Nociceptive fingertip stimulation inhibits synergistic motoneuron pools inthe human upper limb

Background Activation of distinct muscle groups organized in a stereotyped manner ("muscle synergies") is thought to underlie the production of moveme nt by the vertebrate spinal cord. This results in movement with minimum eff ort and maximum efficiency. The question of how the vertebrate nervous syst em inhibits ongoing muscle activity is central to the study of the neural c ontrol of movement. Objective: To investigate the strategy used by the huma n spinal cord to rapidly inhibit muscle activation in the upper limb. Metho ds: The authors performed a series of experiments in 10 healthy subjects to assess the effect of nociceptive cutaneous stimulation on voluntarily cont racting upper limb muscles. They recorded the electromyogram (EMG) with sur face electrodes placed over various upper limb muscles. Results: The author s found evidence of a simple inhibitory strategy that 1) was dependent on t he intensity of the stimulus, 2) was maximally evoked when stimulation was applied to the fingertips, 3) preceded the earliest onset of voluntary musc le relaxation, and 4) produced inhibition of EMG activity in specific upper limb muscle groups. Nociceptive fingertip stimulation preferentially inhib ited contraction of synergistic muscles involved in reaching and grasping ( intrinsic hand muscles, forearm flexors, triceps) while having little effec t on biceps or deltoid. Conclusions: Neural circuitry within the human spin al cord is organized to inhibit movement by rapidly deactivating muscles th at constitute distinct muscle synergies. This strategy of selective and con current deactivation of the same basic elements that produce synergistic mo vement greatly simplifies motor control.