FOR YEARS, research into the cellular malfunctions that lead to cancer has focused on the cell nucleus, with its store of vulnerable DNA. In recent years, however, some biomedical researchers have turned their attention to the outer layer of the cell -- the membrane -- and to the narrow spaces between cells.

Intercellular spaces, they discovered, are filled with a fluid that conducts the flow of electricity easily. The membrane, however, is an extraordinary insulator -- perhaps a millionth of an inch thick but capable of protecting the cell's interior from a charge equivalent to 100,000 volts per inch.

Nonetheless, the cell remains extremely sensitive to electrical signals. If the terminals of a 1.5-volt flashlight cell were connected to two wires dipped in the Pacific Ocean -- one at San Diego and one at Seattle -- the cells of a surfer at Long Beach or a fish off Monterey could still detect the electromagnetic field -- about one ten-millionth volt per inch.

The signal is transferred to the cell through the membrane, which is covered with protein strands that extend into the intercellular spaces like a field of corn with waving ears. Even a very weak signal can set off a domino effect.

The strands thus work as amplifiers, allowing cells to respond to astonishingly weak electromagnetic fields. Tissue fields as small as a hundred-millionth of a volt per centimeter, for example, can affect a shark's ability to navigate.

Slightly higher tissue fields can affect human circadian rhythms, the daily "body clock" that regulates metabolic and sleep patterns. Disruption can produce a host of stress-related symptoms, the most familiar of which is jet lag.

Circadian rhythms are regulated by hormones, particularly melatonin from the pineal gland of the brain. Low-frequency EM fields have been shown to disrupt melatonin cycles and production of other body chemicals in rats, raising questions about the implications of long-term disruptions in man.

The protein strands on the cell membrane also transmit signals outward from the cell into the intercellular spaces, where they are picked up by the protein strands on other cells. Thus the cells communicate or "whisper together" in a language that appears to be shaped by physics rather than chemistry.

The potential for trouble arises when EM fields interfere with or alter the normal flow of signals.

But the effects may not always be negative. Deciphering the faint language of cells might also lead to broader medical applications of electromagnetic fields, which already are used in some diagnostic and therapeutic procedures such as magnetic-resonance imaging.

Armed with the knowledge that certain enzymes that control cellular growth are sensitive to EM fields, for example, researchers have been able to use pulsed magnetic fields to speed healing of bone fractures.