The existence of the nose and brain is not enough for the sense of smell to occur. In order to smell in a healthy way, many auxiliary mechanisms need to be working besides these two organs. Olfactory cells and the olfactory bulb are among these auxiliary mechanisms.
When one says “smelling organ” it is the nose that occurs to mind immediately. However, very few people know that the smelling process is carried out only by 5% of the nose. (P. Whitfield, D.M. Stoddard, “Hearing, Taste, and Smell; Pathways of Perception”, Torstar Books, Inc., New York, 1984.) The statement of Professor Gordon Shepherd of Yale University stresses this fact: “We think we smell with our noses but it is like saying we hear with our earlobe”.
The Chemical Analysis Facility in the Nose
We breathe average of 23.040 times a day. During this process we repeat continuously our nose turns the air into the most suitable condition for the lungs. While doing this, it also carries out another very important function: it detects smells. Every time we breathe, the gas mixture we call “air” gets inside the nostrils. A single breath of air is composed of millions times trillions of molecules. These scent particles, which are so small that we cannot see with the naked eye, are inside this group made up of a massive amount of molecules.
After we breathe, the special bones (turbine bones) inside the nose direct part of the air to the olfactory region. This is seven centimeters inside and up the nostrils. When we approach a flower to our nose to smell it and take a deep breath, more smell molecules reach the olfactory region.
Smell molecules that form the source of smell are of different shapes and sizes, and they are smaller in comparison to other molecules. The charming smells of the flowers in the garden, the attractive smell of delicious food or the foul scent of rotten fruit is composed of different molecules. The chemical facility in our nose can easily differentiate between all of these molecules. It can even immediately recognize molecules that have the same atoms. For instance, a tiny difference between the “L-carvone” and “D-carvone” molecules is due to the different arrangement of atoms. Our nose can easily differentiate the two molecules despite the smallness of the difference; it tells us that one of these is cumin and the other is mint.
Excellent Messengers: Olfactory Cells
Olfactory cells are actually nerve cells. Their main task is to take the messages carried by scent molecules and to carry them to the olfactory bulb. There are different thoughts about their total numbers in the world of science: Some researchers say that their numbers are around 10 million and some say they are around 50 million. Millions of olfactory cells are placed in an amazing order in the olfactory region.
The olfactory cell has a striking distribution of roles in itself. Stuart Firestein, a well-known researcher, draws attention to this special arrangement in the cells, by saying “The olfactory cell is divided into many parts in terms of both structure and function, like all sensory receptors.” This special system stands out at the first glance in the drawings made according to electron microscope images.
The olfactory cell is composed of three main sections; the cell body in the middle, hairs calls cilia on one end, the extension called axon on the other end. The cell body is the section where many complex cellular functions are carried out, the axon where electrical signals are carried and cilia where contact with scent molecules is made.
The numbers of the olfactory hairs at one end of the cell change between ten and thirty, their length being from 0.1 to 0.15 millimeters. The difference of olfactory hairs from their kindred in other sections of the nose is that they do not move and they have olfactory receptors. In other words, the olfactory hairs have a different structure from the other hairs in the body special to them. The olfactory hairs also have a task of forming a skeleton for the receptors. If observed, it is seen that the shape of these hairs is the most efficient model there could ever exist; so in such a small area, there forms a wide space where olfactory molecules can communicate with receptors. In addition, the latest research has shown that every olfactory cell only has one of the thousands of different types of olfactory receptors.
A Unique Messaging Center: The Olfactory Bulb
The olfactory bulb is just on top of the olfactory region and the bone that makes up the skull. There are actually two olfactory bulbs, the main olfactory bulb, and an auxiliary with respect to two olfactory regions; the size of every one is as small as a pea. However, despite this smallness, it looks like a giant communication center or base in terms of the tasks it does. All signals coming from the olfactory receptors are collected first in this center. Millions of pieces of information are re-arranged and sent on to the olfactory cortex, hippocampus, amygdala and hypothalamus in the brain via special olfactory nerves. So this tiny organ is the place where the perfect coordination among millions of olfactory cells is carried out.
The olfactory cells bring the messages to the olfactory bulb Mitral cells are the ones, which carry the messages they receive from the olfactory bulb to the brain and in a grown human being their numbers are around 50.000. The communication between the two groups of cells is established in the communication units called “glomerulus” located in the olfactory bulb. Let us remind you that this communication unit, which has a circular shape has a diameter of 0.1 millimeter. There are almost 2000 glomerulus in a single olfactory bulb. The axons of almost 25.000 olfactory cells and the dendrites of almost 25 million mitral cells meet in every glomerulus.
When we look at the numbers above as a whole, even more amazing numbers emerge. Messages coming from millions of olfactory cells are transferred to tens of thousands of mitral cells. Millions of pieces of information change places among the cells in a flawless way and in time intervals on the order of one millionth of a second.
Furthermore, as a result of the collection, organization and arrangement of all the information coming from every single receptor, sensitivity of smell increases; that is better results are achieved. We can describe the flawless communication here as follows:
Let us assume that certain information is carried over one million phone lines and the number of these lines is suddenly decreased to one thousand In case of such a decrease, it would be impossible not to have a loss of or error in information. No matter how advanced the technology used is, it is impossible to prevent this. On the other hand, olfactory cells continue to do the same task in a flawless manner for as along as we live. This is a certain fact that the message transfer in the olfactory bulb is the result of an amazing system.
