Relative roles of the S cell network and parallel interneuronal pathways in the whole-body shortening reflex of the medicinal leech.

The whole-body shortening reflex of the medicinal leech Hirudo medicinalis is a withdrawal response produced by anterior mechanical stimuli. The interneuronal pathways underlying this reflex consist of the S cell network (a chain of electrically coupled interneurons) and a set of other, parallel pathways. We used a variety of techniques to characterize these interneuronal pathways further, including intracellular stimulation of the S cell network, photoablation of the S cell axon, and selective lesions of particular connectives (the axon bundles that link adjacent ganglia in the leech nerve cord). These experiments demonstrated that the S cell network is neither sufficient nor necessary for the production of the shortening reflex. The axons of the parallel pathways were localized to the lateral connectives (whereas the S cell axon runs through the medial connective). We used physiological techniques to show that the axons of the parallel pathways have a larger diameter in the anterior connective and to demonstrate that the parallel pathways are activated selectively by anterior mechanosensory stimuli. We also presented correlative evidence that the parallel pathways, along with activating motor neurons during shortening, are responsible for inhibiting a higher-order "command-like" interneuron in the neuronal circuit for swimming, thus playing a role in the behavioral choice between swimming and shortening.