4.1.7 What does the Stirrup Muscle do?
The stirrup muscle reduces loudness of sound reaching the inner ear and tunes pressure on the inner ear system.
The stirrup muscle (stapedius) adjusts the pressure of the stirrup bone on the oval window, leading to the inner ear. This affects the fluid pressure in the inner ear and the semi circular canals. It also affects pressure on the ear drum through leverage on the ossicular chain (the three little bones.) As the stirrup bone vibrates, the vibration is passed through the oval window into the inner ear fluid.
The stirrup muscle can control and modify how well sounds are transmitted to the inner ear.
This means that whether consciously or unconsciously, subtle changes in the middle ear will affect which sounds are admitted to the inner ear.
When the stirrup muscle contracts it dampens the vibration of the stirrup bone by pulling on its neck.
There is a reflex called the stapedial reflex, or acoustic reflex, which causes the stirrup muscle to flex when there is a sudden sound. Until recently it was believed that this reflex occurred in order to protect the ear from very loud sounds. However, it was then discovered that the reflex is not fast enough to achieve this protection. So there is now uncertainty about the purpose of the stapedial reflex.
Tomatis would see it as part of the ear’s continual adjustment to sound, contributing to the conscious or unconscious decisions about which sounds we will tune into and which ones will be barred from the ear.
Audiologists test the stapedial reflex to diagnose the source of hearing related problems. It can help in diagnosing if a problem is related to the middle ear, the inner ear, or the neural pathways beyond the ear.
However, this test will not detect the subtle function of this muscle as a part of the highly responsive and wholistic auditory feedback system. In other words, the acoustic reflex may test as normal, but there could still be room for improvement in the way this muscle responds to the full spectrum of sound perception through the frequency range and the complexity of language.