Effect of chemically activated fine muscle afferents on spinal recurrent inhibition in humans.
OBJECTIVE
To test the hypothesis that 'metabolites released during fatiguing muscle contractions excite group III-IV muscle nociceptive afferents, inhibiting homonymous motoneurones via Renshaw cells,' by recording changes in recurrent inhibition of soleus motoneurones when high-threshold, small-diameter afferents (group III-IV fibres) from the same muscle were tonically activated.


METHODS
Experiments were performed in 7 healthy subjects at rest and during weak isometric voluntary contraction of the soleus muscle. Muscle nociceptive afferents were activated by local standardized injection of levo-ascorbic acid. Renshaw cells were orthodromically activated by a conditioning H reflex and the resulting recurrent inhibition of the soleus motoneurones was assessed by a subsequent test H reflex. An additional H reflex of the same size as the test reflex was used to assess motoneurone excitability.


RESULTS
At rest, muscle nociceptive stimulation produced transient facilitation of both test H and reference H reflexes. Under weak voluntary contraction, muscle nociceptive stimulation produced long-lasting extra-inhibition and extra-facilitation of the test reflex and reference reflex respectively, the time course of which closely resembled that of the subjective muscle pain curve.


CONCLUSIONS
Discharge of putative group III-IV muscle afferents facilitated homonymous recurrent inhibition. The filtering property of recurrent inhibition may contribute to limit motoneurone activity during muscle pain and/or adapt motoneurone firing rate to the modified contractile properties of motor units as muscle fatigue developed.
Rossi
A
A
Dipartimento di Scienze Neurologiche e del Comportamento, Università degli Studi di Siena, Siena, Italy. rossiale@unisi.it
Mazzocchio
R
R
Decchi
B
B
D000328:Adult; D001205:Ascorbic Acid / Q000494:pharmacology; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D009119:Muscle Contraction / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology*; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology; D010146:Pain / Q000503:physiopathology; D012146:Rest / Q000502:physiology; D013116:Spinal Cord / Q000166:cytology / Q000502:physiology*; D013268:Stimulation, Chemical
D001205:Ascorbic Acid