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Chapter 13

Chapter 13 - PSYB51

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Department
Psychology
Course
PSY100H1
Professor
Mathias Niemier
Semester
Summer

Description
Chapter 13 – Olfaction  Humans have 2 main chemical detection systems – one for molecules floating in air, and another for molecules that we put in mouths  Olfaction: sense of smell  Gestation: sense of taste  Another chemical sensing system important for both smell and taste is trigeminal system, innervated by trigeminal nerve o Trigeminal system enables us to feel gustatory and olfactory experiences Olfactory Physiology  Odors and Odorants o Odors: general smell sensation of particular quality. By contrast, when referring to specific chemical entity, term odorant should be used o Odorant: any specific aromatic chemical -22 o To be smelled, molecule must be volatile (able to float through air), small (less than 5grams)0 and hydrophobic (repellent to water). Excpetions – CH and CO 4  Human Olfactory Apparatus o Human olfactory system is tacked onto an organ that serves another purpose o Primary function of nose is to filter, warm and humidify air we breathe o Olfactory cleft: narrow space at back of nose into which air flows, where main olfactory epithelium is located o Olfactory epithelium: secretory mucosa in human nose whose primary function is to detect odorants in inspired air. Located on both sides of upper portion of nasal cavity and olfactory clefts, olfactory epithelium contains 3 types of cells – olfactory sensory neurons, basal cells and supporting cells. Each epithelium measures about 1-2 square inches (depending on size of nose)  Supporting cells: 1 of 3 types of cells in olfactory epithelium. Cell type provides metabolic and physical support for OSNs  Basal cells: precursor cells to olfactory sensory neurons  Olfactory sensory neurons (OSN): main cell type in olfactory epithelium. OSNs are small neurons located beneath watery mucous layer in epithelium. Cilia on OSN dendrites contain receptor sites for odorant molecures o Cilia: hairlike protrusions on dendrites of olfactory sensory neurons. Receptor sites for odorant molecules are on cilia, which are first structures involved in olfactory signal transduction o Olfactory receptor (OR): region on cilia of olfactory sensory neurons where odorant molecules bind o Interaction between odorant and OR stimulates cascade of biochemical events producing action potential that’s transmitted along axon of OXN to olfactory bulb o To initiate action potential, 7-8 odor molecules must bind to receptor and takes about 40 for smell sensation to be reported o Have ~20 million OSNs, split between epithelia of right and left nostrils – more than any other sensory system except vision and enable to distinguish thousands of odors o Axons on ends of OSNs opposite cilia (dendrites) pass through tiny sievelike holes of cribriform plate o Cribiform plate: bony structure riddled with tiny holes at level of eyebrows that nose from olfactory sensory neurons pass through tiny holds of cribiform plate to enter brain o Anosmia: total inability to smell, most often resulting from sinus illness of head trauma o Stem cells in olfactory epithelium can form new OSNs and all OSNs die and regenerate once every 28 days. o Most common cause of olfactory loss is upper respiratory tract infection  Second most common cause is sinonasal disease followed by head trauma; only 30% of anosmias are caused by head trauma  30% of anosmias are caused by head rauma  Likelihood of recovery is best when anosmia is caused by infection or disease o Connection between psychiatric depression and sense of smell – people who lose smell can fall into clinically serious depressive states o Olfactory nerve: cranial nerve I. axons of olfactory sensory neurons bundle together after passing through cribiform plate to form olfactory nerve o Olfactory bulb: blueberry sized extension of brain just above nose, where olfactory info is first processed. 2 olfactory bulbs, one in each brain hemisphere, corresponding to right and left nostrils o Ipsilateral: same side of brain, meaning that right olfactory bulb gets info from right nostril and left olfactory bulb gets info from left nostril o Within olfactory bulb are globular tangles of axons from OSNs that are synapsed with dendrites from mitral cells and tufted cells o Mitral cells: main projective output neurons in olfactory bulbs o Tufted cells: secondary class of output neurons in olfactory bulbs o Glomeruli: spherical conglomerates containing incoming axons of olfactory sensory neurons. Each OSN converges onto 2 glomeruli (1 medial, 1 lateral) o Molecular genetic studies in mice have shown that all neurons expressing particular OR type, no matter where they are on nasal epithelium, converge onto one glomerulus pair o OSNs converge onto small number