The ability to detect odors varies widely across the animal world.The numbers game can be misleading, but there is a general correspondence between the number of olfactory neurons (ONs)—cells specialized for odorant detection—that an animal has and the probability that any given odorant will come in contact with an appropriate receptor. Humans have approximately 5-10 million olfactory neurons in the olfactory epithelium, enabling us to detect an estimated 10,000 or more different odorants, but this is a pittance when compared with dogs, who have more than 200 million ONs. By contrast, Drosophila have about 1,300 ONs. For many years, the conventional wisdom was that birds do not have much of a sense of smell, but recent work suggests that the acuity of the avian sense of smell is related to the size of a bird's olfactory bulb relative to its cerebellum (Malakoff 1999). In some species, such as songbirds, it is very small, on the order of 3 percent of the size of the cerebellum, whereas in others, such as many seabirds, it is closer to 40 percent. Although olfaction in aquatic species is of water-borne chemicals, the process works in a largely similar way. It is thought that salmon, a fish that returns to its birthplace every year to spawn, may find their way home with a well-developed sense of smell and a form of imprinting (Barinaga 1999).
Mammalian odor detection begins with olfactory receptors embedded in the olfactory membrane on the roof of the nasal cavity (see Figure 13-1). The olfactory membrane is composed of three layers of cells: (1) supporting cells, in which (2) olfactory receptors cells are embedded, and (3) basal cells, which produce mucus and which are the source of new olfactory receptors. One end extends from the layer of supporting cells to the central nervous system (CNS) as an afferent neuron, and the other end extends to the epithelium where it forms a knob, with cilia projecting from it. This makes the olfactory epithelium the most direct connection from the outside world to the CNS. Another distinction is that, unlike most neurons, olfactory receptors are continuously replaced during life. Olfactory receptor cells are bipolar.
Odorant molecules are detected by transmembrane receptor proteins in the cilia, which are covered by mucus produced by the supporting cells. Odorants in the air or (for aquatic animals) in the water diffuse through the mucus layer to reach the receptor, although hydrophobic odorants must be transported. Among the soluble proteins found in the aqueous medium around the olfactory receptor neurons are odorant-binding proteins (OBPs), to be discussed below. The detection of the presence of an odorant is based on ligand-receptor binding and depends on a class of diverse receptor genes as well as the number of sensory ONs.
How can an organism detect (have the right ligand for) smells it could not have been preprogrammed to detect? The discovery of odorant (or olfactory) receptor (OR) genes, made first in rats in 1991 by Buck and Axel (Buck and Axel 1991), opened the way to the understanding of the molecular basis of olfaction, which has become a model system for many aspects of sensory neurobiology. ORs are found in the cell surface of ONs in the olfactory epithelium and are encoded by members of the 7TMR G protein-coupled receptor (GPCR) gene family, which also includes opsin genes for photoreception and various hormones and many other receptors. The familiar general structure of a typical olfactory receptor is shown in Figure 13-2A.
Pyriform and entorhinal cortex (primary olfactory cortex)
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