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?

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

• 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

• 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)