  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. Here, we propose a mechanism for
the integration of this molecular information. In our model, amplification
arises due to proximity to a dynamical bifurcation, separating a regime with
frequent and regular action potentials (APs), from a regime where APs are
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 timing of APs even in the presence of readout noise. While
proximity to such bifurcation points typically requires fine-tuning of
parameters, we propose that 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.
