Central connections of anamniote auditory otolith endorgans

Abstract

The ascending auditory pathways of anamniotes (fish and amphibians) are organized similarly to those of amniotes (reptiles, birds, and mammals). A common pattern for the flow of acoustic information—inner ear to first-order nuclei to higher-order auditory nuclei in the medulla, midbrain, thalamus, and cerebrum—probably evolved early in vertebrate history and was retained in extant fish and in terrestrial vertebrates (with taxon-specific modifications) despite variability in inner-ear acoustic receptors. Otolith endorgans subserve hearing in jawed fish; auditory fibers segregate from otolith endorgan vestibular fibers and supply auditory divisions of functionally mixed (auditory-vestibular) first-order nuclei. Amphibian auditory otolith endorgans are primarily seismic detectors complementing new sound-pressure receptors of the amphibian and basilar papillae. Otolithic and papillar auditory endorgans supply common areas within functionally mixed (auditory-vestibular) first-order nuclei. Uniquely in anurans, they also supply a discrete first-order auditory nucleus analogous to the amniote cochlear nuclei. Discrete first-order auditory nuclei are correlated with the evolution of a tympanic ear, an independently evolved condition in anurans and among amniotes. Thus, at the first-order level, the amphibian auditory pathway retains primitive organizational features present in fish auditory pathways. Convergent neuroanatomical specializations in anurans and amniotes may reflect common functional requirements.