Taste and the tongue
Taste cells throughout the mouth and throat cluster together to form taste buds. Isolated taste buds are scattered on the surface of the palate and throat, but on the tongue they are grouped in specialized structures called papillae: mushroom-shaped fungiform papillae in the front of the tongue, leaf-shaped foliate papillae protruding from the sides, and circumvallate papillae arrayed in a chevron at the back of the tongue. The tips of the taste cells reach toward a tiny opening on the tongue - the gustatory pore - through which food chemicals can fall. Nerve endings for the taste system and for the touch/temperature/pain system surround the base of the taste bud cells.
From tongue to brain
Three nerves bring taste messages to messages to the brainstem: the facial nerve, shown in the picture above, which brings messages from the anterior two-thirds of the tongue; the hypoglossal nerve, which brings messages from the posterior tongue; and the glossopharyngeal nerve, which brings messages from the throat area and the palate. The nerve carrying messages from the touch/temperature/pain system is called the trigeminal nerve. All of these nerves bring their messages to the brainstem, where they combine their signals in areas of the brainstem that are involved with arousal (for example from sleep), and are - according to Antonio Damasio in The Feeling of What Happens - the locus of our sense of self. As taste messages move further through the brain, they join up with smell messages to give the sensation of flavor.
Smell, the nose, and the brain
Molecules of the food you are eating move through the back of the throat and reach olfactory nerve endings in the roof of the nose. The molecules bind to these nerve endings, which then signal the olfactory bulb to send smell messages to two critical parts of the brain:
- the section of the temporal lobe (T in the image above) involved with memory, particularly memories of place - this is the reason why smells can evoke powerful memories of your experiences;
- the part involved with speech, also in the temporal lobe - in fact, smelling some perfumes can actually inhibit your ability to speak or even to think in words, so you are left with visual thinking.
Of course, we have to know what we are eating, so taste, smell, and trigeminal messages meet in a part of the brain called the insula (not shown in the image above), which identifies ìwhatî the flavor is.
But knowing ìwhatî is not enough. We also need to react emotionally to what we are eating - do we like it? is it poison? should we enjoy it or spit it out? Flavor messages go to the emotional centers in the temporal lobe and the cingulate gyrus. Messages from all of these areas then reach the orbitofrontal cortex in the frontal lobe of the brain, right above the eyes. This part of the brain evaluates experience and choose among alternatives.