analysis of standard, restrictive and supplemental fluid administration in colorectal surgery. Br J Surg 2009; 96: 331-41 3 Varadhan KK, Lobo DN. A meta-analysis of randomised controlled trials of intravenous fluid therapy in major elective open abdominal surgery: getting the balance right.

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Pain is an unpleasant sensation that can range from mild, localized discomfort to agony. The International Association for the Study of Pain's widely used definition states, "pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage" [1][2] . Pain mechanism is a complex phenomenon in which pain is induced by numerous pain modulating systems [2] , sensitized by the neurophysiological systems. Pain can be influenced by the different factors like psychological conditions, health status and sociological factors [3] .Pain is classified into two types, acute and chronic pain. If the pain persists for less than a month is called as acute pain. Clinically, if the pain persists more than three months after the injury, it is called as chronic pain [4,5] . This type of pain may or may not be driven by the tissue injury. The two forms of pains are nociceptive and neuropathic pain [5][6][7] . Stimulation of the pain receptors is considered as the nociceptive pain whereas damage or dysfunctional to the central and peripheral nervous system is considered as the neuropathic pain. Pain signalling and inhibition pathways:Inflammatory mediators like bradykinin, serotonin, prostaglandins and cytokines [8] are released from damaged tissues, which stimulate nociceptors [8][9][10] . Nociceptors are the specialized sensory receptors responsible for the detection of noxious (unpleasant) stimuli, transforming the stimuli into electrical signals, which are then conducted to the central nervous system (CNS) by primary afferent Aδ, C and Aβ fibers that are distributed throughout the body [10] . They can be stimulated by mechanical, thermal or chemical stimuli, called as primary sensitization. Aδ and C fibres transmit information to nociceptive specific neurons in Rexed laminae I and II of the dorsal horn in spinal cord, in addition to projections to other laminae. Primary afferent terminals release a number of excitatory neurotransmitters including glutamate and substance P [10][11][12][13] .Spinothalamic and spinoreticular tract are the two important pathways that carry signals to the higher centers of the brain. Spinothalamic tract is important

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