The finding helps explain why the loss of smell is irreversible

Jun 8, 2012 14:44 GMT  ·  By
This is KI's Tobia Foundation professor of stem cell research, Jonas Frisén
   This is KI's Tobia Foundation professor of stem cell research, Jonas Frisén

According to Swedish researchers, humans exhibit a far weaker sense of smell than other animals simply because neurons in a region of the brain that process data input from the nose do not get replenished over time.

The number of nerve cells in the olfactory bulb remains constant or decreases throughout a person's life time, experts from the Karolinska Institutet (KI) have discovered. They used the carbon-14-based dating method to assess the age of neurons in this region of the brain.

Details of their investigation were published in the latest issue of the esteemed scientific journal Neuron. In most other mammals, the olfactory bulb can be replenished with new nerve cells.

In the past, neuroscientists believe that all areas of the brain were formed with a fixed number of neurons that could not increase. Over the past few years, it has been demonstrated that numerous regions can in fact replenish their neuron count. However, the same is not true for the olfactory bulb.

“I've never been so astonished by a scientific discovery. What you would normally expect is for humans to be like other animals, particularly apes, in this respect,” expert Jonas Frisén says.

The scientist, who was the lead investigator on the new research, holds an appointment as the KI Tobia Foundation professor of stem cell research. He explains that, in mammals, the hippocampus and the olfactory bulb are the two main regions where new neurons form.

The hippocampus is an area of the brain responsible for encoding memories. The olfactory bulb, which interprets smells, works with the hippocampus to create associations between various smells and memories the mammal has of past encounters with an object, or another creature.

Analyzing how many new neurons grow in the human brain is a very complex task, which is why scientists have thus far had a tough time conducting such research. What enabled them to move forward was the realization that Cold War nuclear tests boosted the amount of airborne carbon-14.

Since the carbon isotope is incorporated into human DNA, they were able to measure how many neurons were in the human olfactory bulb at the time of birth. They discovered no significant neural regeneration in this region of the brain.

“Humans are less dependent on their sense of smell for their survival than many other animals, which may be related to the loss of new cell generation in the olfactory bulb, but this is just speculation,” Frisén concludes.