Noninvasive Stimulation of Human Corticospinal Axons Innervating Leg Muscles

These studies investigated whether a single electrical stimulus over the thoracic spine activates corticospinal axons projecting to human leg muscles. Transcranial magnetic stimulation of the motor cortex and electrical stimulation over the thoracic spine were paired at seven interstimulus intervals, and surface electromyographic responses were recorded from rectus femoris, tibialis anterior, and soleus. The interstimulus intervals (ISIs) were set so that the first descending volley evoked by cortical stimulation had not arrived at (positive ISIs), was at the same level as (0 ISI) or had passed (negative ISIs) the site of activation of descending axons by the thoracic stimulation at the moment of its delivery. Compared with the responses to motor cortical stimulation alone, responses to paired stimuli were larger at negative ISIs but reduced at positive ISIs in all three leg muscles. This depression of responses at positive ISIs is consistent with an occlusive interaction in which an antidromic volley evoked by the thoracic stimulation collides with descending volleys evoked by cortical stimulation. The cortical and spinal stimuli activate some of the same corticospinal axons. Thus it is possible to examine the excitability of lower limb motoneuron pools to corticospinal inputs without the confounding effects of changes occurring within the motor cortex.

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Phase-dependent reflex reversal in human leg muscles during walking

Journal of Neurophysiology ◽

10.1152/jn.1990.63.5.1109 ◽

1990 ◽

Vol 63 (5) ◽

pp. 1109-1117 ◽

Cited By ~ 220

Author(s):

J. F. Yang ◽

R. B. Stein

Keyword(s):

Stimulus Intensity ◽

Rectus Femoris ◽

Leg Muscles ◽

Middle Latency Response ◽

Reflex Pathways ◽

Activation Level ◽

Reflex Reversal ◽

Latency Response ◽

Middle Latency ◽

Stimulation Of

1. Reflex responses during walking were elicited in humans by stimulation of the tibial nerve at the ankle. The stimulus intensity was controlled by monitoring the M-wave from an intrinsic foot muscle. Responses were observed in the ipsilateral tibialis anterior (TA), soleus (SO), and rectus femoris (RF) muscles. The most reproducible responses were observed at a middle latency between 50 and 90 ms. The responses were most likely of cutaneous origin, because they closely resembled the responses to stimulation of a purely cutaneous nerve, the sural nerve. 2. A reversal in the direction of the middle latency response from excitation to inhibition was observed for the first time within single muscles during walking. Evidence for a reversal was seen in all three muscles examined and in all seven subjects. 3. The reflex reversal could not be elicited in standing. An inhibition whose amplitude varied in a linear fashion with stimulus intensity and background activation level was always observed at middle latency. The responses elicited during standing resembled those during the stance phase of walking. The two tasks shared some common movement goals and appeared to make use of similar reflex pathways.

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A further examination of effects of cortical stimulation on primate spinothalamic tract cells

Journal of Neurophysiology ◽

10.1152/jn.1983.49.2.424 ◽

1983 ◽

Vol 49 (2) ◽

pp. 424-441 ◽

Cited By ~ 70

Author(s):

R. P. Yezierski ◽

K. D. Gerhart ◽

B. J. Schrock ◽

W. D. Willis

Keyword(s):

