On the Coexistence of Monitoring and Dampening of Pain Perception During the Fight-or-Flight Response.

Pain-stimulus input is transmitted by mechanisms that respond to real or perceived tissue injury and propagate the information to the spinal cord for reflex withdrawal and to the brain for intentional withdrawal. To be useful, perception of the pain signal needs to be a reliable indicator of the magnitude and duration of the sensory input, i.e., threat, so that appropriate action can be taken, such as fight or flight. Two components of signal transmission are the "ascending pathways," mediating sensory input → perception, and the "descending pathways," mediating perception → response. Ascending pathways are often modulated by descending pathways. We previously introduced Lotka-Volterra-style coupling to model ascending and descending pain pathways as a coupled control and feedback loop. Here, we apply the model to examine the value of modulatory feedback to the fight-or-flight response and identify separate response functions that allow for both a quantitative assessment of the immediate threat, i.e., the magnitude of the acute pain signal, and damping of the signal in order to preclude sensory overload leading to counterproductive inaction. As a result of increasing inhibitory modulation that accompanies increasing pain-stimulus magnitude, pain perception rises rapidly as a function of stimulus intensity and then begins to decay as modulation ramps up, but not to zero. Instead, it saturates or plateaus at a value intermediate between zero and peak/maximum perception, at a level that is related to the stimulus intensity. Although the contribution of this work is conceptual and application-focused rather than presenting new mechanistic insights, the observed response functions, i.e., pain perception and modulation, allow for both a quantitative assessment of the immediate threat and damping of the pain signal in order to preclude sensory overload that might lead to counterproductive inaction. Moreover, the fact that plateau pain perception is less than peak/maximum pain perception for respective sensory inputs prevents obscuration of subsequent new, i.e., cumulative, injury.