Modulation of plasticity of the soleus area of the motor cortex using paired associative stimulation

Abstract

Paired associative stimulation (PAS) repeatedly combines single somatosensory nerve stimuli with single transcranial magnetic stimuli to induce bidirectional changes in the excitability of the cortical projections to the target muscle. PAS and motor training have been shown to share common neural mechanisms, suggesting that PAS tests functionally relevant neuronal circuits. While PAS has been used extensively to target the hand area of the motor cortex, few studies have targeted the leg area of the motor cortex. The optimal interstimulus inter- val (ISI) to induce plasticity in the cortical projections to lower limbs is still not well established. Thus, the first purpose of this thesis was to define an optimal ISI to induce long-term potentiation-like plasticity in the cortical projections to the soleus muscle. Next, this PAS protocol was used as a tool to study the effect of training background on motor cortex plasticity. The functionality of the ef- fects following PAS was evaluated in the third experiment, by quantifying fa- tigue resistance during a 15 s sustained maximal isometric contraction prior to and after the PAS intervention. The fourth purpose of the thesis was to place the principle of PAS in a more natural context by replacing electrical stimula- tion with a natural stretch reflex volley (PASreflex). The optimal ISI for the PAS intervention when targeting soleus muscle was the latency of somatosensory evoked potential plus 18 ms (P32 plus 18 ms), which resulted in an 88 ± 105% increase in amplitude of the soleus motor-evoked potential. With the optimal PAS protocol, skill trained athletes exhibited significantly greater motor cortex plasticity compared to endurance trained athletes. The reason for differential motor cortex plasticity is likely related to the different training-induced adapta- tions. On average, fatigue resistance did not change following PAS and conse- quently, the functionality of PAS was not evident. However, PAS-induced ex- citability changes correlated significantly with changes in fatigue resistance. The effect of PASreflex was different immediately after and 30 min following the cessation of the intervention, and thus there were most likely several different phenomena taking place in the motor cortex due to the nature of the stretch re- flex. In conclusion, the findings of this thesis will help to understand the behav- ioral and neural signals that drive function and learning in the motor cortex.