ABSTRACT
The organization of the auditory system in different sarcopterygians varies considerably. In, amphibians have two auditory endorgans, the basilar papilla and the amphibian papilla, which have different frequency ranges and are in different parts of the inner ear. In frogs, there is a tonotopic organization in the central projections of the amphibian papilla (tonotopic organization is unclear in the basilar papilla), which end up in the dorsolateral nucleus. In contrast, amniotes, including birds and mammals, the auditory endorgan is the basilar membrane of the organ of Corti, which is tonotopically organized and can respond to a wide range of frequencies. The sensory cells in the basilar papilla and organ of Corti connect to specific neurons that project to the cochlear nuclei in the brainstem. In birds and mammals, there are many more cochlear nuclei neurons, with distinct populations of neurons receiving input from different parts of the basilar membrane. Sauropsids have a different organization of their auditory nuclei compared to mammals. They have a distinct population of neurons called the angular nucleus, which receives input from a different part of the basilar membrane compared to the magnocellular nucleus. Mammals have a ventral cochlear nucleus and a dorsal cochlear nucleus, which have different functions in processing auditory information. Binaural commissural connections exist in some vertebrates, such as frogs and mammals, and may play a role in directional hearing. In addition, the loss of lateral line and electroreception in amniotes is associated with the transformation of granule cells into granular neurons.
