The sensory space

The sense of smell is a primeval sense, originating in early single-cell organisms. In principle, it functions by taking a sample of the ambient environment and analyzing its chemical contents. In air-breathing organisms, volatile odorant enter the nasal cavity, where the primary organ of smell, the olfactory epithelium, resides. This pseudostratified neuroepithelium contains 10-100 million bipolar sensory neurons, each having a few dozen mucus-bathed hair-like cellular extensions known as olfactory cilia. The ciliary membranes harbor the olfactory receptor (OR) proteins , as well as components responsible for the chemoelectric transduction process. ORs have all been identified as belonging to the 7-transmembrane superfamily of G-protein coupled receptors. The stereospecific binding of odorant molecules to the ORs initiates a cascade of biochemical events that result in action potentials that reach higher brain centers. The number of distinct types of ORs, r, called the olfactory repertoire size, is believed to be around 1000 in all mammals .Only recently, the full sequence of more than 900 human OR genes has been reported, based on genomic databases . Only about 300 of them are functional in humans, and the rest are pseudogenes. However, in other mammals

the pseudogene fraction could be much smaller. The recognition of odorant molecules occurs in the brain by a non-covalent binding process akin to that encountered in many other receptor types, including hormone and neurotransmitter receptors. However, while for “standard" receptors there is usually only one, or very few, natural ligands, olfactory receptors are functionally promiscuous. Therefore, when an odorant (o; c) approaches the epithelium, it interacts with many receptor types, and can be characterized by the vector




with Ri(o; c) being the response of the i'th type of receptor molecule to the odorant (o; c).We deliberately do not specify the details of the response, which can be the fraction of bound receptors, the concentration of some second messenger, or some other relevant entity. It is often, in fact, a dynamic function of time. We shall see later that the exact definition of Ri(o; c) is irrelevant to our algorithm. The r-dimensional odorant vector dB(o; c) describes the way by which the biological sensory machinery responds to the odorant, so that terming this odor space the sensory space is appropriate. An important observation is that all the 105 to 106 OR molecules in the same sensory cell are of the same type, and thus r is also the number of distinct types of olfactory sensory neurons.

The olfactory neurons send their axons to the olfactory bulb (OB), passing in bundles through the cribriform plate. Here, the first, and rather significant stage of the higher processing takes place. It is widely believed that important aspects of odor quality and strength (concentration) perception are carried out in the OB, and studies have in fact shown that the OB responds with odor-specific spatio-temporal patterns. Successive stimulations with the same odorant have been shown to lead to reproducible patterns of activity. Patterns evoked by low concentrations were

topologically nearly identical to those evoked by high concentrations, but with reduced signal amplitude. Within the OB, the OR axons form contacts with secondary neurons inside ellipsoidal synaptic conglomerates, called glomeruli. A glomerulus serves as a synaptic target for neurons expressing only a single OR type. Consequently, it is not surprising that the number of glomeruli, estimated to be between 1000 and 2000, is of the same order of magnitude as r. From our point of view, the important conclusion is that the OB is stimulated by approximately r distinct types of nerve cells, which tells us that the entire olfactory pathway is triggered by the vector dB(o; c).