of glomeruli o Central brain structures that process olfactory info from olfactory bulb include  Primary olfactory cortex: neural area where olfactory info is first processed, which includes amygdale-hippocampal complex and entorhinal cortex  Amygdale-hippocampal complex: conjoined regions of amygdale and hippocampus, which are key structures in limbic system. Complex is critical for unique emotional and associative properties of olfactory cognition  Entorhinal cortex: phylogenetically old cortical region that provides major sensory association input o Limbic system: encompassing group of neural structures that includes olfactory cortex, amygdala, hippocampus, piriform cortex, and entohinal cortex. Limbic system is involved in many aspects of emotion and memory. Olfaction is unique among senses for direct and intimate connection to limbic system o Olfactory receptor cells are different from all other sensory receptor cells in that they aren’t mediated by protective barrier and instead make contact with brain o One consequence of fact that olfactory sensory neurons are direct conduits into brain is that many drugs can be inhaled – OSNs are thinnest and slowest in body o Lag time between sniffing and brain’s registering a smell varies - ~400ms o Distinctions bring up subtle differences between sensation and perception in olfaction  Genetic Basis of Olfactory Receptors o Certain specific chemicals will smell stronger to one person than they do to another – perception may be different o In general, more receptors expressed, more sensitive one is to odorants o Genetic variation in receptor expression may be one innate factor in modulation of odor likes and dislikes o Gilad observed that Old World primate species have about 30% OR pseudogenes, but most New World exception is howler monkey which also has 30% OR pseudogenes – howler monkey has something in common with Old World primates – trichromatic vision  Feel of Scent o Trigeminal nerve: Cranial nerve V. which transmit info about ‘fee’ of odorant o In many cases, impossible to distinguish between sensations traveling up cranial nerve I from olfactory receptors and traveling up cranial nerve V from somatosensory receptors o Trigeminal stimulation accounts for why our eyes tear when we chop onions and why we sneeze when we sniff pepper o High levels of trigeminal stimulation can produce severe burning sensation, and trigeminal achivity has been linked too facial head pain felt in migraine headaches From Chemicals to Smells  Theories of Olfactory Perception o Best accepted biochemical theory is based on match between shapes of odorants and odor receptors – shape pattern theory o Shape pattern theory: current dominant biochemical theory for how chemicals come to be perceived as specific odorants. Shape pattern theory contends that different scents – function of odorant shape to OR-shape fit – activate different arrays of olfactory receptors in olfactory epithelia. Various arrays produce specific firing patterns of neurons in olfactory bulb, which determine particular scent we perceive o Most recent molecular research suggests scents are detected by means of combinatorial code o Different scents activate different arrays of olfactory receptors in olfactory epithelia, producing specific firing patterns of neurons in olfactory bulb – specific pattern of electrical activity in olfactory bulb then determines particular scent we perceive o Specific anosmia: inability to smell one specific compound amid otherwise normal smell perception o Most specific anosmias are to steroidal musk compounds and condition appears to be genetic – 50% of population has a specific anosmia to androstenone o Presumed that specific anosmia and various perceptions of chemical androsternone are genetically determined o Individuals with specific anosmia, receptors that detect androstenone are nonfunctional 9coded by psudogenes0 and different receptors detect compound in those who perceive floral scent than those who perceive ruinous odor o Studies have shown that increased sensitivy to some common odorants – benzaldehyde and citralva can be induced through different exposure to these chemicals o Stereoisomers: isomers in which spatial arrangement of atoms are mirror like rotations of one another – optical isomers o According to shape theory, different arises because rotated molecules don’t fit same receptors o Vibration theory can’t explain why stereoisomers smell different, because vibration of stereoisomers is same o It may be that neither shape pattern nor vibration theory is complete theory of smell as shape pattern theory currently has better general explanatory value, but true that all molecules vibrate and all receptors are made of atoms that vibrate and all receptors are made of atoms that vibrate  Importance of Patterns o We can discriminate many t
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