MORPHOLOGY AND AXONAL PROJECTION PATTERNS OF AUDITORY NEURONS IN THE MIDBRAIN OF THE PAINTED FROG, DISCOGLOSSUS-PICTUS

Acoustic signals are extensively used for guiding various behaviors in frogs such as vocalization and phonotaxis. While numerous studies hav e investigated the anatomy and physiology of the auditory system, our knowledge of intrinsic properties and connectivity of individual audit ory neurons remains poor. Moreover, the neural basis of audiomotor int egration still has to be elucidated. We determined basic response patt erns, dendritic arborization and axonal projection patterns of auditor y midbrain units with intracellular recording and staining techniques in an isolated brain preparation. The subnuclei of the torus semicircu laris subserve different tasks. The principal nucleus, the main target of the ascending auditory input, has mostly intrinsic neurons, i.e., their dendrites and axons are restricted to the torus itself. Ln contr ast? neurons of the magnocellular and the laminar nucleus project to v arious auditory and non-auditory processing centers. The projection ta rgets include thalamus, tegmentum, periaqueductal gray; medulla oblong ata, and - in the case of laminar neurons - the spinal cord. Additiona lly, tegmental cells receive direct auditory input and project to vari ous targets, including the spinal cord. Our data imply that both audit ory and premotor functions are implemented in individual toral and teg mental neurons. Their axons constitute parallel descending pathways to several effector systems and might be part of the neural substrate fo r differential audiomotor integration. (C) 1998 Elsevier Science B.V. All rights reserved.