Last updated: Tue, Nov 19, 2024
Stimulation of sensory nerves in one part of the body can create the sensation of pain in distant areas. This is called referred pain. A well-known example is the left shoulder and arm pain that is often felt by victims of heart attack or angina. Either of these heart conditions is painful, which is easy to understand, but why should it also be felt in the arm?
The left shoulder/arm and the heart both receive their innervation from the same thoracic spinal nerves. Their sensory nerves come together in lamina V of the spine's dorsal horn. Intense stimulation by the sensory nerves of the heart activates neurons in the spine that the brain associates with the left arm.
We saw that allodynia can develop in a cutaneous injury. Stroking in the area of secondary hyperalgesia becomes painful. The same thing happens when heart pain is referred to the left upper extremity; the extremity becomes tender to the touch. The nerve impulses received from the skin of the arm are summed with the impulses from the distressed heart by the spinal cord.
Pain referred from viscera to the skin is so predictable that doctors use the patterns of referred pain to diagnose internal problems. The appendix, when it is first inflamed, causes the skin about the navel to become painful. Later, if it becomes more inflamed, the inflammation spreads to the lining of the abdominal cavity, which is sensitive in its own right. At this later stage, palpation over the appendix becomes painful.
Muscles also commonly refer pain to the skin in predictable patterns. The small multifidus muscles that span the vertebrae, for example, refer pain to areas of skin in the front near the navel and groin, and in the rear in areas of the lower back, buttocks, and back of the upper leg.
Like other instances of spreading sensitization, the areas of referred pain will expand to the degree that the spinal neurons involved are sensitized.
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Radiating (Spreading) Pain (Last updated: Fri, Jul 26, 2024)
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Counterstimulation (Last updated: Fri, Jul 26, 2024)