Neurobiology of Olfaction
Terry Acree

All organisms detect chemicals in their environment and behave differently as a result. Multi-celled Eukaroytes usually detect extra-cellular chemicals as they bind to special receptor proteins floating in the membrane of sensory cells. These receptor proteins (RP) are almost exclusively seven trans-membrane proteins i.e. they have seven distinct helical regions that are more soluble in the membrane than either the inside or the outside of the sensory cell. Chemicals are detected when they bind to the extra-cellular portion of the RP and cause a change in the shape of the protein on the inside of the cell. This change in protein structure at one site induced by a ligand binding (stimulant binding) at a distant site is called an allosteric effect and it is the initial step in the process of chemical detection. Eukarotes use a number of mechanisms in response to the allosteric change in the RP. When mammals detect chemicals in the air and in their food the allosteric effect causes chemical changes in the sensory cell that ultimately causes an adjacent neuron to depolarize. Taken together this process is called sensory transduction because the chemicals are not metabolized but simply detected. Their presence and not their chemical potential is "transduced" into the cell. What is transferred is not the molecule itself but information about its presence outside the cell.

The most important feature of sensory transduction in taste and smell is its amplification mechanisms. A very few molecules binding to the RP outside the cell results in the presence of thousands of molecules or ions inside the cell. This allows an organism to know about the presence of toxic chemicals before the concentration in the environment becomes toxic. For the location and evaluation of food, mates or other desirable chemical environments the ability to detect extremely low concentrations of chemicals is very useful and drives much of the behavior of mammals.

Aroma

The transduction system for smell differs from that used to detect taste molecules in that the olfactory sensory cell is itself a neuron: the olfactory sensory neuron (OSN). When the odorant ligands reach a threshold concentration at the olfactory RP on the outer surface of the olfactory sensory cell the whole cell depolarizes (changes voltage across it's membrane) and this causes release of neurotransmitter at the distal end of the cell which is buried in a part of the brain that sits just above the nose. The OSN in humans, as well as other mammals, is a single cell that begins in the nasal cavity with several cilia (finger like projections) that lie in a layer of mucus exposed to air drawn in through the nose or exhaled from the lungs. From there, the olfactory epithelium, the OSN projects up through the skull via small channels to a part of the brain called the olfactory bulb. The bulb has two lobes one for each side of the nose. From this anatomy you might expect there to be a "stereo-olfactory" effect but there isn't. You cannot tell which nostril is detecting odor; this is not true of trigeminal stimulants as anyone who has had too much wasabi would know. As a student in this class pointed out: if humans evolved to use "stereo-olfactory" perception as fish have, we would have a different looking nose. Perhaps something like the organ arrangement hammerhead sharks use to detecting water pressure gradients. Not a pretty thought.

From the olfactory epithelium the OSNs project into spherical structures in the olfactory bulb called the glomerulus. There are hundreds of these glomeruli in each lobe of the olfactory bulb each one associated with a particular ORP. It seems that during development ORN grows from the olfactory epithelium in the nose up to a particular glomerulus depending upon which RP is expressed by the cell. Each cell expresses only one type i.e. one genetically distinct RP. The result is a one to one correspondence between a RP and a glomerulus.

This organization means that information used by the brain to distinguish odors is based on the topology of the olfactory bulb. A caricature would be that the brain creates its awareness of odor from the three dimensional pattern of activity in the olfactory bulb. How this information is processed into perceptions is still a mystery but it clear that the mixture of stimulants released into the spaced bordering the olfactory epithelium produce a 3-D pattern of neural activity in the brain. A sensory map translated from some of the chemical features in aroma mixtures.

Psychophysics

Individual differences

The sensory map.