A Fast, “Zero Synapse” Acoustic Reflex: Middle Ear Muscles Physically Sense Eardrum Vibration

Abstract

The middle ear muscles may be inconspicuous, but they are special. Silently standing guard at the entrance to the inner ear, their role is to spring into action whenever sound input rises, protecting the highly sensitive cochlea from overload. Such a task requires the utmost speed, for sounds can reach damaging levels within milliseconds. Neural-mediated mechanisms are slow, with the acoustic reflex arc taking up to a hundred milliseconds or more. Here, evidence is assembled that the middle ear muscles have recruited an additional, faster mechanism. The proposal is made that these muscles have developed a preflex mechanism -a zero-synapse system inherent to muscle fibres which, in response to vibration, rapidly stiffens the muscles. Preflexes are a developed form of sensitivity to perturbation common to all muscles, and have recently been identified in leg muscles, for example. However, the advantages that preflexes confer to an animal's auditory system have not yet been recognized. Applied to the middle ear muscles, heightened sensitivity to vibration means that any loud sound entering the middle ear causes the muscles to immediately stiffen, providing instant, on-the-spot overload protection. The muscles are therefore self-reflexive -they are both sensors and actuators. It is shown here how the middle ear muscles appear to have the special anatomical and physiological properties required for preflex action. There are strong resemblances to the superfast muscles of bats, birds, and fish, and to the fast flight muscles of insects.