Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.
Whenever a tissue injury occurs it causes release of local pain producing substances into the extracellular fluid surrounding the injury. They are K+, H+, prostaglandins, histamine, leucotriens, bradykinines, substance P, slow reacting substances of anaphylaxis (SRS-A), etc. Substance-P may cause the local edema or swelling.
These local chemicals activate the nerve endings and transmit painful stimulation to central nervous system through spinal cord. Theses nerves are called "nociceptors". Two types of nociceptors (C fibres and A-delta) are present as free nerve endings in the periphery and viscera or internal organs.
C fibres are activated by chemical, thermal, mechanical and cold stimulation. A-delta fibres transmit at a rate of 10 - 25 times faster than C fibres and are activated by both mechanical and thermal stimulation. Somatic structures like skin, muscle, joints are rich with both C and A-delta fibres. Visceral or internal organs are rich in C fibres along with some A-delta fibres. As A-delta fibres transmit stimulation at faster rate, they give precise location of injury and due to slower rate transmission of C fibres from visceral structures, pain is ill defines.
So, there is an ascending track which takes this pain information to the different parts of brain like, reticular formation, hypothalamus, thalamus, limbic system which influence emotion and memory.
Pain is more than just transmission of a signal from the periphery to the cerebral cortex (deep brain structures). It is a multidimensional process involving past experience, emotion, cultural background, motivation and family and social dynamics. Automatically this pain influences or activates autonomic reflexes which affects on respiratory rate, pulse and blood pressure. That's why chronic pain affects the psychology of the patients due to continuous stimulation for long duration.
Considerable research has focused on descending pathway which passes to the different areas of spinal cord and inhibit pain sensation by serotonergic and norepinephrine neurotransmitters. During pain, body releases some endogenous pain killers which are called "endorphins" which act on different opioid receptors in the spinal cord like, mu (μ), kappa (κ), delta (δ), and epsilon (έ) to reduce pain. If these endogenous pain killers are not enough to reduce your pain then you have to use some endorphins like drugs which will act on these receptors in the spinal cord and reduce your pain.
Severe chronic pain is a massive medical, economic and social problem, which is a silent epidemic, the disease of the 21st century. The American Pain Society (APS) has created the phrase "Pain: the fifth vital sign" to evaluate awareness of pain treatment among healthcare professionals (other vital signs are: Pulse, Blood pressure, Respiration & Temperature, which are also highly related to pain). As real pain relief is a basic human right, we must continue to work to alleviate pain for the benefit of mankind.