The papilla neglecta of turtles: a detector of head rotations with unique sensory coding properties.

The turtle papilla neglecta (PN) is a small organ lying in the ventrolateral utricular wall between the posterior crista (PC) and the utriculosaccular foramen. Innervated by a branch of the posterior ampullary nerve, the organ is covered by a cupula extending only a small distance into the endolymphatic space. Although most rotation-sensitive units in the posterior division of the eighth nerve have sensory coding properties expected of PC fibers, a few have unique properties. Intra-axonal labeling studies show that the former are PC units and the latter are PN units. PC units are maximally responsive to head rotations in the posterior canal plane and are sensitive to a combination of angular velocity and angular acceleration. PN units respond maximally to pitch rotations and are sensitive to a combination of angular acceleration and angular jerk. A maximal response to pitches can be related to the location of the PN, which allows it to sample endolymph flow from both vertical semicircular canals. Differences in response dynamics may reflect macromechanics. Because the cupula of each vertical canal occludes the endolymphatic space, its displacement should be proportional to endolymph displacement. In contrast, the PN cupula is probably coupled to endolymph flow by viscous forces, in which case its displacement should be proportional to endolymph velocity. In many vertebrates, the PN is similar to that seen in turtles in its location and in the size and shape of its cupula, which suggests that its function in these other species is also similar.