Muscle afferent fibers innervating endings activated by potentially damaging (nociceptive) stimuli also transmit their impulses to the spinal cord via small myelinated and unmyelinated afferents. These fibers are generally identified by the Roman numeral system in keeping with D.P.C. Lloyd's original classification of muscle afferents (I-IV). Thus, the small myelinated afferents are known as group III (equivalent in fiber diameter to Ad-fibers) and unmyelinated afferents are known as group IV (equivalent to C-fibers). These small fibers innervate receptors in muscle whose adequate stimulus is best described as squeezing the muscle (normally quite painful). Those with group III axons tend to be activated at lower stimulus intensity than those with group IV axons. Many of these receptors can also be excited by chemical stimuli such as bradykinin or 5-HT. These substances are released when the muscle is under stress due to ischemia, lactic acid buildup (lowered pH), etc. Some of these receptors also exhibit responses to noxious temperature (> 43°C). Thus, nociceptors in muscle resemble those in skin in responding to one or more modalities of stimulation.

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