The electrical connectivity in the inferior olive (IO) nucleus plays an important role in generating well-timed spiking activity. Here we combined electrophysiological and computational approaches to assess the functional organization of mice IO nucleus. Spontaneous fast and slow subthreshold events were commonly encountered during in vitro recordings. We show that the fast events represent a regenerative response in unique excitable spine-like structures in the axon hillock, whereas the slow events reflect the electrical connectivity between neurons (‘spikelets’). Recordings from cell pairs revealed the synchronized occurrence of distinct groups of spikelets; their rate and distribution enabled an accurate estimation of the number of connected cells and is suggestive of a clustered organization. This study thus provides a new perspective on the functional and structural organization of the olivary nucleus, insights into two different subthreshold non-synaptic events, and a novel experimental and theoretical approach to the study of electrically-coupled networks.
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