Forward loop

Mangan, S. and Alon, U., Structure and function of the feed-forward loop network motif, Proc. 

The behaviour of clock-gene products in the SCN provides strong support to the current model of the circadian clockwork as an autonomous intracellular mechanism based upon interlocked feedback and feed-forward loops. Intercellular signalling, more specifically convergent mechanisms for up- and down-regulation of Per expression, is critical to circadian entrainment, which can now be explained in mechanistic terms rather than described by formalisms. 

In general, backwards forwards looping has fallen out of favor in the sampler design community, probably because it does not offer enough advantages to be worth the small extra effort to implement. 

Interestingly, the requirement for an non-clicking backwards-forwards loop is even symmetry around the loop points. A signal x[n] is referred to as even if x[n] = x n. A signal is odd if x[-n = —x[ri. A cosine wave is even around the origin and a sine wave has odd symmetry around the origin. 

Figure 8.11 Backwards forwards loop at a loop point with even symmetry.

Backwards Forwards Crossfade Looping. A backwards forwards loop at any arbitrary point can also be created (or improved) by modifying the loop data. Here the crossfade is performed not between the data at the beginning and end of the loop, but from before and after the loop points. 

Mangan, S. Zaslaver, A. Alon, U. The coherent feedforward loop serves as a sign-sensitive delay element in transcription networks. J Mol Biol 2003, 334 197-204. Kalir, S. Mangan, S. Alon, U. A coherent feed-forward loop with a S UM input function prolongs flagella expression in Escherichia coli. Mol Syst Biol 2005, 1 2005-2006. 

Mangan, S., Itzkovitz, S., Zaslaver, A. andAlon, U. (2006) The incoherent feed-forward loop accelerates the response-time of the gal system of Escherichia coli. J. Mol. Biol. 356, 1073-1081. 

In this configuration, a CSTER is imbedded between two perfectly-mixed reactors 1 and 3 in the forward loop. As in model A above, electrolyte recycling is represented by a perfectly-mixed reactor 4 in the feedback loop shown in Fig.5.2-3(6). Electrolysis takes place in reactor 2 and the collector is reactor 5. 

In packed columns, response to liquid changes appears faster, and the speed of response of the schemes 16.46 and d is less troublesome. In one case (388), considerable improvement in the speed of response of a butene splitter was experienced after trays were replaced by packings. A feed-forward loop, used for speeding the response of trays, was found unneeded and undesirable following the revamp. 

The theorem, which easily can be extended to multivariable systems [14], covers all possible zero crossings due to the recycle feedback. To make the relationship to individual unit properties more clear, one can note that detG(s) is a measure of couplings between the variables in the forward loop. This becomes even more clear by the fact that we can write [14]... 

Fig. 2. Four NMs discovered in yeast (A) auto-regulatory motif (B) feed-forward loop (C) single input module and (D) multi-input module.

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