The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or thunder. Because its reaction time is not fast enough, the muscle cannot protect against hearing damage caused by sudden loud sounds, like explosions or gunshots.
Structure
The tensor tympani is a muscle that is present in the middle ear. It arises from the cartilaginous part of the auditory tube, and the adjacent great wing of the sphenoid. It then passes through its own canal, and ends in the tympanic cavity as a slim tendon that connects to the handle of the malleus. The tendon makes a sharp bend around the processus cochleariformis, part of the wall of its cavity, before it joins with the malleus. The tensor tympani receives blood from the middle meningeal artery via the superior tympanic branch. It is one of two muscles in the tympanic cavity, the other being the stapedius.
The tensor tympani acts to dampen the noise produced by chewing. When tensed, the muscle pulls the malleus medially, tensing the tympanic membrane and damping vibration in the ear ossicles and thereby reducing the perceived amplitude of sounds. It is one of the muscles involved in the acoustic reflex.
Voluntary control
Contracting muscles produce vibration and sound. Slow twitch fibers produce 10 to 30 contractions per second. Fast twitch fibers produce 30 to 70 contractions per second. The vibration can be witnessed and felt by highly tensing one's muscles, as when making a firm fist. The sound can be heard by pressing a highly tensed muscle against the ear, again a firm fist is a good example. The sound is usually described as a rumbling sound. Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear. The rumbling sound can also be heard when the neck or jaw muscles are highly tensed as when yawning deeply. This phenomenon has been known since 1884.
Involuntary control (tympanic reflex)
The tympanic reflex helps prevent damage to the inner ear by muffling the transmission of vibrations from the tympanic membrane to the oval window. The reflex has a response time of 40 milliseconds, not fast enough to protect the ear from sudden loud noises such as an explosion or gunshot. system|alt=|350x350px Thus, the reflex most likely developed to protect early humans from loud thunder claps which do not happen in a split second. The reflex works by contracting the muscles of the middle ear, the tensor tympani and the stapedius. This pulls the manubrium of the malleus inwards and tightens it. This tightening prevents the vibrations from disturbing the perilymph. Withdrawal from drugs such as benzodiazepines had been known to cause tonic tensor tympani syndrome during withdrawal. The tympanic reflex will also activate when loud vibrations are generated by the person themselves. The tensor tympani can often be observed vibrating while shouting at an increased volume, dampening the sound somewhat.
Clinical significance
In many people with hyperacusis, an increased activity develops in the tensor tympani muscle in the middle ear as part of the startle response to some sounds. This lowered reflex threshold for tensor tympani contraction is activated by the perception/anticipation of loud sound, and is called tonic tensor tympani syndrome. In some people with hyperacusis, the tensor tympani muscle can contract just by thinking about a loud sound. Following exposure to intolerable sounds, this contraction of the tensor tympani muscle tightens the ear drum, which can lead to the symptoms of ear pain/a fluttering sensation/a sensation of fullness in the ear. The mechanisms behind dysfunction of the tympanic tensor muscle and their consequences are hypotheses. However, in a published study, researchers studied the case of an acoustic shock whose mechanisms suggest dysfunction of the tympanic tensor muscle. This study appears to be the first to provide experimental support suggesting that middle ear muscles may behave abnormally after an acoustic shock. It is suggested that abnormal contractions of the tympanic tensor muscle may trigger neurogenic inflammation. Indeed, fibers with substances P and CGRP were found in close proximity.
