This column highlights two scientific advances, 60 years apart. The first established the movement of the electrical signal down a nerve, and the second how the electrical signals of the brain can be harnessed to move a robotic arm.
Sir Andrew Huxley died recently at the age of 94. With his long-standing colleague, Adam Hodgkin, they began work in the late 1930’s asking how an electrical signal travels along a nerve fiber. After being interrupted by military service, they returned to the problem, performing the crucial experiments and, in 1952, publishing their model of what might be happening.
Prior to their work it was thought that the electrical signal travelled down the center of the nerve fiber, and the nerve fiber was functioning like a battery. Their modeling suggested that this interpretation was impossible, so they switched attention to the membrane of the nerve fiber, like the covering of a straw. To do this they used the giant axon of a squid, one of the largest nerves known. With this large nerve it was possible to put an electrode inside the nerve and thus be able to measure the movements of ions such as sodium and potassium across the nerve membrane. They measured how movement of sodium out of the axon and potassium into the axon generated changes in electrical state. This state became known as the “action potential”: an electrical charge traveling down the nerve fiber. They also postulated that there would be specific channels in the nerve membrane through which specific ions could flow. These channels were not identified until years later. One of the first computational models of a biological process, “The Hodgkin-Huxley Model” earned the duo the Nobel Prize in 1963.
The second advance involves acquiring signals from the brain, transferring them to a computer for decoding, and translating them to movement of a robotic arm. This report indicates the continuing progress in brain-driven computer responses, led by John Donoghue at Brown University. The first step is to gather the brain-derived electrical signals (the signals discovered by Hodgkin and Huxley) from an area of the brain specific to a particular function, such as arm movement. This is done through a series of small electrodes placed in that area. These signals are transferred to a computer system that decodes them. The decoded signals are then transferred to an output such as a signal on a computer screen, or ultimately to a robotic arm.
Initially the responses were only in two dimensions, moving a signal across a screen. The major addition has been the ability to generate a signal in three dimensions, so that the computer arm can be moved through space. The idea that a paralyzed person could control a robotic arm by thinking about a movement—and, say, get a robotic arm to pick up a soda bottle and move it into position to have a drink—is scientific fantasy come true.
The implications of the application of this research are enormous. The thousands of veterans with disabled or missing limbs may have the basis for commanding a prosthetic device. Further, it is conceivable that injuries to the spinal cord or peripheral nerve can be by-passed, allowing the brain signal to be delivered to the remaining undamaged nerves.
To go from identifying the nature of nerve signals by Hodgkin and Huxley to the using these signals to control events outside the brain may not have been an expected progression of neuroscience, but both are remarkable achievements.