• C Fibers are small, unmyelinated nerves with slow conduction velocities that carry dull, aching burning pain impulses.

• Thinly myelinated A afferent fibers carry fast, sharp, shooting pain sensations and are most integral to the propagation of mechanical pressure stimuli from muscles, joints, and bone.

• Compared with these fibers, C afferent fibers have a higher threshold for mechanical stimuli and a smaller field of reception.

• Both these classes of nociceptive fibers ultimately synapse with neurons in the dorsal horn of the spinal cord.

• The dorsal horn contains several types of neurons that become hyperexcited by distinct types of stimuli from primary afferent nociceptors.

  • Class I cells respond to low-threshold mechanical and thermal impulses and class III respond only to stimuli resulting from tissue damage (ie, they are nociceptive-specific)

  • class II neurons have a wide dynamic range.

  • Nociceptive stimuli that activate these neurons are transmitted to the brain via the spinothalamic, spinoreticular, or spinomesencephalic tracts (STT, SRT, SMT, respectively).

courtesy of Roxane Pain Institute used with permission http://www.roxane.com