Properties of human spinal interneurones: normal and dystonic control

The muscles that control wrist posture receive large inputs from reflexes driven by hand afferents. In several studies, we have investigated these reflexes by electrical stimulation of cutaneous (median nerve) and proprioceptive (ulnar nerve) afferents from the hand. Median stimulation produced short latency inhibition in all motor nuclei investigated, possibly through inhibitory propriospinal-like interneurones. Ulnar stimulation produced similar inhibition but only in wrist extensors. In the other motor nuclei, ulnar stimulation produced short latency excitation mediated by group I motoneuronal drive through both monosynaptic and non-monosynaptic pathways involving excitatory propriospinal-like interneurones. This was followed by late excitations mediated through spinal group II and trans-cortical group I pathways. These results show that these pathways are concerned with the integration of afferent inputs, proprioceptive and cutaneous, to control of wrist posture during hand movements. Patients with focal hand dystonia exhibit abnormal postures. To investigate whether these spinal pathways contribute to these conditions, the effects of ulnar stimulation on wrist muscle activity during voluntary tonic contraction were examined in patients who suffer writer's cramp. Ulnar-induced inhibition of the wrist extensors was reduced on the dystonic side of patients compared with their normal side and controls. In patients who exhibited abnormal wrist posture, group II excitation of the wrist flexors was also modified on the dystonic side. Cutaneous stimuli, by contrast, increased wrist flexor EMG on both sides and only in patients who exhibited normal posture. We conclude that spinal interneurones have a significant role in integrating afferent inputs from the hand to control wrist posture during hand movements and that altered function in these spinal networks is associated with the complex pathophysiology of writer's cramp.