Biological Olfaction - The Electronic Nose Artificial Olfaction Technology - page 67

Fig. 3.1 Functional components of olfactory system

outsiders). The architecture and dynamics of the olfactory system have evolved to
solve the major problems in olfactory information processing.
The chemical sensors in the olfactory system act as receptors and the response
forms a pattern (a set of signals) that is processed in the brain to produce an
appropriate response due to a stimulus. Figure 3.1 shows a block diagram of the
olfactory system. In a human, there is an array of 100 million odor sensors. Within
this array, there are a number of different types of sensors, which display differing
odor specificity to particular classes of odors. Individual elements in the array
show broad and overlapping selectivity to chemical species. Importantly, olfactory
receptors are not highly selective, but selectivity is achieved by the unique patterns
of responses from numbers of such receptors.
Taste and smell are highly associated with each other, and flavors are composite
sensations, derived from primary taste and smell sensations by processes within
the limbic brain areas that participate in emotion. The human emotional system
emerged in evolutionary terms from the olfactory brain of earlier animals and
remains closely linked to taste, smell, and eating behaviors. Taste and smell
evaluation may be an important diagnostic screen for early brain changes destined
to result in serious dementia [ 3 ]. The odors of coffee, chocolate, almond and oil of
lemon are often used for casual testing of olfactory function. Diversity of different
methods has been used to understand olfaction. Operations of olfaction can be
divided roughly in five parts: sniffing, reception, detection, recognition, and
cleansing.
The olfaction starts with sniffing that mixes the odorants into a uniform con-
centration and delivers these mixtures to the mucus layer in the upper part of nasal
cavity. Next these molecules are dissolved in this layer and transported to the cilia
of the olfactory receptor neurons. Reception process includes binding of these
odorant molecules to the olfactory receptors. Odorant molecules are binded tem-
porarily to proteins that transport molecules across the receptor membrane with
simultaneous stimulation of the receptors [ 4 ]. During this stimulation the chemical
reaction produces an electrical stimulus. These electrical signals from the receptor
neurons are transported to the olfactory bulb. From the olfactory bulb the receptor