The effect of afferent electrical stimulation on synaptic plasticity within the sensorimotor cortex will be discussed. Afferent electrical stimulation induces a down regulation of inhibitory neural circuits and plays a critical role in strengthening excitatory synapses. Synaptic modifications such as long-term potentiation (LTP) mechanisms could be a crucial mechanism underlying this stimulation-induced cortical plasticity. LTP and long-term depression (LTD) of synaptic transmission are crucial factors for activity-dependent changes in the strength of synaptic connections. Many studies demonstrated that these pathways play an important role in cortical synaptic plasticity. Repeated activation of excitatory synapses induces both short-term potentiation (STP) and LTP. Both types of synaptic potentiation affect N-methyl-D-aspartate glutamate receptors leading to the formation of new synapses or the unmasking of excitatory amino acid receptors on motor neurons. This increased excitability localized within the sensorimotor cortex may reflect an increase in neuronal activity as a result of a dynamic interaction of various synaptic and cellular mechanisms due to the local processing of afferent electrical input to the sensorimotor cortex. The chapter reviews also the large number of studies using fMRI and TMS to examine the effects of afferent electrical input from the hand on the excitability of human sensorimotor cortex
