SN4 SHORT NOTE 5M

Describe the modalities of taste, the taste buds and the nerve supply of the tongue.

Physiology · 1st Professional — Pre-Clinical

Taste (gustation) is mediated by taste buds, chiefly on the tongue, and requires the substance to be dissolved in saliva.

Modality	Typical stimulus	Mechanism
Sweet	Sugars	G protein (gustducin) → second messenger
Salt	NaCl	Direct Na+ entry through ENaC
Sour	Acids	H+ blocks K+ channels
Bitter	Alkaloids, quinine	G protein; lowest threshold — protective
Umami	Glutamate (MSG)	Metabotropic glutamate receptor

Bitter has the lowest threshold because most natural poisons are bitter — a protective arrangement
The old "tongue map" is incorrect: all modalities are detected over the whole tongue, with only slight regional differences

Region	Nerve
Anterior two-thirds of tongue	Chorda tympani, a branch of the facial nerve (VII)
Posterior one-third	Glossopharyngeal (IX)
Epiglottis and pharynx	Vagus (X)

Taste receptors → VII, IX, X → Nucleus of the tractus solitarius → VPM of thalamus → Taste cortex — insula and postcentral gyrus

Ageusia — loss of taste; hypogeusia — reduced taste
Most of "taste" is actually smell — which is why food is tasteless during a common cold; only the five basic modalities are true taste
Bell palsy involving the chorda tympani causes loss of taste on the anterior two-thirds of that side — a useful localising sign
Zinc deficiency and drugs (metronidazole, captopril, chemotherapy) distort taste

All questions in Special Senses