Brain, Vol. 122, No. 7, 1327-1340,
July 1999
© 1999 Oxford University Press
Corticomotoneuronal synaptic connections in normal man
An electrophysiological study
University Department of Neurology, Hôpital de la Citadelle, Liège, Belgium
Correspondence to:
Alain Maertens de Noordhout, MD, University Department of Neurology, Hôpital de la Citadelle, Boulevard du XIIème de Ligne, 1, B-4000 Liège, Belgium E-mail: al.maertens{at}chu.ulg.ac.be
In order to determine the mono- or oligosynaptic character of connections between pyramidal axons and individual spinal motor neurons, we constructed peri-stimulus time histograms (PSTHs) of the firing probability of voluntarily activated single motor units (SMUs) of various upper and lower limb muscles upon slightly suprathreshold transcranial anodal electrical stimulations of the motor cortex in normal subjects. Weak anodal cortical stimuli are known to activate preferentially fast-conducting pyramidal axons directly, bypassing cell bodies and cortical interneurons. A narrow bin width (0.1 ms) was chosen to measure precisely the duration of the PSTH excitatory peak, which corresponds to the rise time of the underlying compound excitatory post-synaptic potentials (EPSP). A short duration PSTH peak indicates sharp-rising EPSPs, most commonly encountered in the case of monosynaptic connections. In flexor carpi radialis and soleus SMUs, the PSTHs of built-in responses to anodal cortical stimuli were compared with those produced by 1A afferent stimulation able to elicit a Hoffmann reflex, which is known to be largely monosynaptic. In all upper and lower limb muscles, excitable SMUs responded to anodal cortical stimuli with a highly synchronized peak of increased firing probability. In flexor carpi radialis and soleus SMUs, the mean duration of this peak was significantly narrower than that evoked by 1A afferent stimulation, indicating that monosynaptic corticomotor neuronal transmission dominates low-threshold motor units, even in proximal arm and leg muscles. In the various muscles studied, and particularly in forearm SMUs, we did not observe broad PSTH peaks against the activation of non-monosynaptic corticomotor neuronal pathways, even with near-threshold stimuli. In some triceps and forearm flexor SMUs, subthreshold anodal pulses caused significant inhibition of their voluntary firing, with a latency consistent with activation of 1A inhibitory interneurons by the descending volleys. Measurements of the maximal number of counts in the excitatory PSTH peak upon anodal cortical stimuli provide comparisons of the strength of monosynaptic inputs to various muscles which seems to be maximal for hand and finger extensor muscles, and also for deltoid.
corticospinal tract; motor neurons; motor cortex stimulation
EPSP = excitatory post-synaptic potential; H-reflex = Hoffmann reflex; PSTH = peri-stimulus time histograms; PTN = pyramidal tract neurons; SMU = single motor unit; TCS = transcranial anodal electrical stimulations; TMS = transcranial magnetic stimulation
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  J. L. Taylor and P. G. Martin Voluntary Motor Output Is Altered by Spike-Timing-Dependent Changes in the Human Corticospinal Pathway J. Neurosci., September 16, 2009; 29(37): 11708 - 11716. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. W. Hoffman, T. Oya, T. J. Carroll, and A. G. Cresswell Increases in corticospinal responsiveness during a sustained submaximal plantar flexion J Appl Physiol, July 1, 2009; 107(1): 112 - 120. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. N. Mileva, J. L. Bowtell, and A. R. Kossev Effects of low-frequency whole-body vibration on motor-evoked potentials in healthy men Exp Physiol, January 1, 2009; 94(1): 103 - 116. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. G. Martin, J. E. Butler, S. C. Gandevia, and J. L. Taylor Noninvasive Stimulation of Human Corticospinal Axons Innervating Leg Muscles J Neurophysiol, August 1, 2008; 100(2): 1080 - 1086. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. G. Martin, N. Weerakkody, S. C. Gandevia, and J. L. Taylor Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans J. Physiol., March 1, 2008; 586(5): 1277 - 1289. