THE S-CELL - AN INTERNEURON ESSENTIAL FOR SENSITIZATION AND FULL DISHABITUATION OF LEECH SHORTENING

Sensory neurons in the leech excite the S interneuron, which in turn e xcites motoneurons that shorten the leech, although activity in the S cell reportedly cannot by itself shorten the animal. Experiments were performed in semi-intact leeches using established dishabituation and sensitization protocols. S-cell activity increased during reflexive sh ortening once the animal was sensitized or dishabituated with a strong shock. S-cell activity otherwise was not associated with shortening. To test the role of the S-cell in dishabituation and sensitization of the shortening reflex, single S cells were ablated in vivo by intracel lular injections of pronase. S-cell lesions reduced but did not elimin ate dishabituation; however, sensitization was completely disrupted. T his was consistent with recent evidence that separate processes contri bute to dishabituation and sensitization. Since the S cell in each gan glion is a link in a rapidly conducting chain along the length of the animal, it may be sufficient to break the chain at a single point to e liminate sensitization.