficial muscle of the human neck that overlaps the sternocleidomastoid. It covers the anterior surface of the neck superficially. When it contracts, it produces a slight wrinkling of the neck, and a "bowstring" effect ...
. Some yellow fat is intermingled with the fibers.
Nerve supply
The depressor labii inferioris muscle is supplied by the
marginal mandibular branch of the
facial nerve
The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...
.
Function
The depressor labii inferioris muscle helps to depress and everts the
lower lip
The lips are the visible body part at the mouth of many animals, including humans. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be ...
.
It is the most important of the muscles of the lower lip for this function.
It is an antagonist of the
orbicularis oris muscle
In human anatomy, the orbicularis oris muscle is a complex of muscles in the lips that encircles the mouth.
It is a sphincter, or circular muscle, but it is actually composed of four independent quadrants that interlace and give only an appearance ...
.
It is needed to expose the mandibular (lower)
teeth
A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
during
smiling
A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth. Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a Duchenne smile.
Among humans, a smile expresses ...
.
Clinical significance
Resection
The depressor labii inferioris muscle may be
resected (cut and removed) using
surgery
Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...
to correct an asymmetry of the lower lip when smiling.
This asymmetry can be caused by paralysis of the
marginal mandibular branch of the
facial nerve
The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...
on one side, so the healthy side may be cut to create symmetry.
Local anaesthesia
Local anesthesia is any technique to induce the absence of sensation in a specific part of the body, generally for the aim of inducing local analgesia, that is, local insensitivity to pain, although other local senses may be affected as well. It ...
may be used, such as by blocking the
mental nerve
The mental nerve is a sensory nerve of the face. It is a branch of the posterior trunk of the inferior alveolar nerve, itself a branch of the mandibular nerve (CN V3), itself a branch of the trigeminal nerve (CN V). It provides sensation to the ...
.
This operation tends to be successful.
History
The depressor labii inferioris muscle has also (mainly historically) been called the quadratus labii inferioris muscle.
See also
*
Facial muscles
The facial muscles are a group of striated skeletal muscles supplied by the facial nerve (cranial nerve VII) that, among other things, control facial expression. These muscles are also called mimetic muscles. They are only found in mammals, alth ...
*
Depressor anguli oris muscle
The depressor anguli oris muscle (triangularis muscle) is a facial muscle. It originates from the mandible and inserts into the angle of the mouth. It is associated with frowning, as it depresses the corner of the mouth.
Structure
The depres ...
Additional images
File:Depressor_labii_inferioris_muscle_animation small.gif, Position of depressor labii inferioris muscle (red).
File:Gray176.png, Mandible. Outer surface. Side view.
References
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Muscles of the head and neck