taste & smell

DENT/OBHS 131Neuroscience

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 excitation3. Identify the cortical regions important for primary gustation4. 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?

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

• 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 (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)