Fascinating Facts About the Facial Muscles and Their Role in Facial Expression
The facial muscles are part of the striated muscular system, like the skeletal muscles.
Skeletal and facial muscles are formed of similar muscle tissues that typically have
visible striations due to their construction, and are usually capable of intentional
control or voluntary action.
Most of the muscles of expression are innervated by the Seventh (VII) Cranial Nerve
and these are sometimes viewed as THE facial muscles.
Other muscles that participate in producing facial expressions are innervated by the
Fifth (V) Cranial Nerve. The nuclei for these nerves lies in the brain stem.
The facial muscles, however, are different from most skeletal muscles in important respects,
and they have several interesting
characteristics that are discussed in the following paragraphs.
|Skeletal muscle showing
horizontal striations due
to structures in cells. Facial
muscles have similar striated
Skeletal muscles typically have muscle spindle fibers, but whether the
facial muscles have spindle fibers has been a disputed issue for a long time,
and this question remains unresolved.
Spindle fibers are part of a neuromuscular feedback system that allows nerves and muscles
to determine what state the muscle is in, for example, how stretched the muscle tissue is.
If the muscles of facial expression have muscles spindles, then they would be a mechanism
for the nervous system to detect what happens to the muscles when activated by
"involuntary" emotional processes. Some theories of emotion,
beginning with William James and C. G. Lange, posit that such sensations could underlie
the feelings of emotion, although the feedback from muscle spindles is not essential.
Skeletal muscles often are matched in pairs that have opposite actions.
For example, the abductor and adductor muscles of the arm move the arm away from
and towards the torso, respectively, and other pairs of muscles flex and extend the arm or leg.
Facial muscles of expression do not have such an arrangement. Instead, the facial muscles
often have no oppositional muscles and the facial tissue they affect returns to a resting
state because the tissue resists deformation (turgor). When muscles produce opposite facial
appearances (e.g., raising versus lowering the lip corners),
they do not have the oppositional arrangement typical of skeletal muscles,
but instead attach to different bones or tissues.
Evolution of Facial Muscles
The facial muscles have a distinctive evolutionary path.
Most of the muscles of expression in the human face originate from the platysma muscle
of ancestral animals. No other animals have evolved as complex a set of facial muscles
as have humans.
Purpose and Functions
The adaptive functions of the facial muscles are multiple.
Images copyright © Corel Corp. - do not copy.