To beat or not to beat: A decision taken at the network level

The cells of the inferior olivary nucleus, the sole source of the cerebella r climbing fibers, form a network of electrically coupled neurons. Experime ntal observations show that these neurons produce a large repertoire of ele ctrical signals, among which sub-threshold oscillations of the membrane pot ential. Simultaneous recordings from pairs of neurons and optical imaging o f voltage sensitive dyes show that sub-threshold activity occurs in synchro ny throughout the network The mechanism underlying the generation of the su b-threshold oscillations is not fully understood. Experimental observations suggest that the electrical coupling is essential but insufficient for the ir generation. Several theoretical mechanisms have been suggested to explai n these observations. Up-to-date, the most realistic model is the heterogen eity model, that assumes a certain degree of heterogeneity among olivary ne urons. The heterogeneity model proposes that sub-threshold oscillations are produced by electrical coupling of neurons with the same types of ionic co nductances, but with different densities. The variability in channel densit ies yield neurons of different functional types. The main prediction of the model is that different functional types of neurons should be found in the inferior olive. Dynamic clamp experiments support this prediction. (C) 200 0 Elsevier Science Ltd. Published by Editions scientifiques el medicales El sevier SAS.