THE PAPILLA NEGLECTA OF TURTLES - A DETECTOR OF HEAD RELATIONS WITH UNIQUE SENSORY CODING PROPERTIES

The turtle papilla neglecta (PN) is a small organ lying in the ventrol ateral utricular wall between the posterior crista (PC) and the utricu losaccular foramen. Innervated by a branch of the posterior ampullary nerve, the organ is covered by a cupula extending only a small distanc e into the endolymphatic space. Although most rotation-sensitive units in the posterior division of the eighth nerve have sensory coding pro perties expected of PC fibers, a few have unique properties. Intra-axo nal labeling studies shaw that the former are PC units and the latter are PN units. PC units are maximally responsive to head rotations in t he posterior canal plane and are sensitive to a combination of angular velocity and angular acceleration. PN units respond maximally to pitc h 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 bot h vertical semicircular canals. Differences in response dynamics may r eflect macromechanics. Because the cupula of each vertical canal occlu des the endolymphatic space, its displacement should be proportional t o endolymph displacement. In contrast, the PN cupula is probably coupl ed to endolymph flow by viscous forces, in which case its displacement should be proportional to endolymph velocity. In many vertebrates, th e PN is similar to that seen in turtles in its location and in the siz e and shape of its cupula, which suggests that its function in these o ther species is also similar.