The vallate, fungiform, and foliate papillae usually have taste buds embedded in their
epithelium. Each bud is a separate entity, set off from the
surrounding epithelium by a delicate CT capsule. The image at left shows four of them. A bud is yet another example of a peripheral transducer structure whose function is to convert some physical phenomenon into a nervous signal. In this case the phenomenon is the binding of tastant molecules to the membrane of the transducer cells. Binding causes charge changes on the cells that are detected by neural fibers wrapped around them.
Not a whole lot is understood about the phenomenon of taste. There are four basic taste sensations: sweet, salt, sour, and bitter. Specific regions of the tongue have specific associations with these sensations. The data is incomplete, but it's been shown that generally the tip of the tongue has receptors for sweet and salty sensation, and sour and bitter receptors lie near the back. There are no morphological differences that correlate with functional ones, however, and you can't tell by looking at a taste bud what kind it is.
It's thought that the different taste sensations are located in and associated with specific cell types in the taste buds, but there are also indications that some cells may respond to more than one taste stimulation. Depending on where they're located, the taste buds may be innervated by fibers from cranial nerves VII (facial), IX (glossopharyngeal) or X (vagal). This is separate from the "general" sensation; that is, touch perception is not connected through the same nerve circuits. A branch of cranial nerve V (trigeminal) handles this input. Taste sensation is found in all vertebrates, and a similar chemoreception ability in most if not all invertebrates. The more or less universal distribution of chemoreception makes it clear that this sense has very high survival value.
This higher magnification view shows the taste bud in some
detail. There are known to be at least two, probably three,
cell types in these structures, and as you can see from this image,
there is a "dark" cell to go with the "light" cells around it.
The nuclear configurations of the two cell types are different, too.
The receptor cells, regardless of type, are more or less banana shaped, and at the apex of each is a small microvillus, called a "taste hair." (This is a misnomer because they aren't "hairs" in any real sense, but we're stuck with this archaic name.) The "taste hairs" collectively come to form a bundle that's exposed to the world via a small opening in the overlying epithelium, the taste pore. The "hairs" are the actual site of chemoreception; the plasma membrane that covers them contains receptor sites for the different sensations.
Integration of the chemoreception signals takes place in the central nervous system. The transducers respond, but the brain actually "tastes" by interpreting the responses. To a great extent, perception of tastes as "good" or "bad" is learned behavior. In some cultures, grilled meat is considered delectable, but in others, nauseating. Similarly, I have lived in places where rotten eggs are regarded as a delicacy, but my cultural perceptions prevented me from eating them. Many animals (humans included) have to learn what foods are edible, and one of the clues they use is taste perception.
I once knew a woman who had no sense of taste. Not that she didn't have "taste," she had no sense of taste. A childhood viral infection had damaged the nerve fibers leading from the buds, and she couldn't taste anything at all. She literally couldn't taste the difference between steak and ice cream! She could tell them apart by texture and temperture and appearance, but the vast array of subtle taste sensations most of us enjoy was totally lacking. The only advantage she derived from this affliction was a remarkable ability to tolerate the stuff served in fast-food restaurants.
Monkey tongue; H&E stain, 1.5 µm plastic section, 440x and 1000x
