The nature of corticospinal paths driving human motoneurones during voluntary contractions.
The properties of the human motor cortex can be studied non-invasively using transcranial magnetic stimulation (TMS). Stimulation at high intensity excites corticospinal cells with fast conducting axons that make direct connections to motoneurones of human upper limb muscles, while low-intensity stimulation can suppress ongoing EMG. To assess whether these cells are used in normal voluntary contractions, we used TMS at very low intensities to suppress the firing of single motor units in biceps brachii (n = 14) and first dorsal interosseous (FDI, n = 6). Their discharge was recorded with intramuscular electrodes and cortical stimulation was delivered at multiple intensities at appropriate times during sustained voluntary firing at approximately 10 Hz. For biceps, high-intensity stimulation produced facilitation at 17.1 +/- 2.1 ms (lasting 2.4 +/- 0.9 ms), while low-intensity stimulation (below motor threshold) produced suppression (without facilitation) at 20.2 +/- 2.1 ms (lasting 7.6 +/- 2.2 ms). For FDI, high-intensity stimulation produced facilitation at 23.3 +/- 1.2 ms (lasting 1.8 +/- 0.4 ms), with suppression produced by low-intensity stimulation at 25.2 +/- 2.6 ms (lasting 7.5 +/- 2.6 ms). The difference between the onsets of facilitation and suppression was short: 3.1 +/- 1.2 ms for biceps and 2.0 +/- 1.5 ms for FDI. This latency difference is much less than that previously reported using surface EMG recordings ( approximately 10 ms). These data suggest that low-intensity cortical stimulation inhibits ongoing activity in fast-conducting corticospinal axons through an oligosynaptic (possibly disynaptic) path, and that this activity is normally contributing to drive the motoneurones during voluntary contractions.
Butler
Jane E
JE
Department of Exercise and Sport Sciences, University of Copenhagen, Blegdamvej 3, 2200 Copenhagen, Denmark.
Larsen
Thomas S
TS
Gandevia
Simon C
SC
Petersen
Nicolas T
NT
D000200:Action Potentials; D004576:Electromyography; D006801:Humans; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000294:innervation*; D009433:Neural Inhibition; D009473:Neuronal Plasticity; D011712:Pyramidal Tracts / Q000502:physiology*; D011930:Reaction Time; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation; D014836:Volition
J Neurophysiol. 1997 Jun;77(6):3401-5
9212286
J Physiol. 2001 Dec 1;537(Pt 2):651-6
11731595
J Physiol. 1998 Apr 15;508 ( Pt 2):625-33
9508823
Exp Brain Res. 1998 Mar;119(2):265-8
9535577
Brain. 1999 Jul;122 ( Pt 7):1327-40
10388798
J Neurophysiol. 1965 Mar;28:216-28
14283057
Clin Neurophysiol. 2006 Dec;117(12):2684-90
17029949
Mov Disord. 2007 Apr 15;22(5):601-10
17260337
J Physiol. 1983 Aug;341:387-410
6620185
J Physiol. 1986 Aug;377:143-69
3795085
Brain. 1987 Oct;110 ( Pt 5):1117-30
3676695
J Physiol. 1987 Aug;389:729-56
3681741
Respir Physiol. 1988 Sep;73(3):325-37
3175362
J Neurophysiol. 1989 Jul;62(1):212-9
2754473
Exp Brain Res. 2002 Mar;143(2):240-8
11880900
J Physiol. 2002 Oct 1;544(Pt 1):277-84
12356898
Exp Brain Res. 2003 Sep;152(1):1-16
12879177
Proc R Soc Lond B Biol Sci. 1976 Oct 29;194(1116):341-73
11492
Monogr Physiol Soc. 1977;(34):v-xii, 1-450
351378
Nature. 1980 May 22;285(5762):227
7374773
J Neurophysiol. 1980 Oct;44(4):751-72
6253604
J Physiol. 1989 May;412:449-73
2489409
Exp Physiol. 1991 Mar;76(2):159-200
2059424
J Physiol. 1991 Aug;439:545-58
1895244
J Physiol. 1992 Mar;448:397-412
1593472
J Neurosci Methods. 1992 Jul;43(2-3):195-200
1405743
J Physiol. 1993 Oct;470:383-93
8068071
J Physiol. 1993 Nov;471:223-43
8120805
J Physiol. 1993 Nov;471:501-19
8120818
J Physiol. 1994 Jun 1;477 ( Pt 2):223-35
7932215
J Physiol. 1995 May 1;484 ( Pt 3):791-802
7623293
Exp Brain Res. 1996 Apr;109(1):127-35
8740215
J Physiol. 1996 Apr 15;492 ( Pt 2):597-628
9019553
J Physiol. 1997 Feb 1;498 ( Pt 3):817-23
9051592
Neurology. 1997 May;48(5):1398-403
9153480
Brain. 1997 May;120 ( Pt 5):839-53
9183254