I Olfaction A Perceiving Odors Chapter 13 1 odorants are molecules that olfactory receptors recognize and respond to by producing neural signals that the brain represents as perceptions of different odors 2 odors are primarily produced by organic substances and all terrestrial plants at normal environmental temperatures B Odor Detection 1 the concentration of an odorants factors into the detection a concentration is measured in the number of odorant molecules per 1 million molecules b concentration depends on source location and confinement c concentration also effects the identification of odor 2 perceived intensity generally increased as the concentration increases C Thresholds 1 strength of an odor is described as the detection threshold 2 concentration must change substantially before people notice a difference in the intensity D Identifying and Discriminating Odors 1 the ability to identify odors as well as the ability to discriminate between similar odors depends not just on context but also on training and experience 2 training and specialization can improve ability to identify odors but not necessarily in detection 3 most relations between smells and memory are formed in the earliest decade of life II Olfactory Impairments A Age 1 olfactory performance declines with ages just as all other senses 2 dramatic decrease typical around age 70 stronger in men 3 anosmia is the loss of smell a smokers at high risks but can recover lost ability 1 a genetic mutation that leads to an inability to perceive pain can also lead to anosmia B Genetic Mutation C Injury 1 injury to the anatomical structures that support olfaction can cause anosmia 2 when receptor neurons are damaged they degenerate over a period of several days 3 stem cells in the nose can become new receptors and reconnect the nose to the brain III Molecular Aspects of Odors A cross Adaptation 1 olfaction is especially sensitive to change but once identified and responded to the scent s presence becomes less important 2 repeated exposure to an odor reduces sensitivity to it but it can also reduce sensitivity to other similar odors known as cross adaptation 3 in an experiment with rats investigation time was greatest for test odorants that were most dissimilar to the adapted odorant a the study suggests that the physical property of molecules is atleast partly responsible for perceptual similarity 4 ORNs have only one type of receptor on their cilia Check Your Understanding What are two factors that determine whether a particular type of molecule will evoke a response in a normally functioning olfactory system 1 whether it s a recognizable odor molecule lipid soluble and whether the concentration is high enough to reach or exceed the detection threshold If two odorants are present at equal concentrations which is more likely to be perceived the odor with the higher detection threshold or the odor with the lower detection threshold The odor with the lower detection threshold T F People who smoke heavily for 10 years or more typically suffer severe permanent olfactory impairments False damage is repaired Olfaction receptors can regenerate from stem cells in the nasal cavity Which choices will make the following statement false training and specialization help people detect identify remember discriminate odors 1 Detection doesn t improve with training and 2 Remember an odor is possible without training B Neural Basis of Odor Perception 1 turbinates bony convolutions of tissue disperse the air evenly throughout each nasal cavity 2 nasal passages join at the pharynx 3 transduction of odorant molecules into neural signals is carried out by olfactory receptor neurons embedded in the olfactory epithelium located a few centimeters behind each eye a each ORN dies after a few weeks and is replaced by a new ORN generated by basal cells b bowman s glands also in epithelium secrete mucus to wash out odorant molecules from the epithelium c mucus also provides protection from irritants and against harmful microorganisms C Transduction 1 each ORN has hairlike cilia projecting into the mucus each covered with olfactory receptors that bind with odorant molecules that dissolve into the mucus 2 olfactory receptors have GPCR ion channels like vision 3 when the membrane opens positive calcium and sodium ions flow in 4 depolarization generates action potentials 5 ORN axons form olfactory nerve that travel to the olfactory bulb through a grid of tiny holes in the cribriform plate 6 ORN axons enter the glomeruli where they make synapses with the dendrites of two types of relay neurons a mitral cells and tufted cells form the olfactory tract D Olfactory Receptors 1 humans have approximately 350 receptors for olfaction 2 each ORN has only one type of receptor on its cilia so there are also about 350 types of ORNS 3 humans have about 10 20k of each type of ORNs who send their axons to about 5000 glomeruli in the olfactory bulb E Coding 1 an estimated 4 of human DNA is devoted to coding for the 350 olfactory receptors 2 olfaction operates on a population code similar to that of vision a based on relations between 350 receptors instead of 3 3 the pattern of ORN responses the particular subset of ORN types activated and the relative strength of their responses determines how an odorant smells 4 responses can be quite different for molecules with similar structure F Upper Processing 1 the piriform cortex is consider the primary olfactory cortex a dedicated purely to olfaction b anterior piriform cortex produces representations of the chemical structure of odorant molecules c posterior piriform cortex produces representations of the quality of an odor as a whole regardless of whether the odor is simple or complex activity in OFC and amygdala didnt reflect differences d evidence of the APC PPC distinction found using fMRI information 2 also involves amygdala and entorhinal cortex involved in other nonolfactory functions a amygdala crucial in emotional responses b most effectively activated by pleasant and unpleasant smells as opposed to visual or auditory information Check Your Understanding Which choices match up to make two different correct statements A olfactory receptor neurons B mitral cells and tufted cells form the C olfactory tract D olfactory nerve A D and B C Why is the piriform cortex considered the primary olfactory cortex and how do the APC and PPC differ in the kind of olfactory information they process its the only cortex that receives