When the pain from an old back injury begins to bother him, Charles Niethold turns on a radio transmitter about the size of a package of cigarettes, which sends a message to his brain. The slight signal of the radio beam causes the brain to kick out a dose of a hormone. It effectively blocks the pain and brings relief from a few hours to a period as long as several days.  "It's like someone taking a load off your back. It is just a pleasant feeling of relief," says the salesman, who has suffered from a back injury for years.