The sense of TASTE* (= Gustation)

1.

The sensors that register “taste” are the taste buds located in the oral cavity (= the mouth!).

2.

These taste buds, some 5,000-10,000 of them, are located primarily on the surface of the tongue but some are also located on the soft palate, the inner surface of the cheek, in the pharynx wall, and on the epiglottis of the larynx.

3.

A typical taste bud consists of 50-100 cells that are of three different types:

1. Taste cells
2. Supporting cells
3. Basal cells

4.

The supporting cells are the strong cells that support the whole structure of the taste bud.

5.

The basal cells are very important because these are the “stem” cells of the taste buds. This is crucial because taste cells do not live very long (10 days or so) and therefore have to be renewed all the time.

6.

The taste cells (also called gustatory cells) are the sensors. They have little hairs sticking out of the surface of the epithelium where they are bathed in saliva. A taste substance to which this cell is sensitive (see later) will attach to a receptor.

7.

The receptor, in turn, will open an ion channel, which then causes an influx of sodium ions.

8.

The sodium influx will depolarize the cell. The degree of depolarization (= receptor potential) is proportional to the concentration of the stimulating substance.

9.

If the depolarization reaches threshold, an action potential is generated which will got to the brain.

1.

2.

For practical purposes however, we can catalogue all that into four primary sensations of taste.

3.
The four primary gustatory sensations are:

1. Sour taste
2. Salty taste
3. Sweet taste
4. Bitter taste

4.

5.

6.

7.
Bitter taste is also caused by many substances, nearly all of them organic substances such as alkaloids that are often included in drugs such as quinine, strychnine, caffeine and nicotine but also in spoiled food.

8.

9.

1.
The taste buds (= papilla) described in panel A are imbedded in different structures: filliform, fungiform, foliate and circumvallate. The filliform is a simple tastebud as shown in previous panel A.

2.

3.
The circumvallate is a whole different structure (see picture). Furthermore, they are not scattered but located as an inverted V-shape at the base of the tongue.

4.

1.

In this diagram, the distribution of the different types of taste buds are shown.

2.

3.

So, in general, sweet and salty are felt at the ‘tip’ of the tongue whereas sour and especially bitter are more felt at the back.

4.

The backward location of bitter is especially important because this may induce gagging or even vomiting!

1.

 2.

From different parts of the tongue, the mouth and the pharynx, the signals go through the facial nerves, the glossopharyngeal nerves, and the vagus nerve  (= cranial nerve X).

3.

 4.

These second-order neurons then send their signals to a small nucleus in the thalamus.

5.

6.

9.
These two nuclei control the glands located in the submandibular, sublingual and parotid glands to produce saliva when required!

1.

We all have our own preferences as to what we like to eat.

 2.

Some of those preferences may also be linked to what the body requires.

3.

This has been tested in experiments with animals.

4.

For example, if an animal has been given a lot of insulin, the blood sugar level will be very low. That animal will then choose automatically the sweetest food.

 5.

Similarly, an animal that has its parathyroid glands removed will develop calcium depletion. That animal will choose to drink water with a lot of calcium

6.

Finally, if the adrenal glands have been removed, which leads to salt depletion, these animals will prefer to drink salty water.