Ann Na, of Kelston Girls' School, asks :-

What is it we detect, and how, through smell?

Murray McEwan, a chemist at the University of Canterbury and who is developing an `artificial nose' for medical diagnostics, responded.

Smell is the result of airborne chemicals that reach sensory chemoreceptors located in the nose.

The organ of smell in humans is the olfactory epithelium which is situated up on the roof of the nasal cavity (the hole behind your nose). The olfactory epithelium is a patch of tissue about the size of a postage stamp that is made up of nerve cells with microscopic hairs or cilia coming out of the cells. The cilia are immersed in a layer of mucus. The chemoreceptors in the cilia are protein molecules folded in such a way that a particular odorant molecule can dock at that site. There are lots of these receptors - at least 10 million of them in your nose. The docking process stimulates the cilia to produce a nerve signal.

The nerve signals move along the receptors to the olfactory nerve. The olfactory nerve transmits the signals to the olfactory bulb that is a stem-like projection under the front part of the brain. It is the brain?s job to interpret the nerve signal and to identify the smell for you.

The human nose, although very sensitive, is not as sensitive as that of many animals that rely on their olfactory organs to locate food. Even so, humans can distinguish between thousands of different odorant molecules. For some odours, e.g. the smell of rotting meat, one component of that smell, ethyl mercaptan, can be detected at concentrations as low as a few parts per trillion. When you have a cold, the cilia are covered with excess mucus and that deadens the sensitivity to smell.