Gustatory Sense Flashcards
What are the 4+1 primary tastes, and their threshold for stimulation (TFS)?
Salty - safe (approach) (TFS = 0.01M)
Sour - unripe/poor nutritional value (avoid) (TFS = 0.009M)
Sweet - nutritious (approach) (TFS = 0.01M)
Bitter - poisonous (avoid - helps identify toxic compounds) (TFS = 0.000008M)
Umami - amino acids (rich protein source, hearty/savoury)
Structure of the tongue
Epiglottis is at the back of the tongue, tonsils at the side
Papillae (groove like structures) are located on all of the tongue > papillae extend down into the tongue and conatin taste buds > taste buds contain taste cells and supporting cells that end in apical microvilli in the taste pore > taste cells are attached to sensory nerve fibres
How many taste cells does a single taste bud contain?
50-100
Mean number of taste buds for a human?
4000 (1 log unit of variation within the human population) (number declines with age)
How do we taste?
Substance is sensed through the saliva by the microvilli at the taste pore > induces depolariation and AP generation > taste cell AP releases neurotransmitter > activates gustatory afferent fibres
Transduction of sour taste
Taste cells express proton-sensitive potassium leak channels
Sour is an acidic taste (H+ protons)
Protons block the K+ channels > reduces gk and depolarises the cell
Newly discovered acid-sensing cation channel gated by H+ > block depolarises cell
Transduction of salty taste
Express amiloride-sensitive sodium channels (eNaC sodium channels)
Sodium (in salts) enters cells > causes depolarisation
Potassium type salts also simulate these cells because of leak potassium channels and changes in Ek
Transduction of bitter taste
Use receptors (7 transmembrane domains) coupled to various G proteins
T2 receptors are a family of related bitter receptors
Coupling to Gg > activates PLC > increases Ca2+ through IP3
Coupling to gustducin > activates cyclic nucleotide phosphodiesterases
A few bitter compounds act through blocking potassium leak channels
Transduction of sweet taste
Use receptors (7 transmembrane domains) coupled to Gs proteins
T1R2 and T1R3 receptors
Sugars > activate Gs > produce cAMP and PKA phosphorylates > closes K+ leak channels > depolarisation
Artificial sweetners > activate Gg proteins > activate PLC > activates IP3 > increased Ca2+
How does ‘Coding for Taste’ work?
Not congruent with label-line principle (one stimulus > activates one receptor > activates one patway to the CNS)
Pattern code - stimulus activates all receptors in different manners and proportions > CNS decodes the signal
Pattern of activation of receptors provides complexity in taste
Intensity of taste cells
Taste cell neurons increase their firing rate with increases in stimulus concentration (mmol)
Innervation of the tongue
Vagus nerve innervates the epiglottis region of the tongue
Glossoparyngeal nerve innervates back of the tongue
Facial nerve innervates the front of the tongue
Ascending taste pathways
Nucleus of solitary tract (NST) in the brain stem receives stimulation from tongue innervation > signals to ventral posterior medial nucleus of the thalamus (VPMN) > signals to the primary gustatory cortex
Physiology of flavour perception
Taste + smell combined in Orbital Frontal Cortex (OFC) > OFC receives input from primary somatosensory cortex + inferotemporal cortex in the visual pathway (bimodal neurons here respond to taste and smell as well as taste and vision) (firing of these neurons is affected by level of hunger for a specific food)
