Abstract
Sensory discrimination requires distributed arrays of processing units. In the olfactory bulb, the processing units for odor discrimination are believed to involve dendrodendritic synaptic interactions between mitral and granule cells. There is increasing anatomical evidence that these cells are organized in columns, and that the columns processing a given odor are arranged in widely distributed arrays. Experimental evidence is lacking on the underlying learning mechanisms for how these columns and arrays are formed. To gain insight into these mechanisms, we have used a simplified realistic circuit model to test the hypothesis that distributed connectivity can self-organize through an activity-dependent dendrodendritic synaptic mechanism. The results point to action potentials propagating in the mitral cell lateral dendrites as playing a critical role in this mechanism. The model predicts that columns emerge from the interaction between the local temporal dynamics of the action potentials and the synapses that they activate during dendritic propagation. The results suggest a novel and robust learning mechanism for the development of distributed processing units in a cortical structure.
Highlights
A recent development in studies of olfactory bulb functional organization is the recognition of the distributed nature of the circuits that process odor maps elicited in the glomerular layer (Cleland and Sethupathy, 2006; Migliore and Shepherd, 2008; Willhite et al, 2006)
The results suggest that different temporal patterns of action potential propagation through the mitral cell lateral dendrites (Egger and Urban, 2006; Migliore and Shepherd, 2008) lead to selective potentiation or depression of mitral–granule synapses belonging to different glomerular units
We focus on the two recording locations indicated by the symbols in Figure 2a, the mitral cell soma and the lateral dendrite at 350 mm from the soma, during the representative odor input activated at constant strength and constant frequency
Summary
A recent development in studies of olfactory bulb functional organization is the recognition of the distributed nature of the circuits that process odor maps elicited in the glomerular layer (Cleland and Sethupathy, 2006; Migliore and Shepherd, 2008; Willhite et al, 2006). While the properties of these synapses have been extensively studied (Balu et al, 2007; Chen et al, 2000; Egger et al, 2005; Isaacson and Vitten, 2003; Lowe, 2002; Pinato and Midtgaard, 2005; Schoppa et al, 1998; Shoppa and Westbrook, 1999), to our knowledge there is no experimental evidence of long term plasticity changes (for short term effects, see Dietz and Murthy, 2005) These changes must occur in order to account for the distributed connectivity between mitral and granule cells that is observed in the tracing (Willhite et al, 2006) and recording (Urban and Sakmann, 2002) experiments. We hypothesize that during development odor exposure is sufficient to potentiate or depress these synapses in such a way as to form the appropriate connectivity between separated glomerular units that is observed
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