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Learning Objectives

• By the end of this lecture you should:

• Describe taste receptors

• List the location of taste buds

• Describe how the olfactory neurons are activated

• Distinguish between the modalities of taste

Taste receptors

• Located in the citrus shaped taste buds

• Taste receptor populations ≥ 5

• Receptor cells microvilli project from surface

• Taste receptor cell not the primary neuron

• Gustatory afferent nerve fibers

• Individual taste receptor cells contacted

LOCATION OF TASTE BUDS

• Papillae

– Along lateral margin and dorsum of tongue

– Junction of the dorsum and base of tongue

– Palate, epiglottis, larynx and oesophagus

Innervation

• Sense of taste mediated by:-

– Facial chorda -tympani serving serves anterior ⅔

– Glossopharyngeal-lingual serves posterior ⅓

– Vagal-internal branch of superior laryngeal serves • Larynx

• Epiglottis

• Oesophagus

Taste pathways

• Central connections terminate in brainstem in the nucleus of tractus solitarius

• Projections of central pathways terminte in the brainstem in the nucleus of tractus solitarius

• Project to the ipsilateral parabrachial nucleus to the pons

• Some fibers bypass the thalamus

Tastants

• Reach receptor cells through the taste pore • Five classes of taste recognized:

– Sweet – Sour – Bitter – Salty

– Umami

Gustatory receptors

• Individual afferent receptors respond to different chemicals

• Response depends on the stimulus that produces the largest response

• For example, sucrose best response and NaCl second best all the time

Across-fiber-pattern theory

• Gustatory coding determined by response to a large number of different chemicals

• Best stimulus analysis led to concept of labeled afferents theory

• LAT determine gross quality while AFPT discriminates chemicals within qualities

Sweetness

• Sweetness may be carried by sucrose-best neurons

• Intensity encodes by the quantity of neural activity

• Discrimination of sucrose from fructose requires a comparison of neurons that are:-

– sucrose-best, – salt-best and – quinine-best

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