The neuronal basis of feeding in the snail, Helisoma, with comparisons to selected gastropods

Research on identified neurons during the last quarter century was forecast at a conference in 1973 that discussed 'neuronal mechanisms of coordinatio n in simple systems.: The focus of the conference was on the: neuronal cont rol of simple stereotyped behavioral acts. Participants discussing the futu re of such research called for a comparative approach: emphasis on structur e-function interactions; attention to environmental and behavioral context: and the development of new techniques. Significantly, in some cases amazin g progress has been made in these areas, Major conclusions of the last quar ter century are that so-called simple behaviors and the neural circuitry un derlying them tend to be less simple, more flexible, and more highly modula ted than originally imagined. However. the comparative approach has, as yet , failed to reach its potential. Molluscan preparations, along with arthrop ods and annelids, have always been at the forefront of neuroethological stu dies. Circuitry underlying feeding has been studied in a handful of species of gastropod molluscs. These studies have contributed substantially to our understanding of sensorimotor organization, the hierarchical control of be havior and coordination of multiple behaviors, and the organization and mod ulation of central pattern generators. However. direct interspecific compar isons of feeding circuitry and potentially homologous neurons have been lac king. This is unfortunate because much of the vast radiation of the class G astropoda is associated with variations in feeding behaviors and feeding ap paratuses, providing ample substrates for comparative studies including the evolution of defined circuitry. Here, the neural organization of feeding i n the snail, Helisoma, is examined critically. Possible direct interspecifi c comparisons of neural circuitry and potentially homologous neurons are ma de. A universal model for central pattern generators underlying rasping fee ding is proposed. Future comparative studies can be expected to combine beh avioral, morphological, electrophysiological, molecular and genetic techniq ues to identify neurons and define neural circuitry. Digital resources will undoubtedly be exploited to organize and interface databases allowing illu mination of the evolution of homologous identified neurons and defined neur al circuitry in the context of behavioral change. (C) 2001 Elsevier Science Ltd. All rights reserved.