Robustness and adaptive flexibility of the pulmonate's brain

The central nervous system (CNS) of adult pulmonates has exquisite regenerative ability after injury, and each neuron can amplify its genomic DNA to meet the increasing demands for macromolecules. This contrasts with the brain of adult mammals, which has a restricted ability to regenerate and whose neurons have already accomplished terminal differentiation. If the tentacle of a pulmonate is cut or removed, it can spontaneously sprout again within several weeks together with sensory organs and the primary olfactory center existing in the tip of the tentacle. Nerve connections are concomitantly restored between the primary olfactory center and the procerebrum, a bilaterally equipped neuronal center involved in olfactory information processing in pulmonates. Furthermore, the procerebrum can spontaneously recover its structure and function if a part of it is physically damaged or lost. Such an extensive regenerativity is owed at least partly to the neuronal precursor cells that continuously produce new neurons throughout the animal's life. Neurons themselves can even amplify DNA to produce a larger amount of gene products when the body of the adult slug grows larger with an ample food supply. As a consequence, the size of the neurons and the brain is enlarged in fat slugs. All of these flexible changes in the nervous system can occur even in adults. In this review article, we introduce the robustness and innate adaptive flexibility of the brain of pulmonates, and discuss the evolution of these traits from several points of view.