The three principal types of pain are nociceptive pain, musculoskeletal pain and neuropathic pain. The most common causes of pain are musculoskeletal problems and inflammatory conditions.1
Nociceptive pain occurs as a result of the activation of the nociceptive system by noxious stimuli, inflammation or disease.2, 3Nociceptive pain originates in the nociceptors, which are highly specialized primary sensory neurons found primarily in the skin or joints or the walls of organs.4 These receptors convert noxious stimuli into electrical current through a process called transduction. The current is then conducted along the nociceptor axons to their cell bodies, located in the dorsal-root ganglion in the spinal cord, and then to their central terminals located in the dorsal horn. Here, the electrical impulses initiate neurotransmitter release from nociceptor central terminals, which relay the signal across synapses to the dorsal horn neurons.
Musculoskeletal pain is initiated when noxious stimuli deform the axonal membrane of muscle nociceptor endings beyond their firing threshold—causing the release of sensitizing substances such as bradykinin, prostaglandins and serotonin from muscle tissue.5
Once sensitized, group IV afferents fire at lower thresholds—responding to innocuous stimuli like weak pressure and muscle movement—causing the exquisite tenderness experienced from pressure to a myofascial trigger point (MTrP). 6, 7, 8
Continuous input from myofascial trigger points can increase excitability of dorsal horn neurons (causing allodynia and hyperalgesia) and open ineffective synapses—resulting in new receptive fields and the referral of pain.9
This dynamic process can fundamentally alter pain threshold, pain intensity and emotional affect.10
Neuropathic pain involves injury or alteration of the normal sensory and modulatory nervous systems. Painful neuropathies can result from a broad range of etiologies, including:11 Metabolic and endocrinologic disorders such as diabetes and liver disease. Etiologies also include infection, demyelinating disorders, stroke and spinal cord injuries, shingles, and other factors.
Visceral pain is defined as pain that originates from deep visceral structures and can be identified by location (e.g., abdomen)12 and by pain features (e.g., dull, cramping).13,14 Relatively minor lesions in viscera such as the stomach, the bladder or the ureters can produce excruciating pain. Many diseases of the liver, the lungs or the kidneys are completely painless and the only symptoms felt by the patient are those derived from the abnormal functioning of these organs.15 Visceral pain is characterized by the following properties:16
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10Zieglgänsberger W, Berthele A, Tölle TR (2005) Understanding neuropathic pain. CNS Spectr 10:298-308.
11Pappagallo M. Neuropathic Pain in Peripheral Neuropathies. Tollison CD, Satterthwaite JR, Tollison JW (Eds). Practical Pain Management, Chapter 29, 3rd ed. Philadelphia, PA: Lippincott-Williams & Wilkins; 2002: 431–48.
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15Cervero F (1994) Sensory innervation of the viscera: peripheral basis of visceral pain. Physiol. Rev. 74, 95-138.
16Wall and Melzack’s Textbook of Pain, 5th Edition. p.721. 2010.