Neural Connections between the Middle Ear, Eustachian Tube and Brain

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Abstract
This study was designed to investigate the neuroanatomic connections between the middle ear (ME), eustachian tube, and brain. The neural tracer, horseradish peroxidase (HRP), was injected into the promontory mucosa of the ME in six adult rabbits. Following a 24–48 hour survival, the brain was perfused with fixative and the HRP reacted by the tetramethylbenzidine (TMB) blue reaction process. Two groups of HRP-labeled neurons were noted in the ipsilateral brainstem: 1) a well-defined cluster of HRP-labeled neurons in the nucleus of the solitary tract, and 2) a more widely scattered group of labeled neurons in the inferior salivary nucleus. The labeled neurons in the solitary tract may represent the direct central sensory projections of the glomus tympanicum. In a second set of animals HRP was injected into both the tubal and palatal muscles to identify the motor innervation of the eustachian tube musculature; HRP-labeled motoneurons were noted in the trigeminal motor nucleus and nucleus ambiguus, respectively. Assuming that the glomus tympanicum cells can sense changes such as oxygen concentration in their immediate environment in the ME like the related glomus cells in the carotid and aortic bodies which also project to the solitary tract, then the neuroanatomic relationships of the HRP-labeled neurons in this study to other respiratory neurons in the medulla suggest the possibility of a polysynaptic ME aeration reflex. The relevance of these results to the pathogenesis of otitis media with effusion is discussed.