Early somatosensory processing during tonic muscle pain in humans: relation to loss of proprioception and motor 'defensive' strategies.
OBJECTIVE
It is known that tonic muscle pain induced by a Levo-Ascorbic (L-AS) solution injected in a foot muscle can transiently modify both regional proprioception and stimulus perception. These findings are paralleled by changes of middle-latency lower-limb somatosensory evoked potentials (SEPs). However, little is known on the behaviourally relevant aspect whether eventual SEP pain-induced changes could be partly due to a sort of 'motor strategy' of subjects in the frame of a self-protective reaction towards the noxious stimulus. Movement and imagery of movements are in fact known to reduce mainly pre-central SEP amplitude (i.e. gating effect).


METHODS
Low-threshold afferents ulnar SEPs, psychophysical pain ratings and fingers' position sense were monitored in the time-course during L-AS injection in the right first dorsal interosseous muscle. Control experiments included SEPs (either following prevalent ulnar nerve low-threshold afferent stimulation or more conventional mixed nerve stimulation) during actual movements execution and imagery of movements of the right hand.


RESULTS
Tonic pain induced a significant reduction of the post-central N(20)-P(25)-N(33) complex and a significant increase of the N(18) wave. These changes, that were paralleled by distortion of the finger position sense, were delayed 2-5 min with respect to the maximal subjective pain sensation. Conversely, movement imagery tasks lead to a significant, selective, reduction of the pre-central N(30) complex. This wave was even more reduced during actual movements, in combination with a reduction of those post-central components peaking after the first activation of the primary sensory cortex.


CONCLUSIONS
Early sensory processing at cortical level is changed during tonic muscle pain, mainly for those components which may be theoretically involved in proprioceptive afferent elaboration. These changes are likely not due to subconscious or voluntary motor strategies of the subjects in the frame of a self-protective aversive reaction towards the noxious stimulus.
Rossi
Simone
S
Dipartimento di Neuroscienze, Sezione Neurologia, U.O. Neurofisiopatologia, Università di Siena, Policlinico Le Scotte, Viale Bracci, I-53100 Siena, Italy. rossisimo@unisi.it
della Volpe
Raimondo
R
Ginanneschi
Federica
F
Ulivelli
Monica
M
Bartalini
Sabina
S
Spidalieri
Raffaele
R
Rossi
Alessandro
A
D000328:Adult; D001205:Ascorbic Acid / Q000009:adverse effects; D001921:Brain / Q000503:physiopathology*; D004576:Electromyography / Q000379:methods; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D005385:Fingers / Q000294:innervation / Q000502:physiology; D006801:Humans; D019018:Imagery, Psychotherapy / Q000379:methods; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D009132:Muscles / Q000187:drug effects / Q000503:physiopathology*; D010146:Pain / Q000139:chemically induced / Q000503:physiopathology*; D010147:Pain Measurement; D010465:Perception / Q000502:physiology*; D010525:Peripheral Nerves / Q000502:physiology; D011434:Proprioception; D011930:Reaction Time; D013997:Time Factors
D001205:Ascorbic Acid