The nociceptive withdrawal response of the foot in the spinalized rat exhibits limited dependence on stimulus location.

The nociceptive withdrawal response (NWR) of the limb is a protective, multi-joint movement in response to noxious stimulation of the homonymous limb. Previous studies in animal models differed as to the dependence of the response direction and magnitude on stimulus location. The specific aim of our research was to use three-dimensional high-speed video to determine whether movement of the foot in response to heat stimuli delivered to the foot and lower leg depended on the location of the stimulus. In particular, we sought to determine whether the movement strategy was categorical or continuous. In spinalized rats, localized, presumably nociceptive heat stimuli were delivered along three dimensions-circumferentially around the lower leg, circumferentially around the foot and along the plantar surface of the foot. Our results demonstrate that in spite of a wide range of stimulus locations over the hind foot and leg, response directions were restricted to two-rostral/medial/dorsal and caudal/medial/dorsal-directions, consistent with a categorical strategy. Further, the preference for these two directions was also reflected in the distance of the movement, which was greatest for stimuli directly opposite the preferred response directions. However, significant but weak dependencies of response direction and distance on stimulus location were found for all three dimensions of stimulus application, supporting a continuous strategy. Together, our results demonstrate, based on movement analysis, that the NWR employs a hybrid categorical-continuous strategy that may minimize the harmful consequences of noxious stimuli.