Motor Cortex ◽

Sensorimotor Cortex ◽

Dynamic Range ◽

Cortical Stimulation ◽

Cortical Inhibition ◽

Sensory Cortex ◽

High Threshold ◽

Spinothalamic Tract ◽

Average Latency ◽

Stimulation Of

1. Stimulation of the sensorimotor cortex was found to excite and/or inhibit nociceptive spinothalamic tract cells. Thirteen wide dynamic range cells were inhibited by cortical stimulation, 6 were excited and 14 were both excited and inhibited. Four of six high-threshold cells were excited and one was inhibited. 2. Intermediate (200 ms) or long (2 s) duration conditioning trains were effective in reducing responses of spinothalamic cells evoked by noxious mechanical or thermal stimuli and by A- and C-fiber volleys in the sural nerve. Preferential inhibition of low-threshold responses with little or no effect on high-threshold discharges was observed in some cases. 3. Inhibitory actions were obtained primarily from stimulation of the SI sensory cortex and area 5, while excitation or excitation followed by inhibition was the dominant effect from motor cortex (area 4). Spinothalamic cells were also excited by stimulation of the medullary pyramid. 4. In eight animals extensive mapping of the sensorimotor cortex showed that for a given cell, stimulation of the sensory cortex produced inhibition while stimulation of motor cortex resulted in excitation. 5. The average latency of inhibition from sensory cortex was 29.8 +/- 10 ms, while the average latency of excitation from motor cortex was significantly shorter, 13.5 +/- 9 ms. The shortest latencies for excitation from pyramidal stimulation in the cases evaluated ranged from 2 to 9 ms. 6. Spinal cord lesions were made in five animals to determine the descending pathway(s) mediating corticofugal effects. Cortical and pyramidal effects were eliminated or considerably reduced by lesions involving the dorsal part of the lateral funiculus. This observation combined with latency data suggest that the corticospinal tract may be involved in the mediation of cortical excitation, while both pyramidal and extrapyramidal pathways are likely to be involved in cortical inhibition.

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Motor cortex excitability does not increase during sustained cycling exercise to volitional exhaustion

Journal of Applied Physiology ◽

10.1152/japplphysiol.00486.2012 ◽

2012 ◽

Vol 113 (3) ◽

pp. 401-409 ◽

Cited By ~ 45

Author(s):

Simranjit K. Sidhu ◽

Andrew G. Cresswell ◽

Timothy J. Carroll

Keyword(s):

Motor Cortex ◽

Evoked Potentials ◽

Cortical Neurons ◽

Cortical Excitability ◽

Motor Evoked Potentials ◽

Vastus Lateralis ◽

Rectus Femoris ◽

Cycling Exercise ◽

Motor Cortex Excitability ◽

Stimulation Of

The excitability of the motor cortex increases as fatigue develops during sustained single-joint contractions, but there are no previous reports on how corticospinal excitability is affected by sustained locomotor exercise. Here we addressed this issue by measuring spinal and cortical excitability changes during sustained cycling exercise. Vastus lateralis (VL) and rectus femoris (RF) muscle responses to transcranial magnetic stimulation of the motor cortex (motor evoked potentials, MEPs) and electrical stimulation of the descending tracts (cervicomedullary evoked potentials, CMEPs) were recorded every 3 min from nine subjects during 30 min of cycling at 75% of maximum workload (Wmax), and every minute during subsequent exercise at 105% of Wmax until subjective task failure. Responses were also measured during nonfatiguing control bouts at 80% and 110% of Wmax prior to sustained exercise. There were no significant changes in MEPs or CMEPs ( P > 0.05) during the sustained cycling exercise. These results suggest that, in contrast to sustained single-joint contractions, sustained cycling exercise does not increase the excitability of motor cortical neurons. The contrasting corticospinal responses to the two modes of exercise may be due to differences in their associated systemic physiological consequences.

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Magnetic stimulation of the human motor cortex evokes skin sympathetic nerve activity

Journal of Applied Physiology ◽

10.1152/jappl.2000.88.1.126 ◽

2000 ◽

Vol 88 (1) ◽

pp. 126-134 ◽

Cited By ~ 31

Author(s):

David H. Silber ◽

Lawrence I. Sinoway ◽

Urs A. Leuenberger ◽

Vahe E. Amassian

Keyword(s):

