動物生理学 III①Handout No. 9②Figure 1: Taste-receptor cells, buds and papillae.
a, Taste buds (left) are composed of 50–150 TRCs (depending on the species),
distributed across different papillae. Circumvallate papillae are found at the veryback of the tongue and contain hundreds (mice) to thousands (human) of taste
buds. Foliate papillae are present at the posterior lateral edge of the tongue andcontain a dozen to hundreds of taste buds. Fungiform papillae contain one or afew taste buds and are found in the anterior two-thirds of the tongue. TRCs
project microvillae to the apical surface of the taste bud, where they form the'taste pore'; this is the site of interaction with tastants. b, Recent molecular andfunctional data have revealed that, contrary to popular belief, there is no tongue'map': responsiveness to the five basic modalities — bitter, sour, sweet, salty andumami — is present in all areas of the tongue

Figure 2: Encoding of taste qualities at the periphery
There are two opposing views of how taste qualities are encoded in the periphery. a,In the labelled-line model, receptor cells are tuned to respond to single taste
modalities — sweet, bitter, sour, salty or umami — and are innervated by individually
tuned nerve fibres. In this case, each taste quality is specified by the activity of
non-overlapping cells and fibres. b, c, Two contrasting models of what is known as the'across-fibre pattern'. This states that either individual TRCs are tuned to multiple taste
qualities (indicated by various tones of grey and multicoloured stippled nuclei), andconsequently the same afferent fibre carries information for more than one taste
modality (b), or that TRCs are still tuned to single taste qualities but the same afferent
fibre carries information for more than one taste modality (c). In these two models, thespecification of any one taste quality is embedded in a complex pattern of activity
across various lines. Recent molecular and functional studies in mice havedemonstrated that different TRCs define the different taste modalities, and that
activation of a single type of TRC is sufficient to encode taste quality, strongly
supporting the labelled-line model.
①～③の出典 ： Nature 444:288-294 （Nature のウェブサイトからオンライ
ンで全文を読むことができます （学内からのみ）。）③④

Figure 4: Summary of receptors for umami,sweet, bitter and sour tastes
Schematic representation of taste receptors(and candidate receptors) mediating four of thefive basic taste modalities. Although not indicatedin the figure, responses to high concentrations ofsugars, but not other sweet tastants, are also
detected by T1R3 alone15. The grey T2R
receptor is designed to illustrate the possibility
that T2Rs, much like T1Rs, may function asheteromeric complexes. Similarly, the grey
receptor next to PKD2L1 depicts a PKD1-family
member as a candidate partner⑤Figure 32-14 Four basic taste stimuli are transduced into electrical signals by different mechanisms.
Salty. Salty taste is mediated by Na+ influx through amiloride-sensitive Na+ channels. Sour. Sour taste canresult from either the passage of H+ ions through amiloride-sensitive Na+ channels or from the blockade ofK+-channels, which are normally open at resting potential. Bitter. Although at least one bitter stimulus,
quinine, may depolarize taste cells by blocking apical K+ channels, most bitter stimuli are thought to bind toG protein-coupled receptors. There is evidence for two different pathways of bitter taste transduction thatinvolve G proteins. In one the G protein stimulates phospholipase C (PLC) to increase production of
inositol 1,4,5-trisphosphate (IP3), which then causes the release of Ca2+ from intracellular stores. In theother pathway the G protein gustducin activates a phosphodiesterase (PDE) that may reduce intracellularlevels of both cAMP and cGMP. Sweet. Some sweet tastants are also thought to bind to receptors that
couple to gustducin or a G protein that stimulates IP3 production. However, other sweet receptors may
couple to a G protein that interacts with adenylyl cyclase, causing an increase in cAMP that leads to thephosphorylation of K+ channels by protein kinase A.

Figure 32-17
Taste information is transmitted from thetaste buds to the cerebral cortex via synapses in the brain
stem and thalamus. Signals carried by fibers that innervate
the taste buds travel through several different nerves to thegustatory area of the nucleus of the solitary tract, whichrelays information to the thalamus. The thalamus transmits
taste information to the gustatory cortex.