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A Glossary for Systems Biology


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CONTROL

see also

proportional control, integral control, differential control, PID control

Meaning

Biology:

Keep a system's property within physiological limits.

Systems Theory:

Distinguishes between feed-forward (open-loop) and feedback (closed-loop) control (see Illustration).

Explanation

There is an ambiguity in the English language that makes using the term control problematic [55]. Without qualifying additions there can be no distinction between open-loop and closed-loop control, which are two completely different things.

Feed-forward control is an open-loop control in which a sequence of predefined actions is triggered by a certain stimulus. It is a simple control method ``that works when possible situations and countermeasures are highly predictable'' [31]. Because open-loop or feed-forward control reacts in a predefined way it is rather inflexible and only works for the limited set of conditions considered during development of the system. Feed-forward control has the advantage of faster reactions, though, because it only has to execute the predefined scheme.

Closed-loop or feedback control on the other hand does not rely on predefined reactions to counter deviations from a desired value. Rather it 'feeds back' the real value of the property to be regulated, compares it to the desired value and acts upon the deviation. It is therefore a lot more flexible and able to react to a wider range of situations that do not have to be individually pre-considered during design. The complex principle makes it more complicated to design and slower to react than a feed-forward control. (see Examples and Illustrations for further explanations)

Both types of control are used in biology as well as in technical systems (see Examples). In biology, the aim often is to keep a system's properties within physiological limits. This is implemented through interconnections or relations between components, e.g. of a metabolic pathway or genetic network. Control is often used without distinguishing between feedback and feed-forward control in biology. In these instances, control is used in the sense of feedback control most of the time, and often 'regulation' is used instead of 'feedback control'.

In systems theory the distinction between feed-forward and feedback control is usually made explicitly. Without further qualification, control is usually meant in the sense of feedback control.

Examples

Biology:

  • chemotaxis: ``The most typical example [for feedback control] is the integral feedback circuits involved in bacterial chemotaxis.'' [31]
  • heat shock: combines feed-forward and feedback control in order to benefit of both principles' advantages. [14,31]; see Illustrations.
  • neuro-regulation circuits [29]

    • sensitivity of muscle spindles, regulated by $ \gamma $ motor neurons
    • muscle tension regulation through 1b inhibitory interneurons: negative feedback
    • flexion withdrawal reflex ('tack-step' reflex): feedback and feed-forward control
    • neural pathways: feed-forward and feedback inhibition

Systems Theory:

  • heating system (integral control for illustration and explanation) [63]
  • fill-level control in a liquid holding tank [20]
  • airplane autopilot

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