Central Lateral Line and Auditory Pathways: A Phylogenetic Perspective

Abstract

The phylogenetic origins of the lateral line electrosensory, lateral line mechanosensory, and auditory components of the octavolateralis system are unknown but each of these sensory modalities appears to have evolved early in vertebrate history. The octavolateralis terminal field occupies a large area of the dorsolateral wall of the medulla and among agnathids, cartilaginous fishes, non-teleost bony fishes and, with modifications, urodeles, consists of a dorsal electrosensory nucleus, a medial mechanosensory nucleus and a ventral octaval nuclear complex. This arrangement of medullary octavolateral nuclei, which differs from that of non-electroreceptive and electroreceptive teleosts, is considered the primitive plan and is retained in that phyletic line leading to tetrapods. Separate and parallel pathways are known, in elasmobranchs and a few teleosts, to ascend from each medullary lateral line center to the midbrain and presumably from midbrain to telencephalic levels via thalamic relays. There is no evidence, with the loss of lateral line senses among some amphibians and all amniotes, that the central neural pathways and nuclei are retained and used to process information from other sensory modalities. The anatomy of the central auditory system of fishes is unknown but is required for an understanding of whether auditory nuclei and pathways are retained during the fishamphibian transition, or whether new ones arise, to process information from independently evolved peripheral receptors.