INVERTEBRATE LEARNING

[Invertebrates are particularly useful for analyzing the neural and molecular events underlying learning and memory. The nervous systems of many invertebrates contain only several thousand cells (compared with the billions of cells in the vertebrate nervous system). Despite the small number of cells, an invertebrate ganglion can control a variety of behaviors. A given behavior may, therefore, be mediated by 100 or fewer neurons, and this small size of the circuit makes complete description easier. Moreover, many neurons are relatively large and can be repeatedly identified as unique individuals, permitting one to examine the functional properties to a specific behavior mediated by the cell. Changes in cellular properties that occur when a behavior is modified by learning can then be related to specific changes in behavior. Molecular and biophysical events underlying the changes in cellular properties can then be determined. This approach has been particularly successful with the bee and the mollusks Aplysia, Hermissenda, Limax, and Tritonia.

Invertebrates are also excellent subjects for a genetic dissection of behavior and learning and memory. Two animals that have been particularly useful are the fruit fly Drosophila and the worm C. elegans. The basic strategy is to alter the genotype with a mutagen and to test for specific defects in the ability of the animals to learn or remember. The role of individual biochemical processes and genes then can be related to specific aspects of learning and memory.

The entries that follow discuss each of these invertebrates except APLYSIA, which is the subject of a separate section. For additional information on insect species, see INSECT LEARNING.]