The stapedius is the smallest

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

in the

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 ...

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 tetrapods 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 f ...

or stirrup bone of the middle ear.

Structure

The stapedius emerges from a pinpoint

foramen In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...

or opening in the apex of the

pyramidal eminence The pyramidal eminence is a hollow conical projection upon the posterior wall of the tympanic cavity of the middle ear. The stapedius muscle arises in the hollow of the eminence and its tendon exits through its apex. The pyramidal eminence is si ...

(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 audit ...

), 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 within the facial canal, opposite the pyramidal eminence. It passes through a small canal in this eminence to re ...

, 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 ta ...

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 from the general external environment to the inner ear.

Clinical significance

Paralysis Paralysis (: paralyses; also known as plegia) is a loss of Motor skill, motor function in one or more Skeletal muscle, muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory d ...

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 to sound vibration. This condition, known as

hyperacusis Hyperacusis is an increased Hearing, sensitivity to sound and a low tolerance for environmental noise. Definitions of hyperacusis can vary significantly; it often revolves around damage to or dysfunction of the Stapes, ''stapes'' bone, stapedius ...

, 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 within the facial canal, opposite the pyramidal eminence. It passes through a small canal in this eminence to rea ...

, 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 f ...

, 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 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 bilaterally paired suprahyoid muscle located under the jaw. Its posterior belly is attached to the mastoid notch of temporal bone, and its anterior belly ...

in mammals). The depressor mandibulae arose from the

levator operculi Levator muscle can refer to: * Levator scapulae muscle * Levator palpebrae superioris muscle * Levator ani The levator ani is a broad, thin muscle group, situated on either side of the pelvis. It is formed from three muscle components: the puboc ...

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 transient structures seen in the embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, the arches support the gills a ...

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 tast ...

References

Rodríguez-Vázquez JF. Development of the stapedius muscle and pyramidal eminence in humans. J Anat. 2009 Sep;215(3):292-9. doi: 10.1111/j.1469-7580.2009.01105.x

External links

{{DEFAULTSORT:Stapedius Muscle Muscles of the head and neck Otorhinolaryngology Auditory system