7. Pain Tracts and Sources
Part 4 of 5

Nachum Dafny, Ph.D.
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Sources of Pain (continued)

Referred Pain

Referred pain is a painful sensation at a site other than the injured one. The pain is not localized to the site of its cause (visceral organ) but instead is localized to a distant site. The axons carry pain information from the viscera enter into the spinal cord by the same route as the cutaneous pain sensation axons. Within the spinal cord there is a convergence of the information on the same nocineurons. This convergence gives rise to the phenomenon of referred pain. For example, pain associated with angina pectoris, or myocardial infarction is referred to the left chest, left shoulder, and upper left arm (Figure 7.7). Pain resulting from distention of the colon is referred to the periumbilical area (Figures 7.8).

The following are some hypothesis to explain referred pain

1. Common dermatome hypothesis. When pain is referred, it is usually to a structure that developed from the same embryonic segment or dermatome as the structure in which the pain originates. Radiating pain down the left arm is the result of a myocardial infarction (Figure 7.7), or pain originating from the shoulder (dermatomes 3-5).
   
   
2. Convergence and facilitation theories (Figure 7.9). Inputs from visceral and skin receptors converge on the same spinal cord neuron (i.e., viscerosomatic neurons). Therefore, visceral pain is referred to skin area because the nociceptors' terminals from the viscera terminate in the spinal cord on the same neurons that receive input from the skin.
   
   

   Figure 7.9

   Convergence in referred pain is carried via the paleospinothalamic tract.

   
   
3. Facilitation or irritable focus. Pain impulses from the viscera alone are unable to pass directly from spinal cord neurons to the brain, but create an "irritable focus". When visceral and skin impulses arrive together, the information transmitted to higher centers and the brain interprets the pain as being from the skin (Figure 7.10).
   
   

   Figure 7.10

   Convergence of referred pain.

   
   
4. Learned phenomenon. Visceral information arrives in the CNS. However, the brain interprets that the impulses originate from the site of a previous surgical operation, trauma or localized pathologic process.

Phantom (illusory) Pain

Phantom or illusory pain is the experience of pain without any signals from nociceptors in a subject with injuries in which the dorsal roots are literally absent from the cord. Even though no sensory signals can enter the cord, the subject often feels extreme pain in the denervated parts of the body. For example, an amputee will often apparently feel pain in a part of his body that has been removed. The phenomenon of phantom limb pain is a common experience after a limb has been amputated or its sensory roots have been destroyed in which the pain is felt in a part of the body that no longer exists. Pain from an amputated arm is referred to the viscera as a result of disruption to the “balance” between different peripheral inputs to the dorsal horn. A complete break of the spinal cord also often leads to a phantom body pain below the level of the break. The source of phantom pain is complex and not well understood. It has been suggested that there may be abnormal discharges 1) from the remaining cut ends of nerves which grow into nodules called neuromas, 2) from overactive spinal neurons, 3) from abnormal flow of signals through the somatosensory cortex, or 4) from burst-firing neurons in the thalamus.

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