Background and Aim: The Gate Control Theory (GCT) proposed by Melzack can interpret different cases of acute and chronic pain according to which, the Substantia Gelatinosa of spinal cord plays a role of gating across the information transfer to brain. Mathematical models have shown the efficacy of the GCT already. However, the abrupt and instantaneous painless states in case such as sudden accident is not consistent with the GCT theory. A person may not feel any pain at all immediately after accident despite large pain stimuli (nociceptors C fibres) whereas no cognitive feedback has been established yet. In this paper, a new structure of pain system in the spinal cord has been presented and simulated. Methods: We have proposed a modified pain system in the spinal cord which In this model,two new categories of synapses are considered in addition to previous works. activating synapses of small diameter fibres with inhibitory cells and deactivating synapses of inhibitory cells to excitatory cells. Adding these synapses, two extra terms appear in the previous mathematical model (Britton et al.).The proposed mathematical model has been simulated regarding different paradigms of pain sensation in the spinal cord . Results: The proposed model has been mathematically simulated using three different cell population in the spinal cord: T-cells (lamina V), inhibotry and excitatory cells of Laminae I-IV. According to simulation, painless state may occur even in the presence of nociceptor signals, before any cognitive or midbrain modulating signal affecting the synapses with spinal cord cells (T-cells). Conclusion: : We have proposed a modified pain system in the spinal cord which can interpret immediate suppression of pain in the presence of small-diameter stimuli.

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