The role of the olfactory bulb in processing sensory information

The olfactory bulb, the second order brain area in the olfactory system, functions to convert relatively slow monophasic excitatory input patterns into complex spatio‐temporal spike patterns that are conveyed to cortical regions. This conversion involves multiple aspects that are governed by the specific intrinsic and synaptic properties of olfactory bulb neurons. The intrinsic properties of mitral cells, the principal cells of the olfactory bulb, play a critical role in regulating spiking timing in response to both excitatory sensory inputs and local inhibitory inputs. Mitral cells express both A and D‐type transient K currents that play complementary roles in regulating spatio‐temporal output patterns. D‐type K channels allow mitral cells to follow the slow phasic sensory drive that occurs during sniffing and generate precisely timed action potentials on each inhalation cycle. A‐type K channels allow inhibitory postysynaptic responses to trigger rebound spikes in mitral cells. The inhibitory control of mitral cell activity also is complex with multiple types of interneurons regulating different aspects olfactory bulb excitability. While GABAergic granule cells provide the primary source of inhibition onto mitral cells, other types of inteneurons, including Blanes cells, modulate mitral cell activity indirectly through synaptic interactions of other interneurons. These second‐order interneurons can enter a stable persistent firing state, suggesting that they may play an important role in maintaining olfactory processing over multiple sniff cycles.