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DENT/OBHS 131 Neuroscience taste & smell 2009 Learning Objectives 1. Describe the cranial nerve / brainstem sensory innervation of the taste buds / tongue 2. Explain the general ionic mechanism of taste cell excitation 3. Identify the cortical regions important for primary gustation 4. Compare and contrast olfaction with other sensory modalities, including its cranial nerve and nature of projection to cortex 5. Discuss how sub-modalities of taste and smell are sorted as they ascend to the cortex 6. Appreciate that taste / smell and other sensory modalities are combined at the level of the cortex along with limbic information to produce full sensation of flavor Gustatory & olfactory systems • Extract information from chemicals in the environment • G-protein coupled receptors • Taste: (+ ion channels) • Taste and olfactory receptor cells undergo continual lifetime turnover • Taste: modified epithelial cells • Smell: neurons • Stimulus information is encoded in populations of neurons Significance • Emotion and memory: limbic system Perception of flavor • The chemical senses act in concert • Multiple components: • Taste buds • Olfactory receptors • Free-nerve endings (CN V) e.g. spiciness & temperature • Emotional and cognitive valence Taste Blue tongue disease Taste buds • Lingual buds: • Foliate • Fungiform • Circumvallate • 2000-5000 buds • 50-150 taste cells Papillae, buds & cells • circumvallate Learning Objective #1 • Describe the cranial nerve / brainstem sensory innervation of the taste buds / tongue Innervation • Chorda tympani (VII) • Taste map - myth? • palate & pharynx Rostral medulla • Why are we here? CNs and solitary nucleus/tract VII IX X • Principal visceral sensory relay • Rostral portion Learning Objective #2 • Explain the general ionic mechanism of taste cell excitation Taste transduction • Specific chemical interaction • microvillae • G-protein receptor • Ion channel • Depolarization • Passive spread is enough • …but can produce APs • Ca2+ entry • Transmitter (glutamate) release G-proteins & ion channels • • • • • • Sweet Salty Sour Bitter “Umami” TRP channels (see PAIN) Tim Jacob (Cardiff University, UK) Anatomical path • Primary gustatory cortex • Insular / frontal operculum • VPM (head - sensory) • Cortical relay • via central tegmental tract (ipsi) • Rostral pons (to other regions) • Parabrachial nucleus (non-human) • Rostral medulla • reflexes, e.g. DMN X Cortical processing • orbitofrontal cortex • integration, e.g., olfactory information • Projections • amygdala • hypothalamus • striatum Learning Objectives #3 & 4 • Identify the cortical regions important for primary gustation • Discuss how sub-modalities of taste and smell are sorted as they ascend to the cortex Broad tuning of taste pathway • Solitarius cell - multiple • Orbital cortex cell - selective Neural coding of taste Olfaction • My dog’s got no nose…. • How does it smell? • Awful Teaching Objective #5 • Compare and contrast olfaction with other sensory modalities, including its cranial nerve and nature of projection to cortex Anatomical points • Olfactory receptor cells are real neurons • CN I & bulb is really part of the CNS • No thalamic relay Olfactory receptor neurons Epithelia - surface area Olfactory transduction • very fine unmyelinated axons Learning Objective #5 • Discuss how sub-modalities of taste and smell are sorted as they ascend to the cortex Glomeruli - olfactory bulb • mitral cells • Convergence (1000’s) & sorting CNS pathways • Anterior olfactory nucleus • Inhibit contralateral bulb • Olfactory tubercule • Primary olfactory cortex • Piriform cortex (temporal lobe) • Periamydaloid cortex • (part of) parahippocampal gyrus • Further projections • Limbic system - amygdala • Thalamus Learning Objective #6 • Appreciate that taste / smell and other sensory modalities are combined at the level of the cortex along with limbic information to produce full sensation of flavor Thalamic relay memory & emotion dorsomedial nucleus smell selectivity & integration damage • Anosmia • Taste loss? • Parkinson’s disease • Seizures (uncinate) • Begin with smell or taste (unpleasant)
