Biology Reference
In-Depth Information
circuits as they are also known from electrophysiology (where they are also
often based on data at the molecular level; cf. Schaffner, this volume).
5
4.1. Cybernetic models in biology
Though cybernetic approaches have been applied at the molecular level in sys-
tems biology, many of the prominent models from biological cybernetics deal
not with molecular but with nervous control systems, often related to the visual
system. Among such nervous control systems is the regulation of the diameter
of the human pupil in response to light conditions. The iris sphincter muscle
contracts in response to stimulation of the retina by light, thus decreasing the
diameter of the pupil and hence the intensity of illumination of the retina. Both
pupils are regulated separately, but not completely independent from each other.
This allows for a cybernetic analysis of the network that brings the regulation
about. As a result, the order of nonlinear transformation of the stimulus and
integration of stimuli from both retinas could be determined and an equivalent
circuit of the system assigned to anatomical structures of the nerve pathways of
the visual system (Stark, 1959; Varjú, 1967, 1969).
That modeling in these cases does not refer to the molecular level of the system
could explain why several authors who have proposed reconstructions of the
history of systems biology do not refer to cybernetics at all. However, biological
cybernetics has investigated other than nervous systems as well, and well-known
cases combine analyses of molecular processes and neural signaling. This is the
case, say, for Hodgkin and Huxley's model of nervous signal propagation in
the giant axon of squid (Hodgkin et al., 1952; Hodgkin & Huxley, 1952; see
Krohs, 2004 and Schaffner, this volume), which today still provides the basis
for electrophysiological modeling. It also holds for the analysis of the lateral
inhibition of visual neurons in a field of receptor neurons, which has resulted in
a model of synaptic processes involving the molecular level (Furman, 1965).
An important instance of an application of systems analysis that refers directly
to the (biochemically unidentified) molecular level is the early model of the
circadian clock by Arthur Winfree (1967, 1977). The circadian clock regulates
sleep-wake rhythm even in the absence of any
Zeitgeber
. The phase of the
rhythm can be shifted by light stimuli and by certain chemical stimuli. This
led Winfree to postulate an endogenous biochemical oscillator of unknown
5
Norbert Wiener was one of the inventors of cybernetics as an approach applicable to the description of
regulatory processes in biology as well as in engineering (Rosenblueth et al., 1943; Wiener, 1948) and
is regarded as one of the forerunners of systems biology (e.g., Kitano, 2002a). Ludwig von Bertalanffy's
General System Theory (Bertalanffy, 1932-1942; 1968) was to some extent incorporated in later cybernetic
and systems-theoretical accounts (see, e.g., Mesarovic, 1968), which today inspire a hierarchical approach to
systems biology (Mesarovic et al., 2004). Other systems theoreticians that became important for biological
modeling are discussed in O'Malley and Dupré (2005).
