Measuring GABA_A function in the primary motor cortex

Thirty per cent of all inhibitory synapses throughout the nervous system are using γs-aminobutyric acid (GABA) as their main inhibitory neurotransmitter. The function of GABAergic interneurons in the primary motor cortex (M1) can be assessed by paired-pulse transcranial magnetic stimulation (TMS). In this method, two pulses are delivered with an interstimulus interval of 1-5 ms to stimulate M1 area and the motor responses in the contralateral targeted muscle will be recorded. It has been suggested that the first subthreshold stimulus can stimulate GABAergic interneurons in this area which in turn can reduce the size of the motor responses elicited by the second suprathreshold stimulus. This effect was termed short interval intracortical inhibition (SICI). Several parameters are involved in this process that can affect the SICI level in M1 area. This includes coil shape, the direction of induced current in the brain, interstimulus interval and stimulus intensity for each pulse. These parameters and their effects on SICI level in M1 area will be discussed in this chapter. It also presents a detailed step-by-step description of the procedure for assessment of SICI before and after therapeutic interventions.