Several condition-test paradigms using transcranial magnetic stimulation （TMS） have been recently developed to study motor cortical （M1） excitability changes in human. The short interval intra-cortical inhibition （SICI），interheispheric inhibition （IHI） and cerebellar inhibition （CBI） are widely used methods to evaluate M1 modulation through intrinsic GABAergic neurons, callosal fibers, and cerebellum-thalamus-M1 pathways, respectively. We studied these modulations in several neurological disorders. The SICI was abnormally reduced in cortical myoclonus, focal dystonia or Parkinson’s disease. In contrast, it was normal in chorea or essential tremor. The GABAergic dysfunction may be produced by GABA reduction within M1 or abnormal M1 regulation from the basal ganglia. The IHI was reduced in mild cognitive imparment patients and cortical myoclonus. This may be due to GABAergic dysfunction within the cortex. The CBI was decreased in patients with many kinds of cerebellar ataxia, progressive nuclear palsy （PSP），and essential tremor （ET）．Because it tests the function of cerebello-cerebellar connection, we first showed cerebellar dysfunction in PSP and ET who have no clinical cerebellar symptoms. TMS has enable us to study the human central nervous system function physiologically
