Exploring critical and bifurcation points across biological contexts: from tuning to function

Many biological systems are very sensitive to small input signals. A tempting hypothesis is that these systems operate close to bifurcation or critical points, where the system's response exhibits a diverging susceptibility to the control parameter, and small signals are amplified into a large collective response. However, a common concern is that being close to these points requires fine-tuning of parameters, which seems impossible for noisy biological systems subject to varying environments. Based on several examples ranging from snake thermosensing and E. coli chemosensing to fly olfaction, we have investigated a feedback motif that robustly maintains these systems close to their respective critical or bifurcation point. The key ingredient is that the collective response feeds back onto the control parameter. Such a feedback scheme works well if just a single control parameter needs tuning. But robust critical behavior has also been found in high-dimensional systems like plasma membranes made up from thousands of components and in mammalian hearing where local activity of hair cells brings the whole cochlea to the edge of instability. Here we argue how these systems could find the critical manifold in the high-dimensional parameter space and why critical behavior might occur naturally in high-dimensional systems.