Transcranial magnetic stimulation studies of sensorimotor networks in Tourette syndrome

Gilles de la Tourette syndrome (GTS) is a sensorimotor disorder where the sensitivity to external and internal stimuli might be increased and unwanted responses to such stimuli cannot be sufficiently suppressed.Transcranial magnetic stimulation (TMS) studies indicate that, at rest, axonal excitability of cortico-spinal neurons and intra-cortical inter-neurons was consistently normal in GTS. However, synaptic excitability in cortico-spinal neurons and the SICI circuit may be lower than normal. In addition, an electrophysiological marker of sensory motor integration, SAI, was reduced in the baseline state consistent with reduced efficiency of synaptic inhibition. Given the possible influence of sensory inputs in triggering the release of tics reduced SAI may be a direct physiological reflection of increased access of sensory input to motor output in GTS. Experiments examining control of voluntary movements revealed that in GTS motor cortex excitability increases less than in controls when preparing a movement even though intra-cortical inhibition (i.e. SICI) normalises. In GTS the gain of many motor circuits may be reduced and hence less sensitive to small changes in input from other areas. These cortical changes may constitute an adaptive response to abnormal basal ganglia-motor cortex inputs.