The stapedius muscle is the smallest muscle of the human body, measuring approximately 6 millimeters in length. It is located in the tympanic cavity in the middle ear, connecting the pyramidal eminence of petrous part of temporal bone to the posterior aspect of the neck of stapes.
Stapedius muscle is innervated by the stapedial branch of facial nerve. These autonomic fibers enable the muscle to be involved in the auditory middle ear reflex, having a crucial role in protecting the auditory system from damage.
In this article, we will discuss the anatomy and function of stapedius muscle.
|Origin||Pyramidal eminence of tympanic cavity|
|Insertion||Neck of stapes|
|Action||Dampens vibrations passed to the cochlea via the oval window|
|Innervation||Nerve to stapedius muscle (of facial nerve (CN VII))|
|Blood supply||Stapedial branch of posterior auricular artery|
The stapedius muscle originates from a small elevation of the temporal bone located in the tympanic cavity called the pyramidal eminence. After its relatively short course through the tympanic cavity, it inserts onto the neck of stapes.
The stapedius muscle is innervated by a small branch that arises from the facial nerve (CN VII), also known as the nerve to stapedius muscle. After branching off from the mastoid segment of facial nerve, the nerve passes posterior to the pyramidal process to innervate the stapedius muscle.
The vascularization is provided by the stapedial branch of posterior auricular artery, which branches from the external carotid artery.
Although it is the smallest skeletal muscle, the stapedius muscle has an important role in sound transmission and hearing as it is the effector component of the acoustic middle ear reflex.
When the incoming sound is loud enough to stimulate the receptor cells in the inner ear, the afferent signal reaches the cochlear nucleus in the brainstem via vestibulocochlear nerve (CN VIII).
From the brainstem, efferent signals are sent to the ipsilateral and contralateral middle ears triggering the contractions of the stapedius muscles. Their contractions result with posterior rotation and placing the base of the stapes into the oval window which closes it and attenuates further vibrations passed to cochlea. Thus the main function of the stapedius muscle is to protect the hearing apparatus when exposed to loud sounds.
In a healthy individual with normal hearing, the sound threshold is approximately at 85 decibels.
In clinical practice, the function of this reflex can be measured by specific instruments, revealing important information about the status of the entire auditory system.
It is important to note that this reflex is slow (especially on the ipsilateral side) which is the reason why the hearing is easily damaged when sudden impulsive sounds occur, such as explosion or gunshot.
Having trouble understanding the function of the auditory system? No worries, we have prepared more articles about auditory ossicles and the auditory pathway.
Hyperacusis is a condition characterized by hypersensitivity to particular sounds that are not usually a problem for others. Patients with severe forms of this condition can perceive everyday sounds (e.g. telephone ringing) as uncomfortable or even painful.
One of the common causes of severe forms of hyperacusis is the injury of the facial nerve distal to the geniculate ganglion. When injured, the possible outcome is the palsy of stapedius muscle that results in failure to attenuate sounds coming from the outside environment.
This is the reason why these patients experience everyday sounds extremely loud. Since one of the most important protective mechanisms fails to perform, the auditory system gets easily injured and it can lead to complete deafness.
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