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Last updated: Sun, Jun 18, 2017
Three main types of somatosensory neurons are important in pain: A-beta neurons, A-delta neurons, and C-fiber neurons.
A-beta neurons are the largest of the somatosensory neurons. A-beta axons range from about 6 to 13 micrometers (millionths of a meter) in diameter and are covered in myelin, a white, fatty protein. Their myelin covering and large size combine to give A-beta fibers the fastest conduction velocity of the three neuron types—from 30 to 70 meters per second.
A-beta neurons are equipped with low-threshold mechanoreceptors. A-beta neurons are responsible for the sensitivity of the small hairs on the backs of your arms—each of these is innervated by a specialized hair nerve ending. A-beta neurons are also responsible for highly-sensitive touch and vibration sensitivity in the hands.
Four types of A-beta fibers have been found in the human fingertip.
A-delta neurons are the smallest of the myelinated (myelin-covered) neurons. A-delta axons are from 1 to 5 micrometers in diameter. They conduct action potentials at a rate of 6-20 meters per second. A-delta neurons are polymodal, meaning that they respond to pressure, heat, and chemicals. Two types of A-delta neurons have been identified.
In early studies of the skin, individual receptors were found for study by pinching the skin. Since the response threshold of Type II A-delta neurons is so high, they were rarely found in these early studies. Type II is called “mechanically-insensitive” because of its higher threshold, while Type I is called “mechanically sensitive.”
Pain that is described as “pricking” or “sharp” is believed to be the result of the activation of A-delta neurons. Dull aching pain is also believed to result from activation of A-delta neurons under different conditions.
C-fiber neurons have unmyelinated axons of just 0.5-2 micrometer diameter. They conduct action potentials at a rate of only 0.5-2 meters per second. C-fiber neurons are polymodal like A-delta neurons.
C-fiber neurons vary a lot in their sensitivity. Low-threshold (mechanically sensitive) C-fiber neurons that do not respond to heat have been found as well as high-threshold neurons. About 30% are the high-threshold neurons. “Quick” and “slow” C fibers have been distinguished based on their response to heat. The quick C fibers deliver a strong response while temperature rises, while slow C fibers deliver a strong response once the heat levels off.
Both fatigue and sensitization have been found in C fibers. If a heat stimulus is repeated after less than ten minutes, the response to the second stimulus will be reduced.
Pain that is described as “burning” is believed to be the result of the activation of C-fiber neurons.
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