After an amputation, the nerves are left like programmed data cables floating in space.  Dr. Todd Kuiken, director of the Center for Bionic Medicine and director of Amputee Services at The Rehabilitation Institute of Chicago, has led a team that has made prosthetic limbs that utilize the body’s own remaining limb-controlling nerves after an amputation to allow them to control prosthetics just by thinking.  The person thinks about what they want to move, which send impulses to the salvaged nerves that have been implanted into chest muscle.  The resulting tiny changes in chest muscle activity are then translated into electical impulses that move the limb in the same manner that the person was thinking to move it.

http://pagingdrgupta.blogs.cnn.com/2011/02/17/bionic-arm-gives-hope-for-amputees/?hpt=C1