An Overview of the Organization of the Brain of Actinopterygian Fishes

SYNOPSIS. The brain of actinopterygian fishes can be subdividedinto five principal parts, rhombencephalon, cerebellum, mesencephalon,diencephalon and telencephalon, each of which contains a numberof separate morphological entities: nuclei, areas or zones.Analysis of the origin and termination of the cranial nervesand their components reveals that many of the morphologicalentities distinguished in the actinopterygian brain can be interpretedin terms of elementary sensory and motor functions. Experimentalanatomical and physiological studies on the fiber connectionsof the entities thus defined have led to a functional interpretationof many other parts of the brain. Thus, the central circuitryrelated to such sensory functions as hydrodynamic pressoreception,electroreception, vision, gustation and olfaction are well-known,and the same holds true for the motor systems related to feedingand locomotion. However, there are still many regions in theactinopterygian brain the functional significance of which ispoorly understood, and it should be emphasized that most ofour knowledge on the organization of the brain of this grouprests merely on observations in teleosts. One of the most interestingaspects of actinopterygian neurobiology is that the telencephalonin this group develops in a way which differs fundamentallyfrom that found in all other craniates, and that the telencephalonshows a marked progressive differentiation in the series: polypterids—chondrosteans—halecomorphs—teleosts.