Interaction between intracortical inhibition and facilitation in human motor cortex.

Abstract

1 In seven normal subjects, subthreshold transcranial magnetic conditioning stimuli (using a figure-of-eight coil) were applied over the motor cortex in order to evoke activity in intracortical neuronal circuits. The net effect on cortical excitability was evaluated by measuring the effect on the size of EMG responses elicited in the abductor digiti minimi (ADM) muscle by a subsequent suprathreshold test stimulus. 2. A single conditioning stimulus suppressed the size of the test response at interstimulus intervals (ISIs) of 1-4 ms whereas the response was facilitated at ISIs of 6-20 ms. The facilitation could be augmented if pairs of conditioning stimuli were given. 3. Inhibition and facilitation appeared to have separate mechanisms. The threshold for inhibition (0 7 active motor threshold) was slightly lower than that for facilitation (0-8 active threshold). Similarly, the inhibitory effect was independent of the direction of current flow induced in the cortex by the conditioning shock, whereas facilitation was maximal with posterior-anterior currents and minimal with lateromedial current. 4. Direct corticospinal effects were probably not responsible for the results since facilitation of cortical test responses could be produced by conditioning stimuli which had no effect on the amplitude of H reflexes elicited in active ADM muscle. 5. Inhibition and facilitation appeared to interact in a roughly linear manner, consistent with separate inputs to a common neurone. 6. We suggest that subthreshold transcranial magnetic stimulation is capable of activating separate populations of excitatory and inhibitory interneurones in the motor cortex. Kujirai et al. (1993) reported that a single subthreshold magnetic stimulus over the motor cortex could suppress the response to a later suprathreshold test stimulus. They postulated that the first stimulus produced this effect by activation of a set of intracortical inhibitory neurones. They also noted a later phase of facilitation, but did not investigate the mechanism in any detail. The present paper addresses this question and shows that the later facilitation is probably caused by the activation of a separate set of facilitatory cortical neurones. The outputs of these two sets of neurones appear to interact independently at or before the final stage of pyramidal output. METHODSThe experiments were performed, with the approval of the joint ethical committee of the National Hospital for Neurology and Institute of Neurology, on seven normal healthy subjects (all men) aged 27-44 years. The subjects gave their informed consent and were seated in a comfortable reclining chair during the procedures.Surface electromyographic (EMG) recordings were made from the right abductor digiti minimi (ADM) muscle with the active electrode placed over the motor point and the reference electrode on the proximal interphalangeal joint of the small finger. The raw signal was amplified and filtered by Digitimer D150 amplifiers (Digitimer Ltd, Welwyn Garden City, Herts, UK) with a time constant of 10 ...

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“…From the paired pulse curve, it is possible to identify two distinct phases that are referred to as short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) [20][21][22][23][24]. The SICI represents an inhibition phase with ISIs from 1 to 6 ms, maximally around 3 ms. ICF refers to a facilitation phase with ISIs greater than 6 ms, generally maximal around 10-12 ms. We calculated SICI and ICF for each subject and hemisphere and compared them across groups.…”

“…At longer intervals (around 10-12 ms), the response to the test stimulus is facilitated by the preceding conditioning stimulus reflecting neuronal hyperexcitability [19,20]. These phenomena of interstimulus interactions have been related to the activation of intracortical neuronal circuits [21,22].…”

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“…Here the investigator delivers two pulses with varying time intervals between the pulses, ranging from 1 to 70 ms. Several paradigms using different interstimulus intervals to assess intracortical inhibition have been described in adult subjects. The short inter-stimulus intracortical inhibition (SICI) paradigm (which has been shown to be of intracortical origin (Kujirai et al, 1993;Mall et al, 2004;Nakamura et al, 1997;Ziemann et al, 1996) and is mediated by GABA A -receptors ]) is the only paradigm that has been used in children.…”

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