The risorius muscle is a highly variable
muscle of facial expression. It has numerous and very variable origins, and inserts into the angle of the mouth. It receives motor innervation from branches of
facial nerve (CN VII). It may be absent or asymmetrical in some people. It pulls the angle of the mouth sidewise, such as during smiling.
Structure
The risorius muscle is highly variable.
Attachments
Its peripheral attachments may include (some or all of): the
parotid fascia,
masseteric fascia, the fascia enveloping the pars modiolaris of the
platysma muscle
The platysma muscle or platysma 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 "bowst ...
, fascia overlying the
mastoid part of temporal bone, and/or the
zygomatic arch
In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...
.
Its apical and subapical (i.e. convergent) attachment is at the
modiolus.
Innervation
The risorius receives motor innervation from the
buccal branch of the
facial nerve (CN VII).
Vasculature
The risorius receives arterial supply mostly from the
superior labial artery
The superior labial artery (superior labial branch of facial artery) is larger and more egregious than the inferior labial artery.
It follows a similar course along the edge of the upper lip, lying between the mucous membrane and the orbiculari ...
.
Variation
The risorius muscle is highly variable. It ranges in form from one or more slender bundles to a wide (yet thin) fan.
It may be absent in a significant minority of people, and may be
asymmetrical
Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
.
Relations
It is superficial to the
masseter muscle
In anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it is the ...
, partially overlying it.
Function
The risorius muscle draws the angle of the mouth lateral-ward.
It participates in producing facial expressions like a
smile
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 d ...
, grin, or laugh.
Clinical significance
Because it partially overlies the masseter muscle, it may be unintentionally affected during botox injections, resulting in unnatural facial expressions.
Other animals
It has been suggested that the risorius muscle is only found in
Homininae
Homininae (the hominines) is a subfamily of the family Hominidae (hominids). (The Homininae——encompass humans, and are also called "African hominids" or "African apes".) This subfamily includes two tribes, Hominini and Gorillini, both having ...
(African great apes and
human
Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
s).
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Additional images
File:Risorius animation small.gif, Position of risorius.
References
External links
PTCentral
{{Authority control
Muscles of the head and neck