When the nervous system develops, its constituent neurones establish connections with certain selected elements while rejecting others in a fashion that gives the completed system the precision it requires to function. Such specificity is also manifested during regeneration in the central nervous system (CNS). For example, regeneration and recognition have been demonstrated with electrophysiological techniques in the leech CNS, where individual sensory neurones can select their normal postsynaptic targets from among apparently hundreds of alternatives, reestablishing chemical synaptic connections. The leech CNS has also provided an example of neurones reestablishing electrical continuity after surgical disruption of a connecting axon. Such electrically connected 'S cell' interneurones, one of which is situated in each of the 21 segmental ganglia of the leech, are linked at the ends of their axons by electrical synapses across which large molecules cannot pass. The synapse between two S interneurones occurs in the axon bundles (connectives) that join ganglia. The discovery of this junction and the use of the enzyme horseradish peroxidase (HRP) as an intercellular marker have permitted a detailed morphological and physiological analysis of the steps by which a regenerating axon reestablished synaptic contact with its proper target. The authors report here that one such step can be the formation of an electrical synapse between the regenerating neurone and its own severed axon segment, thereby restoring the neurone's functional integrity.
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