Excitability of the motor cortex after sleep deprivation: a transcranial magnetic stimulation study

Objectives: Here we study changes of corticospinal excitability as a function of a sleep deprivation, using several TMS paradigms that have been already used to assess motor cortical excitability during sleep (1). Methods: Corticospinal excitability was studied in 33 normal subjects (18 M and 15 F; age = 24.6 ± 2.4 years) before and after a 36h sleep deprivation (SD); in both cases the recordings started at 10.00 p.m. Intracortical facilitation (ICF) and inhibition (ICI) was measured by the paired pulse TMS technique with a subthreshold conditioning shock (CS) followed by a suprathreshold test one (TS). Inter-Stimulus Intervals (ISIs) between CS and TS were: 1, 3, 7, 10, 12, 15 ms. Besides standardized threshold (ST), lower threshold (LT), defined as the highest stimulus intensity evoking responses in the relaxed ADM muscle with a probability of zero, and upper threshold (UT), defined as the lowest intensity evoking responses with a probability of one were also determined. Results: STs (F1,31 = 9.73; P = 0.004), LTs (F1,31 = 3.87; P = 0.05), and UTs (F1,31 = 9.73; P = 0.004) all increased as consequence of SD. ICI and ICF showed a significant dissociation in females: SD heightened ICF, while ICI changes were not significant. No significant change was observed in males. Conclusions: Corticospinal excitability, as indicated by MTs, decreases after SD. On the other side, cortical facilitation, expressed by the augmentation of MEP amplitude elicited by paired pulses at longer ISIs, increases at least in females. This finding partially confirms the hypothesis that cortical responses evoked by TMS should be higher as a consequence of a longer wakefulness, that produces a net increase in the synaptic weight impinging on many cortical neurons, and as a consequence of a lack of sleep that would be needed to redress the balance (2).