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 mus ...

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, an ...

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 foo ...

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 (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 audito ...

), and inserts into the neck of the stapes.

Nerve supply

The stapedius is supplied by the nerve to stapedius, 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 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 ...

Clinical significance

Paralysis of the stapedius allows wider oscillation of the stapes, resulting in heightened reaction of the auditory ossicles 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, 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 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 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 arch ...

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 taste ...

References

External links

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