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 the muscles have developed a preflex mechanism – a zero-synapse system inherent to muscle fibres which, in response to vibration, stiffens the muscle almost instantaneously. Preflexes are a developed form of sensitivity to loss of equilibrium common to all muscles, and have recently been identified in muscles in the leg, 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 high-level 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 – providing the utmost speed. Preflex action requires special anatomical and physiological muscle properties, and it is shown here how the middle ear muscles appear to have them. There are strong resemblances to the superfast muscles of bats, birds, and fish, and to the fast flight muscles of insects. Show more