Phylogenic Oto-stomatognathic Connection of the Mammalian Jaw: A Novel Hypothesis for Tensor Tympani Muscle and TMD-related Otologic Symptoms

Abstract

Otologic complaints, including otalgia, tinnitus, vertigo, and hearing loss, are known to be related to temporomandibular disorders (TMDs). There have been several hypotheses regarding the clinical correlation between otologic complaints and TMDs, based on clinical phenomena with corresponding symptoms, the close neurological relationship between otic and masticatory structures, and anatomical features of the tympanic cavity and jaw joint. Function of the tensor tympani muscle seems to be crucial to understanding TMD-related otologic symptoms. The tensor tympani inserts into the handle of the malleus and it modulates sound transduction in situations of excessive noise. This muscle is innervated by the trigeminal nerve, like the masticatory muscles. Voluntary eardrum movement by pathological tensor tympani contraction results in various otologic symptoms. Thus, co-contraction of the tensor tympani with the masticatory muscle could be a possible cause of TMD-related otologic symptoms. The tensor tympani is rather unrelated to the acoustic reflex, in which the stapedius is strongly involved. The tensor tympani seem to be controlled by proprioceptive information from the trigeminal sensory nucleus. The peripheral innervation pattern of the tensor tympani and masticatory muscles is also supposed to be interconnected. The middle ear structure, including the malleus, incus, and tensor tympani, of mammals had been adapted for acoustic function and lacks the masticatory role seen in non-mammalian jawed vertebrates. The tensor tympani in non-mammals is one of the masticatory muscles and plays a role in the modulation of sound transduction and mastication. After the functional differentiation of the mammalian middle ear, the nervous connection of the tensor tympani with other masticatory apparatus still remains. Through this oto-stomatognathic vestige, the tensor tympani seems to contract unnecessarily in some pathological conditions of the TMD in which the masticatory muscles contract excessively. We hypothesized that the phylogenic relationship between the tensor tympani and masticatory apparatus is a significant and logical reason for TMD-related otologic complaints.