Functional organization of exteroceptive inhibition following nociceptive electrical fingertip stimulation in humans

To investigate the functional organization of inhibitory spinal reflexes mediated by cutaneous afferents (=cutaneous silent periods, CSPs) in order to assess differential strategies used by the human spinal cord to inhibit movement in upper limb muscles within the same myotome.

Twenty healthy subjects underwent assessment of the effect of electrical finger-tip stimulation with different intensities on voluntarily contracting hand muscles. The rectified and averaged electromyogram (EMG) was recorded with surface electrodes placed over abductor pollicis brevis (APB), abductor digiti minimi (ADM), and first dorsal interosseous (FDI) muscles following recurrent digit II and digit V stimulation.

Finger-tip stimulation resulted in a series of inhibitory and excitatory EMG responses in all 3 hand muscles. Stimulus intensity exerted a significant influence on various CSP parameters (probability, onset and end latency, duration, index of suppression). CSPs were most pronounced in APB following digit II and, somewhat less, following digit V stimulation. FDI and ADM differed in CSP onset latency following digit II stimulation, and in the overall amount of suppression following digit V stimulation, despite being supplied by the same nerve and the same myotome. Excitatory long-loop reflexes seemed to be suppressed by increasing stimulus intensities, yet they interfered with CSPs, even when using noxious stimulation, in FDI following digit II stimulation, and in both ADM and FDI following digit V stimulation.

The findings are consistent with the presence of an inhibitory neural circuitry within the human spinal cord which can rapidly restrain distinct muscle synergies. Neurophysiology indicates a more functional - task-related - organization of protective reflexes in the human upper extremity, rather than a merely metameric - anatomically based - order of activation of this neuronal circuitry.