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Assignments “What is real? How do you define real? If you're talking about what you can feel, what you can smell, what you can taste and see, then real is simply electrical signals interpreted by your brain." - Morpheus from "The Matrix" - Complete Study Guide 1 for Quiz Section on Friday - Exam 1 coming up on Monday, October 19. This exam will cover introductory material, spot check questions on neuroscience, psychophysics, and chemical senses. - submitted by Jawad Husain Odors reach the olfactory epithelium through the nose or through the mouth The olfactory epithelium The olfactory epithelium is located on the roof of the nasal cavity. It contains the olfactory receptor cells, supporting cells, and axons that lead to the olfactory bulb. Olfactory receptor cells The olfactory epithelium The olfactory epithelium lies at the top of the nasal cavity. The axons of receptor cells enter the skull through tiny perforations in the bone. Olfactory receptor cells, unlike other sensory receptors, are true neurons. They are also the only neurons that die (life span 5-8 weeks) and are replaced. 1 Mucus performs a useful function! Olfactory receptors are true neurons Odorant molecules are picked up by special protein molecules in the mucus and transported to receptor sites on the cilia. The axons of olfactory receptors project directly to the olfactory bulb The dendrites of olfactory receptors have cilia that stick out into a layer of mucus Olfactory receptors are true neurons The axons of olfactory receptors project directly to the olfactory bulb The dendrites of olfactory receptors have cilia that stick out into a layer of mucus Olfactory capabilities vary among species • Mice have more than 1400 different types of olfactory receptor proteins, humans have about 350 different types, and fish about 60 different types. • Humans have about 10-12 million olfactory neurons, but dogs have about 4 billion! Olfactory neuron responses to odorants Binding of odorants causes a receptor potential at the dendrites, an integration signal at the cell body, and action potentials in the axon. The olfactory system of the mouse contains about 1400 different receptor proteins. Each receptor neuron expresses just one protein. Humans have about 350 different receptor proteins 2 Olfactory pathways in the CNS The olfactory epithelium Has no clear topography of receptor types, but all receptors of the same type project to the same glomerulus. This pattern of projections creates a map that is conserved up to the level of the cortex. Olfactory nerve to olfactory bulb Olfactory bulb to olfactory cortex Olfactory cortex to olfactory region of thalamus Thalamus to olfactory cortex Buck & Axel, 1991 Olfactory pathways in the CNS: Another view Orbitofrontal cortex is a site for integration across multiple sensory modalities The olfactory system is the only sensory system that synapses in the cortex before the thalamus. How is information about odor represented in the nervous system? One idea is that a different population of neurons is active for each odor (or class of odor). However, it cannot be this simple because each olfactory receptor, nerve fiber and central olfactory neuron responds to multiple odorants. How is information about odor represented in the nervous system? One idea is that a different population of neurons is active for each odor (or class of odor). However, each olfactory nerve fiber and central olfactory neuron responds to multiple odorants. The spatial distribution of activity in the salamander olfactory bulb is the same for three very different odors (although absolute levels of activity are different). 3 A single olfactory receptor or nerve fiber responds to many different odorants As in the taste system, the temporal pattern of response is different for different odorants. Interaction between taste and smell How is information about odor represented in the nervous system? Another idea is that each odor elicits a unique pattern of activity across time as well as space. Olfactory preferences are affected by many factors including what has recently been eaten White bars = taste only Black bars = taste + smell Age differences in olfactory performance Sex differences in olfactory performance It!s not as bad as the graph makes it look! The 99-year olds got over half of the odors right. 4