Motor Cortex ◽

Sympathetic Nerve ◽

Sympathetic Nerve Activity ◽

Cortical Stimulation ◽

Motor Cortex Stimulation ◽

Nerve Activity ◽

Human Motor Cortex ◽

Skin Sympathetic Nerve Activity ◽

Motor Cortical ◽

Stimulation Of

Single-pulse magnetic coil stimulation (Cadwell MES 10) over the cranium induces without pain an electric pulse in the underlying cerebral cortex. Stimulation over the motor cortex can elicit a muscle twitch. In 10 subjects, we tested whether motor cortical stimulation could also elicit skin sympathetic nerve activity (SSNA; n = 8) and muscle sympathetic nerve activity (MSNA; n = 5) in the peroneal nerve. Focal motor cortical stimulation predictably elicited bursts of SSNA but not MSNA; with successive stimuli, the SSNA responses did not readily extinguish (94% of discharges to the motor cortex evoked SSNA responses) and had predictable latencies [739 ± 33 (SE) to 895 ± 13 ms]. The SSNA responses were similar after stimulation of dominant and nondominant sides. Focal stimulation posterior to the motor cortex elicited extinguishable SSNA responses. In three of six subjects, anterior cortical stimulation evoked SSNA responses similar to those seen with motor cortex stimulation but without detectable movement; in the other subjects, anterior stimulation evoked less SSNA discharge than that seen with motor cortex stimulation. Contrasting with motor cortical stimulation, evoked SSNA responses were more readily extinguished with 1) peripheral stimulation that directly elicited forearm muscle activation accompanied by electromyograms similar to those with motor cortical stimulation; 2) auditory stimulation by the click of the energized coil when off the head; and 3) in preliminary experiments, finger afferent stimulation sufficient to cause tingling. Our findings are consistent with the hypothesis that motor cortex stimulation can cause activation of both α-motoneurons and SSNA.

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Corticospinal contributions to lower limb muscle activity during cycling in humans

Journal of Neurophysiology ◽

10.1152/jn.00212.2011 ◽

2012 ◽

Vol 107 (1) ◽

pp. 306-314 ◽

Cited By ~ 43

Author(s):

Simranjit K. Sidhu ◽

Ben W. Hoffman ◽

Andrew G. Cresswell ◽

Timothy J. Carroll

Keyword(s):

Motor Cortex ◽

Muscle Activation ◽

Corticospinal Tract ◽

Cortical Excitability ◽

Motor Evoked Potentials ◽

Vastus Lateralis ◽

Inhibitory Interneurons ◽

Leg Muscles ◽

Static Contractions ◽

Stimulation Of

The purpose of the current study was to investigate corticospinal contributions to locomotor drive to leg muscles involved in cycling. We studied 1) if activation of inhibitory interneurons in the cortex via subthreshold transcranial magnetic stimulation (TMS) caused a suppression of EMG and 2) how the responses to stimulation of the motor cortex via TMS and cervicomedullary stimulation (CMS) were modulated across the locomotor cycle. TMS at intensities subthreshold for activation of the corticospinal tract elicited suppression of EMG for approximately one-half of the subjects and muscles during cycling, and in matched static contractions in vastus lateralis. There was also significant modulation in the size of motor-evoked potentials (MEPs) elicited by TMS across the locomotor cycle ( P < 0.001) that was strongly related to variation in background EMG in all muscles ( r > 0.86; P < 0.05). When MEP and CMEP amplitudes were normalized to background EMG, they were relatively larger prior to the main EMG burst and smaller when background EMG was maximum. Since the pattern of modulation of normalized MEP and CMEP responses was similar, the data suggest that phase-dependent modulation of corticospinal responses during cycling in humans is driven mainly by spinal mechanisms. However, there were subtle differences in the degree to which normalized MEP and CMEP responses were facilitated prior to EMG burst, which might reflect small increases in cortical excitability prior to maximum muscle activation. The data demonstrate that the motor cortex contributes actively to locomotor drive, and that spinal factors dominate phase-dependent modulation of corticospinal excitability during cycling in humans.

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