The mammalian olfactory bulb has a shallow, layered structure of few well-defined neuron types compared with other sensory systems. Nevertheless, it is unknown how its relatively simple circuit discriminates odors and odor concentration. The mitral cells, the primary projection neurons between the olfactory sensory neurons and the pyramidal cells of the olfactory cortex, extend lengthy lateral dendrites which make reciprocal dendrodendritic synapses with granule cell interneurons. Mitral cells excite granule cells, but granule cells inhibit mitral cells through the same synapse. It has been assumed that the role of this dendrodendritic circuit is to synchronize sets of mitral cells and also provide contrast enhancement to olfactory signals through lateral inhibition between sets of mitral cells. Computational models and theory describe how synchrony and lateral inhibition can occur when mitral cells mutually inhibit each other. Current models of olfactory processing assume mutual inhibition if two mitral cells share a granule cell. It has been shown recently, however, that granule cells which synapse in the distal dendrite do not affect the firing of the mitral cell. This implies that mutual inhibition between mitral cell pairs can only occur when the granule cell they share synapses close to the soma of each. This a bidirectional gate. When a granule cell synapses distally to the soma of one mitral cell, but proximally to the soma of another, then the mitral cell with the distal synapse can inhibit the other mitral cell, but the inverse is not true. This a unidirectional gate. Finally, a granule cell may synapse distally to both mitral cells. This is an inconsequential gate. While a granule cell may be inconsequential between two mitral cells, the granule cell may synapse proximally to a third mitral cell. This can introduce scenarios where mitral cell A and mitral cell B do not inhibit mitral cell C until both A and B are active. The goal of the proposed project is to use computational models to quantify and clarify the role of these gates between mitral cell pairs (Aim 1) and in larger networks (Aim 2). It has also been found that synapses in the distal dendrite can prevent action potentials from propagating down the lateral dendrite. The goal of Aim 3 is to explore the role of attenuating action potentials in the lateral dendrites on network dynamics. The relevance of the proposed work is to gain a better understanding of the neuronal operations of mammalian sense of smell.
| Restrepo, Diego; Doucette, Wilder; Whitesell, Jennifer D et al. (2009) From the top down: flexible reading of a fragmented odor map. Trends Neurosci 32:525-31 |
