The sense of SMELL

1.

The olfactory (= smell) membrane or epithelium is located in the superior part of the nose (= nostril).

2.

The area covered is about 2-3 cm2 in each nose and contains millions of sensors.

3.

The sensors are actually nerve cells! They have a bipolar shape.

4.

At one end, there are numerous olfactory hairs = cilia) sticking out into the mucus of the nose.

5.

At the other end of the olfactory cells, non-myelenated axons run through openings of the cribriform plate into the overlying olfactory bulb.

6.

Essentially, these axons run through the base of the skull into the brain!

7.

As in the taste buds, the olfactory cells are supported, surrounded and cushioned by sustentacular cells

8.

And, as in the taste buds, there are also basal cells, which are really stem cells to make new olfactory cells.

9.

This is probably the only known situation in which nerve cells (as the olfactory cells really are) are renewed. All other nerve cells cannot do so.

10.

Olfactory cells live about 60 days.

11.

We should also mention that in the olfactory epithelium there are also olfactory glands, which makes the mucus.

12.

This mucus is very important because it ‘catches’ and contains the chemicals from the air that will stimulate the cilia of the olfactory cells.

1.

The odours that we smell are actually chemicals floating in the air that stick to the mucus and dissolves in it.

2.

Therefore we can only smell chemicals that are volatile. It also helps if the chemical is also water-soluble.

3.

These chemicals bind to a receptor, which is actually part of a G-protein complex

4.

The G-protein activates in turn adenylyl cyclase, which opens sodium-channels.

5.

The influx of sodium ions will depolarize the olfactory cell and if that reaches threshold, will induce one or more action potentials. If the smell is strong, there will be more action potentials.

6.

As the olfactory cell is actually a nerve cell, the action potential will propagate along the un-myelenated axon towards the olfactory bulb.

1.

As with taste, people have and are searching for the basic components of smell.

2.

One attempt, based on psychological studies, came up with the following list of smells:

3.

1. Camphoraceous
2. Musky
3. Floral
4. Pepperminty
5. Ethereal
6. Pungent
7. Putrid

4.

Biochemical and gene studies are starting to show that there are hundreds if not thousand primary smells.

5.

Also important is the fact that the threshold for smell is very very low. Some compounds can still be detected at extreme low concentrations (just a few molecules in 1 mml air).

6.

Finally, the cells will also adapt to a stimuli. This occurs already in the first seconds. There is a second adaptation but that occurs in the brain.

1.

The olfactory bulb contains glomeruli, in which thousands of axons from the olfactory cells synapse to.

2.

In addition, there are other nerve cells, such as the mitral cells that receive the axons from the olfactory cells.

3.

The glomeruli, the mitral and the tuft cells send their axon, though the olfactory tract towards several centres in the brain.

4.

The olfactory bulb and the olfactory tract together form cranial nerve I (= the first). Further on in the brain, the tract divides into two parts.

5.

One part of the tract radiates into the medial olfactory area in the lower brain.

6.

The other part of the tract goes to the lateral olfactory area.

7.

From the lateral olfactory area, fibres go to the limbic system, especially to the hippocampus. This area is important because there the smell stimuli are associated with emotions!

1.

Anosmia is when you cannot smell. Other names are hyposmia (= less sensitive), dysosmia (= distorted smell) and hyperosmia (= increased sensitivity).

2.

Anosmia (= no smell at all!) may happen following a trauma to the head.

3.

In that case, the axons from the olfactory cells that run through the cribriform plate to the olfactory bulbs, have been broken. This is (unfortunately) irreversible!

4.

Other, less traumatic, events may also reduce our sense of smell such as a cold, an allergy, smoking, etc.

5.

It is also interesting that taste is so much dependent on smell. If there is no smell, then food becomes much less tasty.

6.

Brain disorders may also affect smell. Following brain surgery or trauma, some patients have olfactory hallucinations!