TMS and Inhibition & Excitation
It is a remarkable feature of TMS that one can selectively stimulate excitatory or inhibitory pathways. Kujirai et al first demonstrated in 1993 that magnetic stimulation over the motor cortex could engage intracortical inhibitory circuits. Excitability and inhibitory thresholds are clinically useful indicators. In order to reduce variability of the thresholds, it is important to notice mental activity, posture, type of magnetic stimulator, involuntary movement, muscle tone and age. Excitability thresholds are usually elevated in patients with stroke and reduced in patients with epilepsy. Enhancement of motor excitability by subthreshold magnetic stimulation of the motor cortex prior to voluntary movement can be observed during a simple reaction time measurement. Reaction time can be divided into a period of resting condition and the premotion facilitatory effect in the 50 approximately 60 ms after the target stimuli. In patients with Parkinson's disease, the prolongation of reaction time is due to a prolongation of a period of premotion facilitatory effect. Jerk-locked motor evoked potential (MEP) is a useful technique to evaluate a change of motor excitability after voluntary or involuntary movement. To record jerk-locked MEP, magnetic stimulation of the motor cortex is delivered at varying intervals after the onset of the EMG activity (Uozumi 1995).
Applications in Drug Monitoring
The effect of drugs on corticomotor excitability can be measured using twin pulse TMS. The threshold of stimulation may be raised or lowered; silent period may be shortened or lengthened; intracortical inhibition may be increased or decreased; intracortical facilitation may be increased or decreased. Many clinics worldwide find this a very useful method for an early assessment of how an individual reacts to a drug. If a drug does not appear to be having the desired effect on a patient, this can be detected early and the drug changed for a more effective alternative. (Ziemann 2004)
References
- Kujirai, J Physiol, 1993.
- Uozumi, Rinsho Shinkeigaku, 1995.
- Ziemann, Clinical Neurophysiology, 2004.
