Toward Sensor to Reproduce Human Sense - The Electronic Nose Artificial Olfaction Technology - page 126

Sensory perception relies on the way sensory signals are transformed by neural
circuits in the central nervous system. The relation between a given stimulus
parameter and perception were always one-to-one; it would be easy to predict the
effects of changing one or more stimulus parameters. However, studies have
revealed that the relation between the parameters of a sensory stimulus and what is
perceived is often quite complex. In other words, what we perceive is not a direct
reflection of what is present in the environment. Olfaction is different from the
other perceptual modalities in ways that have serious import for the study of
cognition and consciousness.
However, in order for an odorant to be smelled, it must first reach the olfactory
epithelium by traversing the nasal passage and the Mucosa layer within our nos-
trils. Odorants reach the olfactory epithelium and olfactory receptor neurons
orthonasally or retronasally. Orthonasally (from the front of the nose via the
nostrils) the odorant reaches the epithelium either through diffusion from high
levels of concentration to lower levels or through actively sniffing the odorant.
Alternatively, an odorant might arrive from the back of the throat via retro-nasal
olfaction [ 15 ].
Sensory receptors convey the energy of stimuli into membrane potentials and
transmit signals to the nervous system. Sensory receptors perform four functions in
this process: sensory transduction, amplification, transmission, and integration.
Humans have several types of sensory nerves: photoreceptors (for light), me-
chanoreceptors (for touch, sound and equilibrium), chemoreceptors (for smell and
taste), thermoreceptors (for heat), and nociceptors (for pain). Classification of
sensory systems is by type of stimulus. Each sense cell is responsive to one sort of
energy change and can cause only one sensation, though the intensity of sensation
depends on the nerve's threshold value. Normal sensing happens by stimuli
interacting with a biological system, which elicits a positive or negative response.
The receptors that are essential to an organism understanding its environment and
surrounding and of most interest to the engineering community for mimicry are
grouped into the class known as extroreceptors. The three classes of receptors are:

Table 5.2 Human senses and their sensory information

Olfaction Source of sensory information Molecules of the substance being sensed

Receptor organ The nose

Receptor cells Receptors in each nostril can sense different

substances on the basis of molecular shapes

Taste Source of sensory information Molecules of the substance being sensed

Receptor organ Taste buds on the tongue (additional receptors

in the mouth and throat)

Receptor cells Taste receptor are sensitive to five basic taste:

Sweet, sour, salty, bitter and umami

Skin senses Source of sensory information Touch, pressure, warmth, cold and pain

Receptor organ The skin

Receptor cells Receptors that code for touch, pressure,

warmth, cold and pain