Hockenbury Discovering Psychology 5th txtbk

106 CHAPTER 3 Sensation and Perceptionolfactory bulb(ole-FACK-toe-ree) The enlarged ending ofthe olfactory cortex at the front of the brainwhere the sensation of smell is registered.How We Smell (Don’t Answer That!)The sensory stimuli that produce our sensation of an odor are molecules in the air.These airborne molecules are emitted by the substance we are smelling. We inhalethem through the nose and through the opening in the palate at the back of thethroat. In the nose, the molecules encounter millions of olfactory receptor cellslocated high in the nasal cavity.Unlike the sensory receptors for hearing and vision, the olfactory receptors areconstantly being replaced. Each cell lasts for only about 30 to 60 days. NeuroscientistsLinda Buck and Richard Axel won the 2004 Nobel Prize for their identificationof the odor receptors that are present on the hairlike fibers of the olfactory neurons.Like synaptic receptors, each odor receptor seems to be specialized to respondto molecules of a different chemical structure. When these olfactory receptor cellsare stimulated by the airborne molecules, the stimulation is converted into neuralmessages that pass along their axons, bundles of which make up the olfactory nerve.So far, hundreds of different odor receptors have been identified (Mombaerts,2004). We don’t have a separate receptor for each of the estimated 10,000 differentodors that we can detect, however. Rather, each receptor is like a letter in anolfactory alphabet. Just as different combinations of letters in the alphabet are usedto produce recognizable words, different combinations of olfactory receptors producethe sensation of distinct odors. Thus, the airborne molecules activate specificcombinations of receptors. In turn, the brain identifies an odor by interpreting thepattern of olfactory receptors that are stimulated (Shepherd, 2006).As shown in Figure 3.9, the olfactory nerves directly connect to the olfactorybulb in the brain, which is actually the enlarged ending of the olfactory cortex at thefront of the brain. Axons from the olfactory bulb form the olfactory tract. Theseneural pathways project to different brain areas, including the temporal lobe andstructures in the limbic system (Shepherd, 2006). The projections to the temporallobe are thought to be part of the neural pathway involved in our conscious recognitionof smells. The projections to the limbic system are thought to regulate ouremotional response to odors.The direct connection of olfactory receptor cells to areas of the cortex and limbicsystem is unique to our sense of smell. As discussed in Chapter 2, all other bodilyBrainOlfactory bulbOlfactory nerveFigure 3.9 (a) The Olfactory System Inhaled throughthe nose or the mouth, airborne molecules travel to thetop of the nasal cavity and stimulate the olfactory receptors.When stimulated, these receptor cells communicateneural messages to the olfactory bulb, which is part ofthe olfactory cortex of the brain, where the sense of smellis registered.(b) The Olfactory Receptor Cells More than 5 millionolfactory neurons make up the moist, mucus-bathed tissueat the back of the nose. Projecting from the olfactoryneurons are the fiberlike olfactory hairs, which can beclearly seen in this photograph. Olfactory neurons arereplaced every month or two.Airborneodor molecules(a)Porous portionof skullOlfactoryreceptorcells(b)