2.3.2 taste

Sweet: Sugars (e.g., fructose and sucrose), certain proteins (monellin), and artificial sweeteners (e.g., saccharin and aspartame).
Sour: Acids.
Salty: Salts.
Bitter: Certain ions (e.g., $K^+$ , $Mg^{2+}$ ) and organic molecules (e.g., quinine, caffeine).
Umami: Savoury taste of the amino acid glutamate.

Tongue papillae containing taste buds:
- Fungiform: Mushroom-shaped; located on the tip, in between filiform papillae, and sides of the tongue.
- Foliate: 4-5 short vertical folds; located on the sides of the posterior tongue.
- Circumvallate: 8-12 dome-shaped structures; located anterior to the sulcus terminalis on the posterior tongue.
Papillae without taste buds:
- Filiform: Cone-shaped and most numerous; cover the front 2/3 of the tongue, and serve touch sensation.
Taste bud distribution and density:
- Taste bud numbers range from 500 to 20,000 (averaging between 2,000 and 5,000).
- Super tasters have a higher density of fungiform papillae on the tip of the tongue compared to non-tasters.
- At normal food concentrations, all basic tastes can be perceived across the entire tongue rather than in restricted taste zones.

Each taste bud contains 50 to 150 taste receptor cells arranged like sections of an orange, along with supporting basal cells.
Taste receptor cells are modified skin cells renewed every 10 to 14 days.
Microvilli (taste hairs) extend from the apical end into the taste pore to contact tastant molecules.
Taste receptor cells are NOT neurons; they signal to gustatory afferent neurons whose cell bodies reside in ganglia.

Functional classification of taste cell types:
- Type I cells: Most abundant; provide glial-like support and detect salt at pleasant, low-to-moderate concentrations via the epithelial sodium channel (ENaC).
- Type II cells: Express G protein-coupled receptors (GPCRs) that detect sweet, bitter, or umami tastes.
- Type III cells: Detect sour tastes and form direct synapses with sensory nerve fibers.
- Type IV cells: Function as stem or progenitor cells that divide and differentiate into other taste cell types.

Salty Transduction (Type I cells):
- $Na^+$ enters directly through epithelial sodium channels (ENaC), depolarising the cell.
- High/aversive salt concentrations additionally recruit Type II and Type III taste cells.
Sour Transduction (Type III cells):
- $H^+$ Ions enter through specific channels, directly depolarising the cell.
- Depolarisation opens voltage-gated $Ca^{2+}$ channels and triggers release of neurotransmitters (serotonin and norepinephrine).
Sweet, Bitter, and Umami Transduction (Type II cells):
- Tastants bind to G-protein-coupled receptors (GPCRs), activating secondary messenger cascades that release $Ca^{2+}$
- Internal $Ca^{2+}$ elevation depolarises the cell and causes ATP release through non-synaptic channels.
Selectivity and comparison across types:
- Direct ion channel entry causes depolarisation in Type I (salty) and Type III (sour) cells, whereas Type II (sweet, bitter, umami) cells rely on GPCR messenger cascades.
- Synaptic neurotransmitter release (serotonin, norepinephrine) occurs in Type III cells, whereas Type II cells release ATP through non-synaptic channels.

Cranial Nerve Innervation:
- Three cranial nerves innervate taste buds across different regions of the tongue (anterior, posterior) and throat (including the epiglottis).
Central Relays:
- Gustatory afferents terminate in the gustatory nucleus (nucleus solitarius / nucleus of the solitary tract) located in the medulla of the brainstem.
- Axons project ipsilaterally through the brainstem to the ventral posteromedial nucleus (VPM) of the thalamus.
- Thalamic neurons project ipsilaterally from the VPM to the primary taste cortex (located in the parietal lobe and insula).
Clinical Relevance:
- Lesions along the central gustatory pathway or in the gustatory cortex cause ipsilateral ageusia (loss of taste perception).
- Unilateral taste loss often goes undetected due to bilateral functional compensation.