In the brain, they may be part of a signaling system that recognizes and eliminates inappropriate neural connections. Expression of MHC class I is regulated by the naturally occurring electrical activity, and sensitive to both natural and pathological changes in the activity. Electrical activity of neurons drives to an establishment of the final pattern of connection. Changes in the strength of individual synapses such as potention and depression leads to stabilization and withdrawal, respectively, of the affected connections. There are data, that in mice with deficiency of MHC class I and CD3-zeta the LTP in the hippocampus is enhanced significantly and LTD is absent. Thus, MHC class I is crucial for translating activity into changes in synaptic strength and neuronal connectivity in vivo. He required for normal activity dependent potentiation, depression, removal of inappropriate connection and responding to injury in the CNS (6).