Journal
CEREBRAL CORTEX
Volume 26, Issue 5, Pages 2167-2177Publisher
OXFORD UNIV PRESS INC
DOI: 10.1093/cercor/bhv048
Keywords
corticospinal volleys; primary motor cortex; spinal cord injury; transcranial magnetic stimulation; voluntary movement
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Funding
- National Institute of Neurological Disorders and Stroke-National Institutes of Health [R01 NS076589, NS0900622]
- Department of Veterans Affairs [3397626]
- Paralyzed Veterans of America [2968]
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Paired-pulse transcranial magnetic stimulation (TMS) of the human motor cortex results in consecutive facilitatory motor-evoked potential (MEP) peaks in surface electromyography in intact humans. Here, we tested the effect of an incomplete cervical spinal cord injury (SCI) on early (first) and late (second and third) MEP peaks in a resting intrinsic finger muscle. We found that all peaks had decreased amplitude in SCI subjects compared with controls. The second and third peaks were delayed with the third peak also showing an increased duration. The delay of the third peak was smaller than that seen in controls at lower stimulation intensity, suggesting lesser influence of decreased corticospinal inputs. A mathematical model showed that after SCI the third peak aberrantly contributed to spinal motoneurone recruitment, regardless on the motor unit threshold tested. Temporal and spatial aspects of the late peaks correlated with MEP size and hand motor output. Thus, early and late TMS-induced MEP peaks undergo distinct modulation after SCI, with the third peak likely reflecting a decreased ability to summate descending volleys at the spinal level. We argue that the later corticospinal inputs on the spinal cord might be crucial for recruitment of motoneurones after human SCI.
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