Olfaction: Feature Detection and Integration

Odorant Feature Detection

Every time we inhale a breath, volatile molecules emitted from odor sources travel up our nostrils and interact with our olfactory epithelium. Located within the epithelium is a thin sheet containing millions of olfactory sensory neurons. Each one of these neurons expresses a single receptor protein type that recognizes and binds with certain molecular structures or features of odorant molecules, such as hydrocarbon chain length or functional group. This feature allows individual odor molecules to interact with a large number of different receptor types in the epithelium. The unique set of receptor neurons that are activated by an odor [Page 669]forms the basis of an odor identity code that is projected to the olfactory bulb.

Within the bulb, sensory neuron axons synapse with bulbar output neurons in distinct spherical regions known as glomeruli that spread out across the entire surface of the olfactory bulb. Each glomerulus only receives input from sensory neurons expressing the same receptor type. This organization allows for odors to be represented in the initial stages of olfactory processing as a set of glomeruli based upon the set of receptors activated by that odor. Thus, each odor creates a specific topographical map of glomerular activity across the surface of the olfactory bulb. However, these maps can be become quite complex and highly overlapping, especially when odors contain similar molecules or features. In this case, the input map of glomerular activity must be fine-tuned by subsequent processing in order for similar odors to be perceived as distinct.

This process begins at the first synapse in the olfactory bulb with a complex network of excitatory and inhibitory neurons that surround each glomerulus (see color insert, Figure 9). This interneuronal network affects odor maps in two main ways. First, a subset of periglomerular neurons that receive direct sensory input provide feedback inhibition back onto the sensory axons within each glomerulus. This is accomplished by blocking neurotransmitter release from the activated sensory neurons and effectively damping the input. This mechanism serves as a way for the system to control the overall strength of the initial input into the bulb. Second, another group of juxtaglomerular neurons excite the inhibitory neurons surrounding adjacent glomeruli, forming a lateral inhibitory network. This center-surround inhibition allows strongly activated glomeruli to block or reduce the excitation of the output neurons associated with other similarly responding glomeruli. By only allowing the strongly activated sensory neurons to pass on their odor information to higher processing levels, glomerular inhibition serves to reduce representational overlap between similar odors and sharpen odor maps at the glomerular level.

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