Figure 5. Homology between vertebrates and invertebrates in neuroanatomical control of arousal.

(A) The basic circuit for vertebrate arousal involves aminergic excitation of the ventral forebrain and thalamus, which in turn excite the cortex. This circuit is well-established in mammals and is probably similar in birds and reptiles. Arousal-controlling nuclei are also found in homologous locations in fish, but in these animals the thalamus projects to the limbic system, including the brain region that is thought to function like the mammalian hippocampus. Thus, fish do not possess the thalamocortical loops that allow for entrainment of SWA, and in fact this form of NREM sleep has not been detected in fish. Instead it is possible that hippocampal sharp wave ripples, which have been detected in fish, serve a rudimentary related function. (B) A homologous circuit can be found in insects and other invertebrates that possess mushroom bodies (MBs), which are believed to be derived from an ancestral circuit that gave rise to the vertebrate cerebral cortex. Like the cortex, MBs also undergo oscillations that are thought to be important for memory. Abbreviations: DP (dorsal pallium); DVR (dorsal ventricular ridge); 5HT (serotonin); DA (dopamine); NA (noradrenaline or its invertebrate equivalent, octopamine).