taste & smell Basic Neuroscience NBL 120 (2008)
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 Multiple components: Taste buds Olfactory receptors Free-nerve endings (CN V) xe.g. spiciness & temperature Emotional and cognitive valence
Taste
Blue tongue disease
Taste buds Lingual buds: Foliate Fungiform Circumvallate buds taste cells
Papillae, buds & cells circumvallate
Innervation Chorda tympani (VII) Taste map - myth? palate & pharynx
Rostral medulla Why are we here?
VII IX X CNs and solitary nucleus/tract Principal visceral sensory relay Rostral portion
Taste transduction Specific chemical interaction microvillae G-protein receptor Ion channel Depolarization Passive spread is enough …but can produce APs Ca 2+ entry Transmitter (glutamate) release
G-proteins & ion channels Sweet Salty Sour Bitter “Umami” TRP channels (see PAIN) Tim Jacob (Cardiff University, UK)
Anatomical path Rostral medulla reflexes, e.g. DMN X VPM (head - sensory) Cortical relay via central tegmental tract (ipsi) Rostral pons Parabrachial nucleus (non-human) Primary gustatory cortex Insular / frontal operculum
Cortical processing orbitofrontal cortex integration, e.g., olfactory information Projections amygdala hypothalamus striatum
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
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
Glomeruli - olfactory bulb Convergence (1000’s) & sorting mitral cells
CNS pathways Anterior olfactory nucleus Inhibit contralateral bulb Olfactory tubercule Primary olfactory cortex Piriform cortex Periamydaloid cortex (part of) parahippocampal gyrus Further projections Limbic system - amygdala Thalamus
Thalamic relay dorsomedial nucleus smell selectivity & integration memory & emotion
damage Anosmia Taste loss? Parkinson’s disease Seizures (uncinate) Begin with smell or taste (unpleasant)