• 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