Abstract
Activation of neurons not only changes their membrane potential and firing rate but as a secondary action reduces membrane resistance. This loss of resistance, or increase of conductance, may be of central importance in non‐invasive magnetic or electric stimulation of the human brain since electrical fields cause larger changes in transmembrane voltage in resting neurons with low membrane conductances than in active neurons with high conductance. This may explain why both the immediate effects and after‐effects of brain stimulation are smaller or even reversed during voluntary activity compared with rest. Membrane conductance is also increased during shunting inhibition, which accompanies the classic GABAA IPSP. This short‐circuits nearby EPSPs and is suggested here to contribute to the magnitude and time course of short‐interval intracortical inhibition and intracortical facilitation.
Highlights
A variety of different transcranial stimulation (TS) techniques are available to modulate brain excitability and explore neuroplasticity in the human brain. They range from brain polarization with transcranial direct current stimulation (Nitsche & Paulus, 2000) to transcranial alternating current stimulation (Antal et al 2008) and short-duration current pulses of about 100 μs produced by transcranial magnetic stimulation (TMS). tDCS and tACS apply small constant or alternating currents via electrodes placed on the scalp
A complex experiment involving the interaction of Short-interval intracortical inhibition (SICI) with short-interval intracortical facilitation (SICF; or I-wave interaction) suggests that any early excitatory inputs activated by a higher intensity S1 are quashed by inhibition but that later ones are not (Peurala et al 2008)
Measuring cortical excitability with TMS to the motor cortex and quantifying motor-evoked potential (MEP) amplitudes, SICI, ICF and recruitment curves has provided a unique framework for understanding concepts as well as parameters needed to induce repetitive transcranial magnetic stimulation (rTMS) as well as tDCS after-effects
Summary
A variety of different transcranial stimulation (TS) techniques are available to modulate brain excitability and explore neuroplasticity in the human brain. If tDCS of motor cortex is applied during voluntary muscle activation, anodal tDCS no longer produces excitation; instead, both anodal and cathodal stimulation induce inhibition as measured by their effects on MEPs (Antal et al 2007). Since there is continuous activity in the brain even when at ‘rest’, it is possible that a behaviour could reduce input to some neurons and increase their membrane resistance, potentially meaning that they become more sensitive to remaining inputs.
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