Microstimulators serve as digital nerves.

Cardiac pacemakers rank among the best-known electrical stimulators. They regularly fire a small electrical impulse into the heart through implanted electrodes to keep the old ticker pumping faithfully. Three labs are now collaborating on a family of related electronic devices to reawaken and command nerve-deadened tissue -- from paralyzed arms to bladders.

"Our new device is as sophisticated as the pulse generators in pacemakers," says Philip R. Troyk of the first working prototype, unveiled last month for the National Institutes of Health, the project's major sponsor. The main difference? This one is only slightly larger than a grain of rice.

"If this were bigger, there'd be nothing special about it," notes Troyk, of the Illinois Institute of Technology in Chicago. Neurologists wanted a small implant they could insert into muscle to fire off electrical messages as needed to specifically targeted tissues. Together with researchers at the Alfred E. Mann Foundation for Scientific Research in Sylmar, Calif., and Queen's University in Kingston, Ontario, Troyk's team developed hermetically sealed glass bottles capped on each end by an electrode. Just 1 centimeter long and 2 millimeters in diameter, the implants are designed for insertion with a large-gauge hypodermic injector.

A tiny capacitor inside each microstimulator stores up the electrical energy delivered by an external, battery-driven power source. The stimulator discharges into adjacent tissue whenever it receives a radioed command to do so. A custombuilt computer chip inside the implant helps the stimulator decode the commands, which tell the stimulator when to pulse, for how long and what amplitude of current to deliver.

If adorned with external sensors, microstimulators might not only listen and fire out commands but also talk back -- perhaps offering highly localized assays of a person's blood pressure, joint angle or tissue oxygenation, says Troyk.

The prototype has yet to undergo testing in animals of humans, but Troyk says trials may begin as early as next year.