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The sense of TASTE* (= Gustation)

 

Purpose: to determine whether the stuff that we eat and drink is nice and safe or potentially toxic and dangerous.

A. Structural components requiredg:

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
taste bud

 

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.

 

B. Primary sensations of taste:

1.

Research has identified at least 13 possible receptors in taste buds and there could be (many) more. Furthermore, several different receptors may be present in a single taste bud.

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.

Sour taste is caused by acids in the saliva. These are usually hydrogen ions (H+). The more H+, the more sour!

5.

Salty taste is caused by ionized salts, mainly sodium (table salts!).

6.

Sweet taste is caused by many chemicals such as sugars, glycols, alcohols, ketones, amides etc etc.

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.

It should be mentioned that if something is very bitter, this might induce rejection of the food or the drink.

9.

And this is important because many toxins from poisonous plants in nature contain alkaloids.

 

C. Different types of taste buds:

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.

The most common structure is the fungiform which is scattered over the entire surface of the tongue.

types of taste buds

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.

The foliate form resembles the circumvallate but is less complex and somewhat smaller

 

D. Distribution of taste buds (= gustatory coding):

1.

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

2.

On the left side, the topography of the different type of taste buds are shown and on the right side, the areas where the four primary sensations are located.

tongue and taste buds

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!

 

E. The gustatory pathway:

1.

This figures shows how the taste signals are transmitted to the brain.


 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).

 

 

pathways of the taste

3.

All these fibers converge into the tractus solitarius, located in the brainstem, where they synapse onto second-order neurons.


 4.

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

5.

From the thalamus, they synaps to third-order neurons that  radiate to a small part of the parietal cortex

6.

In the brainstem itself, the fibers from the tractus solitarius are also connected to the superior and inferior salivatory nuclei.

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

 

F. Taste and Diet:

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.

G. Taste Pathologies:

1.

As you get older (> 45 years), the number of taste buds decrease. Sorry old man!

2.

But there are also abnormalities in taste possible. These are called: .geusias

3.

Ageusia absence of taste (genetic?)

4.

Hypogeusia diminished taste sensitivity. May happen for example after usage of certain drugs


5.

Dysgeusia disturbed sense of taste. Eating some plants may make acids taste

6.

Hypergeusia increased ability to taste!!

 

* The word “Taste” comes from the Latin “taxare” which means to touch, to evaluate. In other words, with our taste we are evaluating what we eat, what we drink.

 

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HumanPhysiology.academy 2014