PAIN ADAPTATION & SENSITIZATION

A. Most nociceptors are either nonadapting or only slowly adapting

B. Sensitization

1. Define: increased response at a given stimulus level (hyperalgesia) or response in the absence of a noxious stimulus (allodynia)

2. Peripheral Sensitization: increased sensitivity of the primary afferent nociceptor; e.g. inflammation mediators, peripheral feedback from Substance P

3. Central Sensitization: increased or spontaneous synaptic excitation on the nociceptive pathway; e.g. action of excitatory transmitters in the spinal dorsal horn

Hypothesis

a. low intensity noxious stimulus: glutamate neurotransmitter; brief, sharp pain sensation

b. high intensity noxious stimulus: substance P released also; increased sensitivity, longer lasting, dull pain sensation

c. high intensity noxious stimulus of long duration: up regulation of post-synaptic excitatory receptors; hypersensitivity/allodynia/chronic pain