The astonishing temperature sensing capability of pit vipers

What is it about?

Many living systems seem to perceive their environment entirely differently from a human. If a mouse was a meter away from us, we would typically only notice it if we see, hear, or maybe smell it. But we wouldn’t be able to feel its presence via the heat it emits – the resulting change in the temperature of our skin would just be too small for us to detect. Certain snakes, however, rely on sensing radiant warmth for finding their prey at night when visual cues are not available. This infrared sensing of snakes is a particularly striking example of how biological systems make sensitive amplifiers: it has been shown that individual nerve fibers in the snake’s sensory organ can reliably respond to milli-degree changes in temperature, far more sensitive than our own thermally sensitive neurons, which only respond to temperature changes a thousand times larger. Perhaps surprisingly, at the molecular scale, we and snakes both use thermally sensitive ion channels to detect temperature, and at the molecular scale these ion channels have a similar sensitivity of a few degrees. How did the snake learn to amplify this molecular response by a factor of a thousand? In this work, we propose a plausible mechanism for the integration of this molecular channel information into a collective, neural response. In our mathematical model, amplification is achieved via proximity to a special point in the parameter space, a bifurcation, where the collective dynamics qualitatively change, and small signals are strongly amplified. There, the snake’s brain can get almost as much information about temperature as if it could read out all the measurements from the individual ion channels and average them. Our study also suggests how the snake can maintain its incredible sensitivity even as the ambient temperature changes by several degrees between day and night. A simple feedback motif can naturally maintain the system close to the bifurcation – without the need for fine-tuning of parameters.

Featured Image

Photo by Dulce Wilson on Unsplash

Photo by Dulce Wilson on Unsplash

Why is it important?

This challenge of detecting very small changes in varying environments is not unique to snakes. Indeed, many organisms are intrinsically noisy and still exhibit incredible sensitivity to weak signals – so weak that information from many measurements must be aggregated to get a statistically significant, amplified readout. Our work shows that proximity to bifurcation or related critical points has a functional benefit for sensory systems: It allows for efficient transmission of noisy, molecular information into a collective response even if the readout is corrupted by additional noise. Our study further indicates that robust self-tuning to these points can be achieved by a simple feedback mechanism. This robustness suggest that similar feedback and design principles might be found in other sensory systems which also need to detect tiny signals against varying backgrounds.