Document Detail

Responses of ankle extensor and flexor motoneurons to transcranial magnetic stimulation.
MedLine Citation:
PMID:  12091538     Owner:  NLM     Status:  MEDLINE    
Transcranial magnetic stimulation (TMS) of the motor cortex excites limb muscles of the contralateral side of the body. Reports of poorly defined, or a complete lack of systematic excitatory responses of soleus motoneurons compared with those of tibialis anterior (TA) motoneurons has led to the proposal that while all ankle flexor motoneurons receive strong corticomotoneuronal connections, very few soleus motoneurons do. In addition, the connections to these few motoneurons are weak. The nature of corticomotoneuronal connections onto these two motoneuron pools was re-evaluated in the following experiments. The leg area of the left motor cortex was stimulated with a large double-cone coil using Magstim 200, while surface electromyographic (EMG) and single motor unit (SMU) responses were recorded from soleus and TA muscles of healthy adult subjects. Under resting conditions, the onset (25-30 ms) and duration of concomitantly recorded short latency motor evoked potentials (MEPs) in surface EMG from both muscles were similar. The input-output relationships of the simultaneously recorded soleus and TA EMG responses showed much greater increases in TA MEPs compared with soleus MEPs with identical increases in stimulus intensity. Under resting and nonisometric conditions, a later peak with onset latency of approximately 100 ms was observed in soleus. During isometric conditions or with vibration of the TA tendon, the second soleus peak was abolished indicating reflex origin of this peak. Recordings from 42 soleus and 39 TA motor units showed clear response peaks in the peristimulus time histograms (PSTHs) of every unit. Two statistical tests were done to determine the onset and duration of response peaks in the PSTHs. With chi(2) test, the duration was 6.9 +/- 4.2 ms (mean +/- SD) for soleus and 5.1 +/- 2.1 ms for TA. Using the criterion of discerning a peak by bin counts being three SDs above background, the duration was 10.0 +/- 4.4 ms for soleus and 7.8 +/- 2.6 ms for TA. Results of these experiments do not suggest a lack of systematic corticomotoneuronal connections on soleus motoneurons when compared with those on TA, though some differences in the strengths of corticomotoneuronal connections onto the two pools do exist.
P Bawa; G R Chalmers; H Stewart; A A Eisen
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't    
Journal Detail:
Title:  Journal of neurophysiology     Volume:  88     ISSN:  0022-3077     ISO Abbreviation:  J. Neurophysiol.     Publication Date:  2002 Jul 
Date Detail:
Created Date:  2002-07-01     Completed Date:  2002-09-06     Revised Date:  2006-11-15    
Medline Journal Info:
Nlm Unique ID:  0375404     Medline TA:  J Neurophysiol     Country:  United States    
Other Details:
Languages:  eng     Pagination:  124-32     Citation Subset:  IM    
School of Kinesiology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
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MeSH Terms
Ankle / innervation*
Electric Stimulation
Middle Aged
Motor Cortex / physiology*
Motor Neurons / physiology*
Muscle, Skeletal / innervation*,  physiology
Neurons, Afferent / physiology
Transcranial Magnetic Stimulation*

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