Attenuation Reflex

Definition: The attenuation reflex, also known as the middle ear muscle reflex or the acoustic reflex, is a protective mechanism of the auditory system that automatically dampens the sensitivity of the middle ear in response to loud sound stimuli.

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1. Function

The attenuation reflex serves to protect the delicate structures of the middle and inner ear from damage caused by excessively loud sounds. It reduces the transmission of sound energy from the outer ear to the inner ear, thereby preventing potential harm to the auditory system.

2. Physiology

When the attenuation reflex is triggered, the contraction of the stapedius and tensor tympani muscles occurs. These muscles are attached to the tiny bones in the middle ear, the stapes, and malleus, respectively. Their contraction stiffens the ossicular chain and reduces its ability to transmit sound vibrations to the cochlea.

3. Triggering Factors

The attenuation reflex is initiated by the loudness and intensity of sound. Typically, it is activated by sounds exceeding a certain threshold, usually around 70-100 dB SPL (sound pressure level). However, the triggering level may vary among individuals and can be influenced by factors such as age and overall hearing sensitivity.

4. Neural Pathways

The attenuation reflex involves a complex network of neural pathways. When sound reaches the cochlea, sensory information is transmitted to the auditory brainstem. From there, signals are relayed to the facial and trigeminal nerves, which innervate the stapedius and tensor tympani muscles, respectively. The contraction of these muscles is coordinated by the brainstem and cranial nerves.

5. Clinical Implications

Abnormal attenuation reflex responses can be indicative of certain auditory disorders or pathologies. Reflex thresholds that are significantly elevated or absent may occur in conditions such as acoustic neuromas, middle ear infections, and conductive hearing loss. Measurement of the attenuation reflex can therefore be useful in diagnostic audiology to assess the integrity of the auditory system.