The stapedius is the smallest

skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

in the

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the

stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foot ...

or strirrup bone of the middle ear.

Structure

The stapedius emerges from a pinpoint

foramen In anatomy and osteology, a foramen (;Entry "foramen"
in

or opening in the apex of the

pyramidal eminence The pyramidal eminence (pyramid) is a conical projection in the middle ear. It is situated immediately behind the fenestra vestibuli (oval window), and in front of the vertical portion of the facial canal; it is hollow, and contains the stapedi ...

(a hollow, cone-shaped prominence in the posterior wall of the

tympanic cavity The tympanic cavity is a small cavity surrounding the bones of the middle ear. Within it sit the ossicles, three small bones that transmit vibrations used in the detection of sound. Structure On its lateral surface, it abuts the external auditory ...

), and inserts into the neck of the stapes.

Nerve supply

The stapedius is supplied by the

nerve to stapedius The nerve to the stapedius is a branch of the facial nerve (CN VII) which innervates the stapedius muscle. It arises from the CN VII opposite the pyramidal eminence and passes through a small canal in this eminence to reach the stapedius muscle T ...

, a 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 stapedius dampens the vibrations of the stapes by pulling on the neck of that bone. As one of the muscles involved in the

acoustic reflex The acoustic reflex (also known as the stapedius reflex, stapedial reflex, auditory reflex, middle-ear-muscle reflex (MEM reflex, MEMR), attenuation reflex, cochleostapedial reflex or intra-aural reflex) is an involuntary muscle contraction that ...

it prevents excess movement of the

, helping to control the amplitude of

sound waves In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...

from the general external environment to the inner

ear An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...

Clinical significance

Paralysis Paralysis (also known as plegia) is a loss of motor function in one or more muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory damage. In the United States, roughly 1 in 50 ...

of the stapedius allows wider

oscillation Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

of the stapes, resulting in heightened reaction of the

auditory ossicles The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (inner ear), labyrinth (cochlea). The ...

to sound vibration. This condition, known as

hyperacusis Hyperacusis is the increased sensitivity to sound and a low tolerance for environmental noise. Definitions of hyperacusis can vary significantly; it can refer to normal noises being perceived as: loud, annoying, painful, fear-inducing, or a combina ...

, causes normal sounds to be perceived as very loud. Paralysis of the stapedius muscle may result when the

nerve to the stapedius The nerve to the stapedius is a branch of the facial nerve (CN VII) which innervates the stapedius muscle. It arises from the CN VII opposite the pyramidal eminence and passes through a small canal in this eminence to reach the stapedius muscle T ...

, a branch of the

, is damaged, or when the facial nerve itself is damaged before the nerve to stapedius branches. In cases of

Bell's palsy Bell's palsy is a type of facial paralysis that results in a temporary inability to control the facial muscles on the affected side of the face. In most cases, the weakness is temporary and significantly improves over weeks. Symptoms can vary fr ...

, a unilateral paralysis of the facial nerve, the stapedius is paralyzed and hyperacusis may result.

Other animals

Like the

bone to which it attaches, the stapedius muscle shares evolutionary history with other vertebrate structures. The mammalian stapedius evolved from a muscle called the

depressor mandibulae Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

in other tetrapods, the function of which was to open the jaws (this function was taken over by the

digastric muscle The digastric muscle (also digastricus) (named ''digastric'' as it has two 'bellies') is a small muscle located under the jaw. The term "digastric muscle" refers to this specific muscle. However, other muscles that have two separate muscle bellie ...

in mammals). The depressor mandibulae arose from the

levator operculi Levator muscle can refer to: * Levator scapulae muscle * Levator palpebrae superioris muscle * Levator ani * Levator labii superioris alaeque nasi muscle * Levator veli palatini * Levator muscle of thyroid gland * Levator labii superioris * Levator ...

in bony fish, and is equivalent to the epihyoidean in sharks. Like the stapedius, all of these muscles derive from the

hyoid arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arc ...

and are innervated by

cranial nerve VII 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 ...

References

External links

{{DEFAULTSORT:Stapedius Muscle Muscles of the head and neck