taste & smell DENT/OBHS 131 Neuroscience 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) 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
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?
VII IX X CNs and solitary nucleus/tract 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 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 (to other regions) Parabrachial nucleus (non-human) Primary gustatory cortex Insular / frontal operculum
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 Convergence (1000’s) & sorting mitral cells
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 dorsomedial nucleus smell selectivity & integration memory & emotion
damage Anosmia Taste loss? Parkinson’s disease Seizures (uncinate) Begin with smell or taste (unpleasant)