Biological Olfaction - The Electronic Nose Artificial Olfaction Technology - page 79

excitable tissues. They are able to develop rapid and transient change in their
membrane potentials.
The process of neural signaling is, in a sense, a cyclic process, so it can be
described anywhere in the cycle and eventually come around to the starting point
again. One logical point at which to start is the process of synaptic transmission,
the transfer of information from one neuron (the presynaptic neuron) to another
(the postsynaptic neuron) by release of a chemical substance, or neurotransmitter,
from the axon terminal(s) of the presynaptic neuron. The neurotransmitter affects
the state of the postsynaptic neuron, making it more or less likely to transmit
information to its targets.
Before any neural processing can take place, there must be some means of
detecting the presence of information in the environment, collecting the different
forms and patterns of energy that represent this information, and converting the
physical energy into a form that can be acted upon and utilized by the nervous
system. The process through which a specific pattern of information (energy) in
the environment (e.g., light, vibrations, dissolved chemicals or airborne chemicals)
is converted to a pattern of electrical activity in the nervous system is called
transduction.
Signal transduction occurs in response to a signal and consists of a series of
post-translational modifications that regulate the activity of proteins. The net result
of this regulatory cascade is a change in cell activity. This change in cell activity
might involve the production of a product, such as activation of the transcription of
a gene. But it might not. It might just result in a different cytoskeletal configu-
ration, vesicle fusion or could result in the repression of a gene's transcription.
Lots of other responses are possible, including cell division, differentiation, or
death. So the easiest way to think about signal transduction pathways is simply as a
mechanism (cascade of posttranslational modifications) that regulates a cellular
activity or activities in response to a signal.
Olfaction (smell), vision (sight), gustation (taste), equilibrium (balance),
hearing, somato-sensory (touch, pain, temperature) are the senses of human sys-
tem. Special sensory receptors are located in sense organs such as the eye or ear,
where the receptors are protected by surrounding tissues. The information these
receptors provide is distributed to specific areas of the cerebral cortex (the auditory
cortex, the visual cortex, and so forth) and to centers throughout the brain stem.
Table 3.2 enlists the sensory system receptor and transduction energy used in the
sensory system.
Beginning with the physical energy of an environmental stimulus, there are
several processes that must occur before a pattern of neural activity is generated.
The steps leading to transduction include the following:
The stimulus energy must reach specialized receptor cells. In taste & touch this
process is relatively simple and straightforward.