Role of middle ear muscles
Middle ear houses two important muscles:
1. Tensor tympani
2. Stapedius
Tensor tympani: Originates from the cartilagenous and bony portions of the eustachean tube. It gets inserted into the handle of malleus after going around the processus cochleariformis. Contraction of this muscle pulls the malleus medially and anteriorly, at right angles to the normal direction of vibration. Contraction of this muscle can be seen as in drawing of the ear drum.
Stapedius: Arises from the pyramid present in the posterior wall of middle ear cavity.
It gets inserted to the neck of stapes.
Contraction of stapedius muscle causes fixation of stapes. It increases the stiffening effects of middle ear conduction mechanism. Contraction of this muscle can reduce transmission by up to 30 dB for frequencies less than 1-2 KHz.
Contraction of these two muscles serves to dampen unwanted resonances in the middle ear system causing spoken words to be heard with clarity.
Stapedius muscle contracts in response to loud sound. The reflex arc of stapedius muscle has 3 - 4 synapses ending in the facial nerve. Since the reflex arc is very small the reaction time is also pretty short i.e. 6-7 milliseconds.
Both these muscles are known to contract in response to non acoustic stimuli also i.e. stimulation of cornea by puff of cold air, vocalisation, touching the skin around the eye etc.
Functions of middle ear muscle reflex:
1. Protection of inner ear from damage due to excess noise. Although the reflex may be pretty slow in protecting the inner ear against sudden noise, it could serve this purpose with longer lasting noise exposures. Infact it has been demonstrated by Zakrisson that patients with Bell's palsy and paralysis of stapedius muscle have a greater incidence of temporary threshold shift when compared with normal controls.
2. Selective attenuation of low frequency sounds emitted from normal body mechanisms like flowing of blood through vessels etc by these muscles have been shown to improve the intelligibility of speech.
