Biological Olfaction - The Electronic Nose Artificial Olfaction Technology - page 75

3.4.1 Steps in Olfactory Transduction

Odorant molecules ? Golf ? adenylate cyclase ? cAMP ? protein Kinase ? Ion
Channel Protein ? Membrane Conductance change ? Receptor potential ?
Electronic propagation ? Action potential.
When odorants bind to the receptor site, the receptor protein changes shape
which in turn triggers the flow of ions across the receptor-cell membrane and an
electrical response is triggered in the cilium. Electrical responses in the cilia
spread to the rest of the receptor cell, and from there are passed onto the olfactory
bulb of the brain in the olfactory nerve. There are about 1,000 different types of
receptor proteins each sensitive to different odorants. Human have a total of about
10 million receptor neurons. Each receptor neuron has about 1,000 similar
receptor proteins. Because there are 1,000 different receptor proteins, there are also
1,000 different receptor neurons.
The olfactory systems gather at the interface of the environment and the ner-
vous system. The olfactory system is responsible for correctly coding sensory
information from thousands of odorous stimuli. Many theories existed regarding
the signal transduction mechanism that mediates this difficult task. The discovery
that odorant transduction utilizes a unique variation (a novel family of G protein-
coupled receptors) based upon a very common theme of the G protein-coupled
adenylyl cyclase cascade to accomplish its vital task emphasized the power and
versatility of this design. Within the compact cilia of the OSNs a cascade of
enzymatic activity transduce the binding of an odorant molecule to a receptor into
an electrical signal that can be transmitted to the brain. Sensory transduction is
shown in Fig. 3.10 . As described in detail, this is a classic cyclic nucleotide
transduction pathway in which all of the proteins involved have been identified,
cloned, expressed and characterized.

Fig. 3.10 Sensory transduction [ 12 ]