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First published online January 19, 2006
Journal of Experimental Biology 209, 493-503 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02030

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Functions of the subesophageal ganglion in the medicinal leech revealed by ablation of neuromeres in embryos

Andrea Cornford¹, William B. Kristan, III^1,2, Sierra Malnove¹, William B. Kristan, Jr¹ and Kathleen A. French^1,*

¹ Division of Biological Sciences, Neurobiology Section, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0357, USA
² Department of Biological Sciences, California State University, San Marcos, San Marcos, CA 92096, USA

^* Author for correspondence (e-mail: kfrench{at}ucsd.edu)

Accepted 12 December 2005

Two general trends in the evolution of the nervous system have been toward centralization of neuronal somata and cephalization of the central nervous system (CNS). These organizational trends are apparent in the nervous system of annelid worms, including leeches. To determine if the anterior brain of the leech serves functions similar to those of the brains of more complex organisms, including vertebrates, we ablated one of the two major regions of the cephalic brain - the subesophageal ganglion (SubEG). For anatomical reasons, ablations were performed in embryos, rather than in adults. At the end of embryonic development, we observed the leeches' spontaneous behaviour and their responses to moderate touch. We observed that, although the midbody ganglia of the leech CNS display a high degree of local autonomy, the cephalic brain provides generalized excitation to the rest of the CNS, is a source of selective inhibition that modulates behaviour, integrates sensory information from the head with signals from the rest of the body, and plays an important role in organizing at least some complicated whole-body behaviours. These roles of the leech cephalic brain are common features of brain function in many organisms, and our results are consistent with the hypothesis that they arose early in evolution and have been conserved in complex nervous systems.

Key words: annelid, anterior brain, central nervous system, cephalic brain, Hirudo medicinalis, leech, subesophageal ganglion