Pathophysiology of acute and chronic pain

N. Forss, R. Hari, E. Kirveskari, M. Pohja, T. Raij, and N. Vartiainen

Collaborators:

Pain is a serious problem in clinical medicine. We have shown that with small modifications to the intensity and area of painful stimulation with a thullium laser (applying a restricting hole at the tip of the handpiece of the laser), we can reliably differentiate brain responses elicited by two nociceptive fibre systems, Ad- and C-fibres, corresponding to first and second pain. The most important source areas in the brain were the second somatosensory cortex SII and, in contrast to many previous brain imaging studies, the posterior parietal cortex PPC and not the primary somatosensory cortex SI reported by many groups; in fact, SI lesions do not affect pain perception.

 We have also shown, by using both the rhythmic 20-Hz activity and the 20-Hz cortex–muscle coherence as tools to probe the functional state of the primary motor cortex MI, that the M1 cortex is automatically activated by laser stimuli, both after  Ad- and C-fibre stimulation, This effect is functionally useful during acute pain but a likely source of a vicious cycle during chronic (tension) pain when the pain turns from a symptom to a disease.

In a recent fMRI study we addressed the very basic question how humans are able to differentiate events surfacing from their own mind from those arising from the external world. We screened among 103 volunteers 14 suggestion-prone healthy subjects who rated during fMRI the experienced reality of pain when the pain was induced either by laser pulses to the skin or by hypnotic suggestion. Both pain states were associated with activation of the brain's pain circuitry but with some interesting differences. During suggestion-induced pain, the reality estimates correlated positively with activation strengths in the rostral and perigenual anterior cingulate cortex and in the pericingulate regions of the medial prefrontal cortex, and other correlation analyses suggested that these areas likely contribute to monitoring of the source of information in the external world.

 Publications

  1. Forss N, Raij T, Seppä M and Hari R: Common cortical network for first and second pain. Neuroimage 2005, 24: 132–142.
  2. Raij T, Forss N, Stancak A and Hari R: Modulation of motor-cortex oscillatory activity by painful Ad- and C-fiber stimuli. Neuroimage 2004, 24: 569–573.
  3. Raij T, Numminen J, Närvänen S, Hiltunen J and Hari R: Brain correlates of subjective reality of physically and psychologically induced pain. Proc Natl Acad Sci USA 2005, 102: 2147–2151.
  4. Stancak A, Raij TT, Pohja M, Forss N and Hari R: Oscillatory motor cortex–muscle coupling during painful laser and nonpainful tactile stimulation. Neuroimage, in press.