Mechanisms of middle ear aeration: anatomic and physiologic evidence in primates

Proper aeration is a prerequisite for normal middle ear function in all terrestrial mammals. Our previous studies in primates provided anatomic evidence of neural circuits between the middle ear, brain, and eustachian tube by which central respiratory neurons can control middle ear aeration. Yet mechanisms that regulate middle ear aeration remain poorly understood. This study extends our research by examining maturation of these neural circuits, and investigating their underlying physiology. Ultrastructural examination of tympanic nerves, the afferent limb of the neural circuit, in an age-graded series of cynomolgus monkeys (Macaca fascicularis) showed substantial differences between newborn, young, and adult animals. These included a twofold increase in average myelin thickness, and greater than threefold increase in the ratio of myelinated to unmyelinated fibers from newborn to adult animals. These marked developmental changes may translate into functional differences in regulation of middle ear aeration in young animals, and possibly explain the extraordinarily high incidence of middle ear disease in early childhood. In physiologic experiments, bilateral electromyographic responses were recorded from eustachian tube muscles, the efferent limb of the neural circuit, in adult monkeys after ipsilateral stimulation of the tympanic nerve. Response latencies were 9 to 28 msec, similar to those of other multisynaptic bilateral brainstem reflexes. These physiologic data strongly suggest a concept of active control of middle ear aeration by respiratory neurons in the brain.