Title: Robust signal amplification and information integration via self-tuned proximity to bifurcation points
In various biological systems information from many noisy molecular receptors must be integrated into a collective response. A striking example is the thermal imaging organ of pit vipers. Single nerve fibers in the organ reliably respond to mK temperature increases, a thousand times more sensitive than their molecular sensors, thermo-TRP ion channels.
We propose a mechanism for the integration of this molecular channel information. Amplification of the signal arises due to proximity to a dynamical bifurcation, separating a regime with frequent and regular firing of action potentials (APs), from a regime where AP firing is irregular and infrequent. Near the transition, AP frequency can have an extremely sharp dependence on temperature, naturally accounting for the thousand-fold amplification.
Furthermore, close to the bifurcation, most of the information about temperature available in the TRP channels’ kinetics can be read out from the times between consecutive APs even in the presence of extrinsic noise. A key model prediction is that the coefficient of variation in the distribution of interspike times decreases with AP frequency, and quantitative comparison with experiments suggests that nerve fibers of snakes are indeed located very close to the bifurcation.
While proximity to such bifurcation points typically requires fine-tuning of parameters, having feedback act from the order parameter (AP frequency) onto the control parameter robustly maintains the system in the vicinity of the bifurcation. This robustness suggests that similar feedback mechanisms might be found in other sensory systems which also need to detect tiny signals in a varying environment.
To illustrate this idea, I’ll also briefly mention other sensory systems, for which we hypothesize a similar amplification mechanism due to self-tuned proximity to a critical point with diverging susceptibility.