Functional morphology of the paratympanic organ in the middle ear of birds.

The paratympanic organ (PTO) is a small sense organ in the middle ear of birds. This luminal organ contains mechanoreceptors (hair cells) with afferent and efferent innervation and may function as a baroreceptor. The hypothesis that elastic ligaments of the middle ear may be involved in the transduction of barometric pressure was tested. Two elastic ligaments are shown to attach to the PTO. The columellar-squamosal ('Platner's') ligament inserts at its caudal pole; the superior portion of the superior drum-tubal ligament attaches at the rostral tip of the organ. To determine if pressure to the tympanic membrane and tension of ligaments may cause lumen changes in the PTO, the length of elastic ligaments, and the length, volume and configuration of the PTO were measured in normal animals and in animals following application of positive or negative pressure to the tympanic membrane. The rostral pole of the PTO changes its shape differentially as a function of the tension of the superior drum-tubal ligament, resulting in volume shifts of about 10% of the total volume in the PTO. With negative pressure, volume moves from the rostral pole to the caudal half of the PTO; with positive pressure, volume shifts rostrally. Displacement of fluid in the lumen of the PTO thus may stimulate the paratympanic hair cells. Tracing of efferent projections to the PTO with the fluorescent compound DiI reveals a cluster of about 40 labeled neurons at the caudal pole of the ventral facial motor nucleus adjacent to the superior olive. These findings support the notion that the PTO may be part of a neural circuit that controls the position of the tympanic membrane and may mediate barometric perception in birds.