Chemical senses

Tip of tongue tastes
Sweet
Back of tongue tastes
Bitter
Sides of the tongue tastes
Salty and sour
Tastes buds
Multiple per papillae (bumps) ~5000 total
Taste receptor cells
~50-150 per taste bud. Varying response profiles even within a single taste bud
Amiloride
A potassium-sparing diuretic, first approved for use in 1967 (then known as MK 870), used in the management of hypertension and congestive heart failure.
Salty taste channels
Specialized Na+ selective Channel.
Non voltage gated
Blocked by amiloride
Stimulus= High external salt concentration such as chicken soup. What happens in the taste receptor pathway?
1) Na+ enters through amiloride channels(inward current).
2) Depolarization Amplified by voltage gated Na+ channels (receptor potential, no action potential)
3) Ca2+ enters through Ca2+ voltage gated channels
4)Synaptic vesicle fusion/ Transmitter release
Sour taste
Sour = acidity = low pH = high concentration of H+ ions. Protons = Tastant
Sour Taste Channels
H+ enters Amiloride sensitive Na+ channels which causes depolarization. H+ also blocks K+ channels which furthers depolarization. (most likely more unknown mechanisms as well)
Bitter, Sweet, and Umami
All use G protein coupled GPCR receptor pathways.
Families of Taste receptor Genes
T1r and T2R
Bitter Taste
~30 different T2R receptors, specialized for poisons.
Sweet
2 particular T1r receptors (T1R2 and T1R3)
Umami
Detects amino acids (savory). Two particular T1R receptors T1R1 & T1R3
Cranial nerves involved in Taste
VII (facial) IX (glosopharyngeal) and X (Vagus)
Pathway Taste travels from tongue to brain
Tongue -> Medula -> Thalmus -> Cortex
Tongue -> gustatory nucleus -> VPM -> Gustatory nucleus
ageusia
Loss of Taste perception. Could be caused by lesions to Gustatory Thalmus, VPM or Gustatory cortex
Individual receptor cells are
Diverse
-Different receptor cells respond differently to the same stimulus.
Broadly tuned
-Receptor cells respond to more than one type of stimulus (eg. both salty and sour)
Complex flavors such as Banana are coded by
Patterns of activation across the population of receptors
Olfactory receptor cells
True nuerons, each has its own axon
Regularly replaced every 4-6 weeks
Humans~12 million Olfactory receptors
Cranial Nerve I = Axon of all the olfactory receptor cells.
Odorant Receptor Proteins (ORP)
More than 100 types of Odorant receptor proteins
Each receptor cell typically expresses only one ORP
G protein coupled receptor pathway
Generate action potentials
Diverse broadly Tuned
Olfactory glomeruli
~2000 on each bulb
Specific mapping
Each glomerus receives input from a large region of the olfactory epithelium
receives input only from receptor cells expressing a particular odorant receptor gene
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Categories: Microbiology