They theorized that when pain signals first reach the nervous system they excite activity in a group of small neurons that form a kind of pain pool. When the total activity of these neurons reaches a certain minimal level, a hypothetical gate opens up that allows the pain signals to be sent to higher brain centers. Chemical Pathways Shortly after the gate theory was promulgated, a landmark discovery about pain-suppressing chemicals was made by scientists in Aberdeen, Scotland and at Johns Hopkins in Baltimore. They were curious about how morphine and other opium-derived painkillers, or analgesics, worked. The transmitter molecules cross the gap separating the two cells and attach to special receptor sites on the neighboring cell surface.
Some neurotransmitters excite the second cell allowing it to generate an electrical pain signal.
Others inhibit the second cell preventing it from generating a pain signal. When the researchers injected morphine into experimental animals, they found that the morphine molecules fit snugly into receptors on certain brain and spinal cord neurons. They therefore reasoned that there might be naturally occurring brain chemicals that behaved exactly like morphine. Both groups of scientists found not just one pain-suppressing chemical in the brain, but a whole family of such proteins. The discovery of the endorphins lent weight to the general concept of the gate theory.