7. Pain Tracts and Sources
Part 5 of 5

Nachum Dafny, Ph.D.
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Acute pain arises from activation of nociceptors for a limited time and is not associated with significant tissue damage (e.g., a pin prick).

Chronic pain is prolonged pain lasting for months or longer that arises from tissue injury, inflammation, nerve damage, tumor growth, lesion or occlusion of blood vessels. Chronic or inflammatory pain can sensitize (see "Sensitization" below) the nervous system, evoking chemical, functional, and even structural changes that serve to “prime the pain-processing pump”. Chronic pain, such as lower back pain, rheumatoid and osteoarthritis, and headache (see "Headaches" below) may result from constant inflammatory activity which activates G proteins. In some cases, the pain persists long after the injury heals, but there is no treatment that will eliminate the pain. This chronic pain does not serve a protective function like other types of pain.

One possible explanation for chronic pain is a phenomenon called sensitization. Following continuation and prolong noxious stimulation, nearby silent nociceptive neurons that previously were unresponsive to stimulation, now become responsive. In addition, some of the chemicals produced and released at the injured site also alter the physiological properties of nociceptors. The nociceptors begin to initiate pain signals spontaneously, which cause chronic pain. In addition, weak stimuli, such as a light touch that previously had no effect on these nociceptors, will further activate the nociceptors which result in severe pain signals. This phenomenon is referred to as “peripheral sensitization.” The outcome of peripheral sensitization results in a greater and more persistent barrage of nerve impulses firing in the CNS. The persistent barrage of nerve impulses results in long-term changes in nerve cell activity at the level of the spinal cord and higher centers in the brain. This phenomena is referred to as “central sensitization”. It appears that peripheral and central sensitization persists after the injury apparently has healed. The sensitization of nociceptive neurons after injury results from the release of different chemicals from the damaged area. It is known that substance P and calcitonin gene-related peptides are released from peripheral nerve ending which stimulate most cells to release algesic substances which further potentiates the pain from the injury. In contrast, central sensitization resulting from severe and persistent injury which cause prolonged release of glutamate on nociceptive dorsal horn cells, this constant glutamate release via G protein dependant phosphorylation cascades results in opening of postsynaptic ion channels gated by the NMDA receptors. This phenomenon is also termed "wind up." This activation produces hyperexcitability of the dorsal horn cells and causes "central sensitization." Pain experts now agree that treating chronic pain early and aggressively yields the best results and prevents patients from developing physical and psychological conditions that could worsen the pain.

Fibromyalgia is characterized by widespread chronic pain throughout the body, including fatigue, anxiety and depression. It is now believed that it has a genetic component which tends to run in families.

A headache is a poorly understood type of pain that can be either acute or chronic. There are about 300 different types and causes of headaches. The following are some categories of disorders associated with headaches:

• Intracranial structural disease
• Infectious disease
• Cerebrovascular ischemia
• Cerebral vein thrombosis
• Metabolic disease
• Toxic exposures
• Medications
• Extracranial pressure disorders
• Sinusitis
• Vasculitis and collagen vascular disease
• Hemorrhage (parenchymal and subarachnoid)
• Trauma
• Withdrawal syndromes
• Severe hypertension
• Dental, cranial vault, TMJ, and myofascial disorders
• Cervical spine and occipitocervical junction disorders

Because of the importance of warning signals of dangerous circumstances, several nociception pathways are involved to transmitting these signals and some of them are redundant.

The neospinothalamic tract conducts fast pain (via A delta fibers) and provides information of the exact location of the noxious stimulus, and the multisynaptic paleospinothalamic and archispinothalamic tracts conduct slow pain (via C fibers), a pain which is chronic and poorly localized in nature. (Figure 7.5)

Pain activates many brain areas, which link sensation, perception, emotion, memory and motor reaction. Therefore, many pain clinics target their treatments to block the perception of pain using psychosomatic means of treatments such as biofeedback, hypnosis, physical therapy, electrical stimulation, and acupuncture.

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