Mediation of late excitation from human hand muscles via parallel group II spinal and group I transcortical pathways.
This study addresses the question of the origin of the long-latency responses evoked in flexors in the forearm by afferents from human hand muscles. The effects of electrical stimuli to the ulnar nerve at wrist level were assessed in healthy subjects using post-stimulus time histograms for flexor digitorum superficialis and flexor carpi radialis (FCR) single motor units (eight subjects) and the modulation of the ongoing rectified FCR EMG (19 subjects). Ulnar stimulation evoked four successive peaks of heteronymous excitation that were not produced by purely cutaneous stimuli: a monosynaptic Ia excitation, a second group I excitation attributable to a propriospinally mediated effect, and two late peaks. The first long-latency excitation occurred 8-13 ms after monosynaptic latency and had a high-threshold (1.2-1.5 x motor threshold). When the conditioning stimulation was applied at a more distal site and when the ulnar nerve was cooled, the latency of this late excitation increased more than the latency of monosynaptic Ia excitation. This late response was not evoked in the contralateral FCR of one patient with bilateral corticospinal projections to FCR motoneurones. Finally, oral tizanidine suppressed the long-latency high-threshold excitation but not the early low-threshold group I responses. These results suggest that the late high-threshold response is mediated through a spinal pathway fed by muscle spindle group II afferents. The second long-latency excitation, less frequently observed (but probably underestimated), occurred 16-18 ms after monosynaptic latency, had a low threshold indicating a group I effect, and was not suppressed by tizanidine. It is suggested that this latest excitation involves a transcortical pathway.
U731 INSERM, Université Pierre et Marie Curie, Paris, France.
George
G
Lourenço
Caroline
C
Iglesias
Paolo
P
Cavallari
Emmanuel
E
Pierrot-Deseilligny
Véronique
V
Marchand-Pauvert
D000328:Adult; D000368:Aged; D001132:Arm / Q000294:innervation; D002540:Cerebral Cortex / Q000502:physiology; D003000:Clonidine / Q000031:analogs & derivatives / Q000494:pharmacology; D003080:Cold Temperature; D004558:Electric Stimulation; D004576:Electromyography; D006225:Hand / Q000294:innervation*; D006801:Humans; D007036:Hypothermia, Induced; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D009125:Muscle Relaxants, Central / Q000494:pharmacology; D009470:Muscle Spindles / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D012018:Reflex / Q000502:physiology*; D013127:Spinal Nerves / Q000502:physiology*; D013569:Synapses / Q000502:physiology; D014459:Ulnar Nerve / Q000502:physiology
D009125:Muscle Relaxants, Central; C023754:tizanidine; D003000:Clonidine