The Leech Local Bending Circuit

William B. Kristan, Jr.

in Handbook of Brain Microcircuits

Published on behalf of Oxford University Press

Published in print August 2010 | ISBN: 9780195389883
Published online September 2012 | e-ISBN: 9780199965137 | DOI:
The Leech Local Bending Circuit

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  • Development of the Nervous System


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The leech is an annelid, a segmented worm, which has a fixed number of segments (32), twenty-one of which are a series of roughly identical midbody segments. The local bend circuitry is repeated in each of the segmental ganglia associated with these segments. The basic circuit consists of three layers of neurons: pressure-sensitive mechanoreceptive neurons (P cells), which excite a layer of local bend interneurons (LBIs), which in turn excite motor neurons (MNs). There are four P cells per ganglion, seventeen identified LBIs, and twenty-two longitudinal MNs. This chapter shows that the connections from the LBIs onto MNs is all to all, with every LBI making a connection to every MN, again with decreasing strengths from LBIs with progressively more distant receptive fields. All the lateral inhibition, therefore, is provided by the connections of iMNs onto eMNs. The iMNs receive inputs from LBIs as though they were eMNs to the opposite side: dorsal iMNs are most strongly excited by inputs from the opposite ventral P cell, and the ventral iMNs get most strongly excited by the dorsal P cell on the opposite side.

Chapter.  1711 words.  Illustrated.

Subjects: Development of the Nervous System

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