Rhythm Generator Circuits in a Simple Nervous System

Abstract

Publisher Summary The comparatively simple central nervous system of the leech consists of a nerve cord of 32 segmentally iterated ganglia. The rostral four and caudal seven segmental ganglia are fused, constituting a head and a tail ganglion, respectively. The other, unfused, or abdominal ganglia, are numbered rostrocaudally from 1 to 21. Each contains about 400 bilaterally paired neurons, as well as a few unpaired neurons. Sensory and effector neurons project their processes to targets outside the CNS via segmental nerves, whose roots emerge from the lateral edge of the ganglion. The anatomy of the leech ganglion is sufficiently stereotyped so that about one-quarter of the neurons of the segmental ganglion of the medicinal leech, Hirudo medicinalis have by now been identified according to various criteria, including function. Thus, some cells have been classified as sensory, others as motor neurons, and yet others as interneurons, and their connectivity has been elucidated. The most complex behavioral routines of the leech, of which accounts have so far been given in terms of networks of identified neurons, are the heartbeat and swimming rhythms.