Sensitization is a form of nonassociative learning in which a strong o
r noxious stimulus persistently enhances the response produced by a we
aker stimulus. In the leech Hirudo medicinalis, the S-interneuron is r
equired for sensitization of the shortening response. A single S-cell
axon was surgically separated from its sole synaptic partner, the neig
hboring S-cell, This consistently eliminated sensitization without imp
airing reflexive shortening itself, as measured in semi-intact specime
ns. Sensitization of the shortening reflex returned after 3 weeks when
the severed axon grew and regenerated its specific electrical synapse
within tile nerve cord, as shown by restored conduction of impulses b
etween S-cells. This confirms the essential role of one neuron, the S-
cell, in sensitization, and it demonstrates that regeneration of the s
ynapse between S-cells restores this example of nonassociative learnin
g.