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)
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
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
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
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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
• Parabrachial nucleus (non-human)
• Rostral medulla
• reflexes, e.g. DMN X
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
• mitral cells
• Convergence (1000’s) & sorting
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
memory &
emotion
dorsomedial nucleus
smell selectivity & integration
damage
• Anosmia
• Taste loss?
• Parkinson’s disease
• Seizures (uncinate)
• Begin with smell or taste
(unpleasant)