How Taste Signals Reach the Brain—and Why It Matters

Whether individual tastes travel exclusively along dedicated pathways or are encoded to allow multiple taste signals to travel through shared nerves is an active argument within science that is still unresolved. This is not just an academic question. The nature of these connections determine the kinds of sensations we perceive. 

To generalize, there are two types of connections from sensory organs to their early processing centers in the brain. One is an unambiguous connection from one particular type of sensory cell; the second connection may handle multiple cell or sensor types, with their signals coded to allow them to send slightly different messages to the brain.

The first type—labeled lines—function like old-time telephone systems: each type of signal on a dedicated wire, in this case each carrying one particular identity: bitter, sweet, etc. In the second, which scientists call “across-fiber” coding, multiple signals can be sent simultaneously on the same line, analogous to the way different channels from your cable TV all travel through a single wire.

The advantage of a labeled line system is that it’s quick and unequivocal. Its disadvantage is that it’s limited in its information content, and in terms of neural wiring, a separate line is needed for each signal type. A coded system like olfaction can carry vastly more information, but the coding/decoding takes neural resources, and takes time. Many other senses encode their information by using some kinds of waves or spiking electrical impulses, so why not here?

Which system does human taste employ?

In mammals, the current evidence mostly favors the labeled-line model. Even if there is some type of intensity or other coding in humans, there definitely are taste-specific nerves from most receptor cells right up to the primary gustatory cortex. This wiring is very specific, so when nerves wear out and need to be replaced, they must be guided to their correct destinations to keep taste signals from getting scrambled. 

Since most taste cells respond to a single taste type, the labeled-line system can work quite well, but there are still open questions. Some taste cells have functional receptors for more than one taste. Sour-sensing (type III) taste cells are a real wild card: some sense sour and salty; others sense sour plus one or more bitter, sweet or umami tastes. It’s not clear how these complex cells work with the labeled line model. 

Things are different in insects. In Manduca seca moths, each different tastant generates a unique pattern of neural “spiking,” which researchers found allowed differentiation between individual bitter chemicals. The researchers found no evidence in the moths of labeled lines or even of basic taste categories. All responses were tastant-specific, allowing identification of individual chemicals by the spiking pattern of the receptor neurons. 

This sensory activity drove behavior; moths preferred sucrose over other sugars, and targeted flowers rich in it. The moths also demonstrated selective tolerance for some bitter compounds, but not others. For these insects, taste works more like smell: it’s not the response of any one receptor, but the pattern that matters.

It may seem obvious that what applies to moths might not be true for mammals, since our last common ancestor was 550 million years ago. However, scientists studying chemical ecology note that mammals and insects have more in common than we might think. We share sensitivity to sweet, bitter, water and CO2, although insects can’t taste salty or sour. Both chemosensory systems analyze a pattern of multiple receptor responses in order to identify smells. Amazingly, elephants and the cabbage looper moth deploy the exact same neurotransmitter for receptor signaling purposes. Our shared terrestrial environment has shaped the evolution of our senses in ways that are similar, even though our bodies are so different.

Some researchers believe some taste specificity extends to vertebrates and even some mammals. Differing response spiking patterns for bitterness have been observed in rats. In mice, different bitter compounds appear to create different neural codes in the brain, suggesting they have some ability to discriminate. 

What does all of this mean for us?

Humans have 26 different receptors responding to a large range of bitter chemicals. This fact would seem to open the door to our being able to discriminate between different bitter sensations. But so far, there appears to be no evidence that different bitter chemicals produce different taste sensations in us. All bitter-sensing cells, no matter the receptors present, feed into the same nerves headed for the brain, so only one type of purely bitter sensation is possible for us.

But that does not mean we can’t discriminate. Many bitter chemicals also display mouthfeel characteristics such as astringency. This is even more true of whole botanicals as opposed to purified bitter compounds as in the case of cinchona vs. its quinine alkaloid. There may also be differences in  timing, with some chemicals being quick to generate sensations and others that linger far longer on the palate, often due to their solubility—or lack thereof—in saliva or other reasons. Since our brains quickly merge different senses onto multimodal sensations, they can sometimes can be a little challenging to pull apart. All of this is actually true for certain acids, which bring a bit of raspiness on top of their sourness, as is true for malic acid compared to citric acid.

References:

Behrens et al., “Gustatory Expression Pattern of the Human TAS2R Bitter Receptor Gene Family Reveals a Heterogenous Population of Bitter Responsive Taste Receptor Cells,” 

Journal of Neuroscience 27, No. 46 (2007): 12630-12640, https://doi.org/10.1523/JNEUROSCI.1168-07.2007.

Chikazoe et al., “Distinct representations of basic taste qualities in human gustatory cortex,” 

Nature Communications 10, No. 1048 (2019), https://doi.org/10.1038/s41467-019-08857-z.

Berenbaum, “Are Mammals Just Furry Bugs with Fewer Legs? Convergences in Mammalian and Insect Chemical Ecology,” Chemical Signals in Vertebrates 13(2014): 3-10,https://doi.org/10.1007/s00429-019-01945-2.

Christian H. Lemon and Donald B. Katz, “The neural processing of taste,” BMC Neuroscience 8, no. 5 (2007), https://doi.org/10.1186/1471-2202-8-S3-S5.

Wilson et al., “Bitter taste stimuli induce differential neural codes in mouse brain,“PLoS One 7, no. 7 (2012): e41597. Epub 2012 Jul 23, https://doi.org/10.1371/journal.pone.0041597.

Lee et al., Rewiring the Taste System,” Nature 548, No. 7667 (2017): 330–333,https://doi.org/10.1038/nature23299.

Reiter et al., “Spatiotemporal Coding of Individual Chemicals by the Gustatory System,” The Journal of neuroscience 35, No. 35 (2015): 12309-21,https://doi.org/10.1523/JNEUROSCI.3802-14.2015.