Chapter 32 Modulation of cortical plasticity

Abstract

One way to explore plasticity in the human cortex is by using transcranial magnetic stimulation (TMS) of the motor cortex. It is shown that motor representations are subject to reorganization, following a cerebral stroke, active physiotherapy after stroke, motor practice and skill acquisition in healthy subjects, repetitive movement, limb amputation, or immobilization. Despite knowing that plasticity occurs in the adult human motor cortex, it remains a largely open question to which extent these forms of lesion- and practice-induced plasticity can be modulated by specific intervention. Experiments in animals showed that one important modulator of cortical plasticity is γ-aminobutyric acid (GABA), the main inhibitory neurotransmitter in neocortex. Local reduction of GABA mediated cortical inhibition may lead to the rapid expansion of motor representations, most likely through unmasking of pre-existent but latent excitatory corticocortical connections. Furthermore, reduction of GABA mediated inhibition is required in many protocols, for the induction of neocortical synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD). On the other hand, recovery of function after the brain lesion was impaired when the lesioned animals were treated with GABA receptor agonists. This chapter explains the TMS experiments that tested the role of GABA, in modulating plasticity of the human motor cortex.