Journal of Neuroscience, Vol 2, 972-985, Copyright © 1982 by Society for Neuroscience
Segmental specialization of a leech swim-initiating interneuron, cell 2051
JC Weeks
The physiological and anatomical properties of an unpaired intersegmental
interneuron designated cell 205 are described. Cell 205 is unusual among
leech neurons in that it combines a variety of functional properties in a
single cell. Constant current depolarization of cell 205 initiates and
maintains swimming behavior in semi-intact leeches or the swim motor
pattern in brainless, isolated nerve cords. During swim episodes elicited
by other stimuli, cell 205 is rhythmically active. Current pulses passed
into cell 205 during swimming reset the pattern, indicting that it has
access to, or may be a member of, the swim central pattern generator (CPG).
Cell 205 is the first interneuron in this system to exhibit both
swim-initiating and CPG functions. Individual touch, pressure, and
nociceptive primary mechanoreceptor neurons polysynaptically excite cell
205, which, in addition, is coupled electrically to the multimodal S
interneuron. These inputs may contribute to the initiation and/or
modulation of swimming in response to sensory stimuli. Cell 205 shares some
common synaptic inputs and outputs with the only other known
swim-initiating interneuron, cell 204, but the two cells differ
fundamentally in that cell 204 exerts only a tonic effect on the CPG. No
synaptic interactions were found between cells 204 and 205, but their
excitatory effects on swimming summate. Unlike other swim neurons which are
segmentally repeated, cell 205 generally is present only in segment 9, and
numerous lines of evidence suggest that it is, in fact, a segmentally
differentiated homolog of cell 204.