A recent paper in Nature (LINK to article) tells us for the first time how taste sensations on the tongue are transmitted to the brain. It turns out that, just as there are dedicated receptor cells on the tongue and palate for each of the five basic tastes, so there are specific ganglions (analogous to the different wires in a telephone exchange) tuned to each of these tastes, and responsible for conveying the message about their presence from the receptors to the brain. One practical upshot (on the assumption that human brains react similarly to the mouse brains that were studied) is that our response to taste mixes should largely reflect the sum of the responses to the individual tastants.
Interestingly, it turns out that a small proportion of ganglions are multi-tuned – in particular, there are some, connected to the tongue’s receptors for bitterness, that can also carry messages about sourness (i.e. acidity). This may help to explain the confusion that sometimes exists when we discuss these two tastes. There are also a few that are multi-tuned to carry messages about umami and sweetness. Stay tuned for future developments in the emerging science of taste and the brain!