Auditory Muscles and their Function

AUDITORY MUSCLES

Two skeletal muscles are attached to ossicles:

i. Tensor tympani

ii. Stapedius.

i. Tensor Tympani

Tensor tympani is larger of the two muscles of tympanic cavity.

Origin, insertion and nerve supply Tensor tympani arises from cartilaginous portion of eustachian tube (see below), adjacent to great wing of sphenoid bone and osseous canal. Its tendon is inserted on manubrium of malleus, which is in turn attached to tympanic membrane. Thus, the tensor tympani is attached to tympanic membrane through malleus. It is supplied by mandibular division trigeminal nerve.

Function

Tensor tympani muscle pulls and keeps the tympanic membrane stretched or tensed constantly. This

constant stretching of tympanic membrane is essential for the transmission of sound waves, which may reach any part of the tympanic membrane. Paralysis of tensor tympani causes hearing impairment.

ii. Stapedius

Stapedius is the smallest skeletal muscle in human body with a length of just over 1 mm. It lies in a conical bony cavity, on the posterior wall of the tympanic cavity.

Origin, insertion and nerve supply

Stapedius arises from interior pyramid of tympanic cavity. Its tendon is inserted into the posterior surface of neck of stapes. It is supplied by branch of facial nerve.

Function

Stapedius prevents excess movements of stapes. When it contracts, it pulls the neck of stapes backwards and reduces the movement of footplate against the fluid in cochlea. Paralysis of stapedius allows wider range of oscillation of stapes, leading to hyperreaction of auditory ossicles to sound vibrations. This condition is called hyperacusis. Paralysis of stapedius occurs in the lesion of facial nerve.

Tympanic Reflex

Tympanic reflex is an attenuation reflex characterized by involuntary contraction of tensor tympani and

stapedius muscles, in response to a loud noise. It has a latent period of 40 to 80 millisecond.

When both the muscles contract, manubrium of malleus moves inward and stapes is pulled outward.

These two actions result in stiffness of auditory ossicles, so that the transmission of sound is decreased.

Significance of tympanic reflex

i. Tympanic reflex protects the tympanic membrane from being ruptured by loud sound

ii. It also prevents fixation of footplate of stapes, against oval window, during exposure to loud

sound

iii. It helps to protect the cochlea from damaging effects of loud sounds. Contraction of tensor

tympani and stapedius during exposure to loud sound develops stiffness of the auditory

ossicles so that, the transmission of sound into cochlea is decreased.

EUSTACHIAN TUBE

Eustachian tube or the auditory tube is the flattened canal extending from the anterior wall of middle ear

to nasopharynx. Its upper part is surrounded by the bony wall and the lower part is surrounded by

fibrocartilaginous plate. Eustachian tube connects middle ear with posterior part of nose and forms the passage of air between middle ear and atmosphere. So, the pressure on both sides of tympanic membrane is equalized.

INTERNAL EAR

Internal ear or labyrinth is a membranous structure, enclosed by a bony labyrinth in petrous part of temporal bone. It consists the sense organs of hearing and equilibrium. Sense organ for hearing is the cochlea and the sense organ for equilibrium is the vestibular apparatus.