Impaired organization of pairedpulse TMS-induced I-waves after human spinal cord injury. Cereb. Cortex. doi: 10.1093/cercor/bhv048. [Epub ahead of print

Paired-pulse transcranial magnetic stimulation (TMS) of the humanmotor 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 influenceof decreased corticospinal inputs. Amathematicalmodel showedthat after SCI the thirdpeakaberrantly contributed to spinalmotoneurone recruitment, regardless on themotor 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. Key words: corticospinal volleys, primary motor cortex, spinal cord injury, transcranial magnetic stimulation, voluntary movemen