Scientists uncover why human sense of smell is so poor
A European team of scientists has discovered that the olfactory bulb, a structure in the brain that processes sensory input from the nose, has something completely unique. It differs from all other mammals because no new neurons develop in this area after birth. This finding could shed light on why humans lack the heightened sense of smell that animals have. Presented in the journal Neuron, the study was funded in part by the European Research Council (ERC) under the EU's Seventh Framework Programme (FP7). This project was led by the Karolinska Institutet in Sweden, in collaboration with researchers in France, Austria and Sweden.
New neurons in the adult mammal are formed in two brain regions: the hippocampus and the olfactory bulb. Memory is connected to the hippocampus and the interpretation of smells is associated with the olfactory bulb. Despite efforts made to increase our understanding about the formation of new nerve cells in the human brain, no clear answer has ever emerged... until now.
The researchers in this study pieced together this puzzle by estimating the age of the cells. For this purpose, they measured how much of the radioactive isotope carbon-14 these cells contained. Carbon-14 is a natural radioisotope of carbon present in organic materials and hence incorporated in the DNA when cells are produced. Its decay is used as a dating method.They observed that the olfactory bulb neurons in the adult human subjects had carbon-14 levels that matched those in the atmosphere at the time of their birth. So no new neurons are produced in this part of the brain, making humans distinct from other mammals.
'I've never been so astonished by a scientific discovery,' explained lead investigator Jonas Frisén, Tobias Foundation professor of stem cell research at the Karolinska Institutet, who also received a ERA Advanced grant for his work. 'What you would normally expect is for humans to be like other animals, particularly apes, in this respect. 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'.
Researchers used to believe that brain neurons were created up to the time of birth, and not beyond. But experts later discovered that nerve cells still formed from stem cells in the mammalian brain. Researchers then changed their position on the plasticity of the brain, opening up the possibility that nerve cells lost during various neurological diseases could be replaced.
Other funding for this study came from the Tobias Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research (SSF), the American Brain and Behaviour Research Foundation (NARSAD), the Swedish Brain Fund, the Knut and Alice Wallenberg Foundation, AFA Försäkring and Stockholm County Council through its ALF agreement with the Karolinska Institutet.