Excitation of the corticospinal tract by electromagnetic and electrical stimulation of the scalp in the macaque monkey.

Abstract

SUMMARY1. The responses evoked by non-invasive electromagnetic and surface anodal electrical stimulation of the scalp (scalp stimulation) have been studied in the monkey. Conventional recording and stimulating electrodes, placed in the corticospinal pathway in the hand area of the left motor cortex, left medullary pyramid and the right spinal dorsolateral funiculus (DLF), allowed comparison of the actions of non-invasive stimuli and conventional electrical stimulation.2. Responses to electromagnetic stimulation (with the coil tangential to the skull) were studied in four anaesthetized monkeys. In each case short-latency descending volleys were recorded in the contralateral DLF at threshold. In two animals later responses were also seen at higher stimulus intensities. Both early and late responses were of corticospinal origin since they could be completely collided by appropriately timed stimulation of the pyramidal tract. The latency of the early response in the DLF indicated that it resulted from direct activation of corticospinal neurones: its latency was the same as the latency of the antidromic action potentials evoked in the motor cortex from the recording site in the DLF.3. Scalp stimulation, which was also investigated in three of the monkeys, evoked short-latency volleys at threshold and at higher stimulus intensities these were followed by later waves. The short-latency volleys could be collided from the pyramid and, at threshold, had latencies compatible with direct activation of corticospinal neurones. The longer latency volleys were also identified as corticospinal in origin.4. The latency of the early volley evoked by electromagnetic stimulation remained constant with increasing stimulus intensities. In contrast, with scalp stimulation above threshold the latency of the early volleys decreased considerably, indicating remote activation of the corticospinal pathway below the level of the motor cortex. In two monkeys both collision and latency data suggest activation of the corticospinal pathway as far caudal as the medulla.5. The majority of fast corticospinal fibres could be excited by scalp stimulation with intensities of 20% of maximum stimulator output. Electromagnetic MS 8118 S. A. EDGLEY AND OTHERS stimulation at maximum stimulator output elicited a volley of between 70 and 90 % of the size of the maximal volley evoked from the pyramidal electrodes.6. Electromagnetic stimulation was also investigated in one awake monkey during the performance of a precision grip task. Short-latency EMG responses were evoked in hand and forearm muscles. The onsets of these responses were approximately 0-8 ms longer than the responses evoked by electrical stimulation of the pyramid. Furthermore, they were comparable in latency to the fastest post-spike facilitation produced in the same muscles by identified cortico-motoneuronal cells.7. It is concluded that in the monkey, both electromagnetic and scalp stimulation of the motor cortex can activate corticospinal neurones directly, but that suprathreshold scalp stim...

Voluntary Motor Output Is Altered by Spike-Timing-Dependent Changes in the Human Corticospinal Pathway

“…TMS produces multiple descending volleys over several milliseconds with some corticospinal neurons firing more than once (Edgley et al, 1990;Rothwell et al, 1991). Our timing was based on estimates of the arrival of the first descending volley at the motoneuron, but clearly timing of later volleys is also likely to be important.…”

Evidence that a long latency stretch reflex in humans is transcortical.

“…Transcranial magnetic stimulation of the human brain causes short latency facilitation of the motoneurones of forearm and hand muscles which has been attributed to the excitation of fast corticospinal neurones (Hess, Mills & Murray, 1987;Day, Dressler, Maertens de Noordhout, Marsden, Nakashima, Rothwell & Thompson, 1989), at the initial segment (Edgley et al 1990). Fast conducting MOTOR CORTEX STIMULATION AND MUSCLE STRETCH corticospinal neurones make monosynaptic connections with motoneurones of hand and forearm muscles in primates, based on physiological (Bernhard & Bohm, 1954;Phillips & Porter, 1964) and histological (Lawrence, Porter & Redman, 1985) evidence.…”

“…This was considered to be compatible with summation occurring at the cortex, although, since there was no measure of the size of the descending volleys and since rectified electromyography does not permit precise timing of events, it was not possible to exclude the excitation of different subsets of motoneurones, non-linear responses of a population of motoneurones to descending volleys, or summation occurring at interneurones (Day et al 1991). This differential facilitation was observed with magnetic stimulation (believed to excite cortical neurones) but not with electrical stimulation over the cortex (thought to excite the axons of cortical cells (Edgley, Eyre, Lemon & Miller, 1990)). This implies that there were changes in the excitability of cortical neurones unless the two forms of stimulation excited different populations of cortical neurones with different effects on spinal neurones.…”

Task dependence of responses in first dorsal interosseous muscle to magnetic brain stimulation in man.

“…Since the CM system can be activated by magnetic brain stimulation (Edgley, Eyre, Miller & Lemon, 1990: see Rothwell, Thompson, Day, Boyd & Marsden, 1991), it is interesting to investigate whether the muscle responses this evokes behave in a task-related fashion. Datta, Harrison & Stephens (1989) found that EMG responses in the first dorsal interosseous muscle (1DI) were larger in an 'isolated' isometric abduction of the index finger than when the same muscle was used in concert with other muscles during a power grip.…”