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. A. Krutky and E. J. Perreault Motor Cortical Measures of Use-Dependent Plasticity Are Graded From Distal to Proximal in the Human Upper Limb J Neurophysiol, December 1, 2007; 98(6): 3230 - 3241. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. E. Butler, T. S. Larsen, S. C. Gandevia, and N. T. Petersen The nature of corticospinal paths driving human motoneurones during voluntary contractions J. Physiol., October 15, 2007; 584(2): 651 - 659. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. G. Martin, S. C. Gandevia, and J. L. Taylor Muscle fatigue changes cutaneous suppression of propriospinal drive to human upper limb muscles J. Physiol., April 1, 2007; 580(1): 211 - 223. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
O. Soto, J. Valls-Sole, P. Shanahan, and J. Rothwell Reduction of Intracortical Inhibition in Soleus Muscle During Postural Activity J Neurophysiol, October 1, 2006; 96(4): 1711 - 1717. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. Taube, M. Schubert, M. Gruber, S. Beck, M. Faist, and A. Gollhofer Direct corticospinal pathways contribute to neuromuscular control of perturbed stance J Appl Physiol, August 1, 2006; 101(2): 420 - 429. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. G. Martin, J. L. Smith, J. E. Butler, S. C. Gandevia, and J. L. Taylor Fatigue-sensitive afferents inhibit extensor but not flexor motoneurons in humans. J. Neurosci., May 3, 2006; 26(18): 4796 - 4802. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. L. Pearce, P. D. Thompson, and M. A. Nordstrom Interhemispheric Coupling of Corticospinal Excitability Is Suppressed During Voluntary Muscle Activation J Neurophysiol, April 1, 2005; 93(4): 2174 - 2182. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 U. Ziemann, T. V. Ilic, H. Alle, and F. Meintzschel Cortico-motoneuronal excitation of three hand muscles determined by a novel penta-stimulation technique Brain, August 1, 2004; 127(8): 1887 - 1898. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. L Pearce, T. S Miles, P. D Thompson, and M. A Nordstrom Responses of single motor units in human masseter to transcranial magnetic stimulation of either hemisphere J. Physiol., June 1, 2003; 549(2): 583 - 596. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Mazevet, S. Meunier, P. Pradat-Diehl, V. Marchand-Pauvert, and E. Pierrot-Deseilligny Changes in propriospinally mediated excitation of upper limb motoneurons in stroke patients Brain, April 1, 2003; 126(4): 988 - 1000. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. T Petersen, J. L Taylor, and S. C Gandevia The effect of electrical stimulation of the corticospinal tract on motor units of the human biceps brachii J. Physiol., October 1, 2002; 544(1): 277 - 284. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Bawa, G. R. Chalmers, H. Stewart, and A. A. Eisen Responses of Ankle Extensor and Flexor Motoneurons to Transcranial Magnetic Stimulation J Neurophysiol, July 1, 2002; 88(1): 124 - 132. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. A. Eyre, J. P. Taylor, F. Villagra, M. Smith, and S. Miller Evidence of activity-dependent withdrawal of corticospinal projections during human development Neurology, November 13, 2001; 57(9): 1543 - 1554. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Olivier, S. N. Baker, K. Nakajima, T. Brochier, and R. N. Lemon Investigation Into Non-Monosynaptic Corticospinal Excitation of Macaque Upper Limb Single Motor Units J Neurophysiol, October 1, 2001; 86(4): 1573 - 1586. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Nicolas, V. Marchand-Pauvert, D. Burke, and E. Pierrot-Deseilligny Corticospinal excitation of presumed cervical propriospinal neurones and its reversal to inhibition in humans J. Physiol., June 15, 2001; 533(3): 903 - 919. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Nakajima, M. A. Maier, P. A. Kirkwood, and R. N. Lemon Striking Differences in Transmission of Corticospinal Excitation to Upper Limb Motoneurons in Two Primate Species J Neurophysiol, August 1, 2000; 84(2): 698 - 709. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Weber, A. Eisen, H. G. Stewart, and P. M. Andersen Preserved slow conducting corticomotoneuronal projections in amyotrophic lateral sclerosis with autosomal recessive D90A CuZn-superoxide dismutase mutation Brain, July 1, 2000; 123(7): 1505 - 1515. [Abstract] [Full Text] [PDF]
|
|