Taste & Smell

Chemical senses

stimulated by chemicals either

in the air we inhale (odours) or in food/drink (tastes)

taste receptors for 5 basic chemical types: Sweet, Sour, Salt, Bitter, Umanmi (meaty, savoury)

taste buds

Clustered In tongue papillae

2000-5000 taste buds, mostly in tongue, some in palate, pharnyx & epiglottis

Types of cells:

Taste cell (detect tastants) - Type II & III

Tastants - dissolve in saliva & act on taste receptors at taste pore, usually turned over approx 20-30 days

1 stimulus type per cell

Type II Mechanism + Transduction

Modality: Have GPCR chemoreceptors for sugar, amino

acids (umami), or bitter stimuli

Transduction:

Sweet, umami, or bitter tastants:

stimulated by sugars and sugar like molecules

Depolarisation & activation of afferent gustatory nerve via atypical non-vescular ATP release via CALHM1/3 channels

AP initiated at afferent nerve

Atypical non-vescular release of neurotransmitters onto gustatory afferent nerve

Type III Modality & Transduction process

Modality: Na+ or H+ sensitive cation channel

Acid:

H+ enter taste cell via cation channel

Activation of voltage-gated Ca2+ channels

Vesicle release of Serotonin (5-HT) onto afferent gustatory nerve fibre to initiate AP

Salt:

Na+ enter taste cell via cation channel (ENaC)

• Via electrochemical gradient

• Depolarise cell

→ Activation of voltage-gated Ca2+ channels

→ Vesicle release onto afferent gustatory nerve

fibre and initiate AP

Taste Pathway

Taste receptors activate sensory nerves

Synapse onto secondary afferent neurons in

the nucleus of solitary tract of medulla.

Synapse onto tertiary afferent neurons in the

thalamus.

Thalamic neurons project to gustatory cortex

for taste sensation and perception.

• Chemotopic organisation

Explain how odorants travel

Inhaled & swirled around in nasal conchae

→ Dissolve in water mucus in roof of nasal cavity

→ Detected by cilia projecting from olfactory sensory neurons

Olfactory signal transduction

Odorant molecules bind to specific

GPCRs on cilia of olfactory sensory

neuron

• We have about 1000 different receptor

types, but can detect >10,000 odours

• Many odorants activate more than one

receptor type (combinatorial logic)

→ Activation of cAMP and opening of

ion channels

→ Na+ and Ca2+ influx leads to

depolarization (graded potential)

→ Upon threshold, action potential

initiated at axon hilloc

Olfactory Pathway (olfactory bulb)

Glomeruli: Spherical clusters of synaptic input and

dendritic architecture, encapsulated by glia.

Mitral and Tufted cells: Encode odor identify and

intensity (second afferent neurons

Activated by input from odour sensory

neuron. Bypasses the thalamus and

projects directly to “olfactory cortex

Flavour

Perceived in brain as sum of:

Input from taste cells

• Combination of taste receptors generate

complex and unique sensory profiles

• Input from odour receptors

• Temperature, texture, spiciness

• Also influenced by appearance