Central nervous system plasticity and persistent pain.

Nerve signals arising from sites of tissue or nerve injury lead to long-term changes in the central nervous system and contribute to hyperalgesia and the amplification and persistence of pain. These nociceptor activity-induced changes are referred to as central sensitization. Central sensitization involves an increase in the excitability of medullary and spinal dorsal horn neurons brought about by a cascade of events, including neuronal depolarization, removal of the voltage-dependent magnesium block of the N-methyl-D-aspartate (NMDA) receptor, calcium entry into neurons, phosphorylation of the NMDA receptor, a change in the cell's excitability, and an increase in synaptic strength. These changes also include activation of other ionotropic and metabotropic excitatory amino acid receptors, neuropeptides such as substance P, neurotrophins, and kinases involved in the phosphorylation process. Central sensitization occurs in trigeminal nociceptive pathways, and more robust neuronal hyperexcitability occurs following deep tissue stimulation than following cutaneous stimulation. By means of Fos protein immunocytochemistry, researchers have found that 2 distinct regions are activated: the subnucleus interpolaris/caudalis transition zone (Vi/Vc) and the caudal subnucleus caudalis. The latter exhibits changes very similar to those in the spinal dorsal horn, but the Vi/Vc zone likely is involved in autonomic nervous system processing and activation of the pituitary-adrenal axis. Descending systems are also an important component of the central sensitization process and provide the neural networks by which cognitive, attentional, and motivational aspects of the pain experience modulate pain transmission. These findings of nociceptor activity-induced neuronal plasticity have important clinical implications in the development of new approaches to the management of persistent pain.