Edgar Adrian and Keith Lucas carried out pioneering experiments with this new instrumentation, using it to amplify the tiny electrical signal produced by a nerve fibre around 2,000 times. Adrian was a strong advocate of the importance of technology, believing that ‘the history of electrophysiology has been decided by the history of electric recording instruments’. He practised what he preached and his laboratory ‘contained the most glorious clutter even seen’. He also said that his results ‘didn’t involve any particular hard work, or any particular intelligence on my part. It was one of those things which sometimes just happens in a laboratory if you stick apparatus together and see what results you get.’ Adrian’s protégé Alan Hodgkin later dryly remarked that when most people ‘stick apparatus together and look around they do not make discoveries of the same importance as those of Adrian’. Interestingly, Hodgkin was one of the few who did.
Adrian began as an assistant to Keith Lucas while still a student at Cambridge in 1912. Lucas’s laboratory was memorable for being sited in a tiny dark dank cellar that flooded every time it rained, so that the scientists were forced to walk about on duckboards in wet weather (not the best environment for electrical experiments and one that would surely be banned on health and safety grounds today). Lucas set Adrian an exciting challenge – to investigate the conduction of the nerve impulse. Earlier experiments had already provided some support for the idea that nerve fibres fire fully or not at all, but there remained room for doubt; however, before they were able to settle the case one way or another, their research was interrupted by World War I. Indeed, Lucas’s career came to a permanent halt as he was killed in a mid-air collision while carrying out instrument tests for the Royal Air Force. After the war was over, Adrian took over his mentor’s laboratory in Cambridge. By painstakingly dissecting a single nerve fibre away from its neighbours in a nerve bundle, he discovered that when he stimulated the nerve, it generated a series of tiny electrical impulses of regular amplitude but variable frequency; the greater the stimulus strength, the higher the discharge rate. In other words, the intensity of a sensation is proportional to the frequency of sensory nerve impulses.
Adrian remarked that constant reminders of this fact come unexpectedly, citing a memorable experiment in which he had placed some electrodes on the optic nerve of a toad. ‘The room was nearly dark and I was puzzled to hear repeated noises in the loudspeaker attached to the amplifier, noises indicating that a great deal of impulse signalling was going on. It was not until I compared the noises with my own movements about the room that I realized that I was in the field of vision of the toad’s eye and that it was signalling what I was doing.’
Chance and Good Fortune1
By the middle of the last century it was appreciated that nerves and muscles transmit information in the form of electrical impulses, but exactly how the nerve impulse was generated and propagated along the nerve fibre was still a mystery.
The pioneering experiments that led to the solution of this problem were carried out using single nerve fibres of the squid, giving this animal a special place in the hearts of physiologists. It was John Zachary Young, a scientific polymath affectionately known as JZ, who discovered that the common squid (Loligo forbesii) has a nerve fibre large enough to be seen with the naked eye. Tall and distinguished, with a thick shock of silver hair and an infectious enthusiasm, JZ was a memorable individual. Every summer he would disappear to Plymouth or Naples to pursue his studies of octopus and squid, and it was while he was there that he first observed the squid mantle is innervated by enormously thick nerve fibres. These giant cells conduct nerve impulses very fast and are responsible for initiating the rapid jet-propelled escape of a squid when confronted by an enemy. They also provided scientists with an invaluable preparation for studying how nerve impulses are generated, and an excellent excuse for spending large amounts of time at the seaside. Two marine laboratories where fresh squid were obtainable became particularly popular: the Marine Biological Laboratories at Plymouth in England and those at Woods Hole on Cape Cod in the USA.
The huge size of the giant squid axon – between a half and one millimetre in diameter – meant that it was possible to insert an electrode inside the axon and for the first time measure the voltage difference between the inside and the outside of the cell. This was achieved by a famous partnership between two young Cambridge scientists, Alan Hodgkin and Andrew Huxley, in early August 1939.2 For Huxley, still a medical student, this was his first taste of research. They carefully dissected out a single giant nerve fibre, hung it vertically from a hook, and threaded a thin silver wire (protected by a glass capillary) down its length, steering it straight down the middle of the axon without touching the sides with the help of a small mirror. A second electrode was placed in the seawater surrounding the axon. This enabled them to measure the voltage difference between the inside and outside of the cell simply by measuring the voltage difference between the two electrodes.
Action potential: showing the negative resting membrane potential and the transient positive overshoot that occurs when the nerve cell fires an impulse.
When they did this, they found that the inside of the resting nerve cell was around 50 millivolts more negative than the outside. This was not entirely unexpected, as a negative resting potential had already been predicted. It is produced by a tiny leak of positively charged potassium ions out of the cell at rest, as described in the previous chapter. The big surprise was when they stimulated the nerve with a
small electric shock and evoked a nerve impulse, for they found this produced a transient reversal in the voltage difference across the membrane such that the inside of the cell became almost 50 millivolts more positive than the outside. This ‘overshoot’ in potential was remarkable for being completely contrary to existing dogma and it necessitated a rethink in how nerves might work.
Hodgkin and Huxley recorded the first action potentials on 5 August 1939 and were tremendously excited about their discovery. They quickly dashed off a brief note to the journal Nature, but with no explanation of their findings. Three weeks later, on 1 September, Hitler marched into Poland, Britain declared war on Germany and the scientists had to abandon their experiments for eight years. It must have been immensely frustrating, but they had little time to brood on it, as they were soon engaged in the more pressing problem of how to win the war.
During the first few months of hostilities, Hodgkin tried to write up their results as a full paper, but he did not get very far as his war duties kept him very busy; and by 1940 ‘the war had gone so disastrously, and the need for centimetric radar was so pressing’ that he ‘lost all interest in neurophysiology’ and worked flat out on radar. This necessitated a considerable investment of time in learning the necessary physics, but it was not without its excitements, for Hodgkin was soon engaged in developing a short-wave airborne radar system for night-fighter aircraft that was capable of detecting enemy bombers in the dark. This involved much in-flight testing of the prototypes in unpressurized aircraft, and the early high-voltage equipment was prone to arc in the rarefied air at altitude, setting fire to the instruments and filling the plane with smoke. Huxley was also kept very busy working on the application of radar to anti-aircraft naval gunnery.
Although the British scientists were unable to continue their experiments during the war, work did not stop in the United States. Kenneth Cole (known as Kacy) and his colleague Howard Curtis also started to record action potentials from squid axons at Woods Hole. Unfortunately, some of their results were misleading. They illustrated their paper with a ‘typical’ recording, but like many scientists they did not show a truly representative trace, but rather the one that they considered their best – and they picked the biggest. But bigger is not always better and unfortunately this recording was flawed. In retrospect, Cole suggested their equipment might have been poorly adjusted because their action potential overshot the zero potential by almost 100 millivolts. This giant action potential did not fit with any known
theory and it held back understanding of how the nerve impulse works for some time: a cautionary tale that serves to remind scientists that a ‘typical record’ really should be typical.
Taming the Axon
After the war was over, Hodgkin and Huxley teamed up again and in 1945 finally wrote up the results of their 1939 experiments in detail. They produced a full-length paper with four possible explanations of their results – all of which were wrong, as they later acknowledged. What was needed was more experiments. However, it was not easy to get the Plymouth laboratories going again as they had been partly demolished in air raids, squid were in short supply and as Hodgkin remarked, he ‘had forgotten much of the technique’. When they were finally able to restart experiments, in 1947, Huxley was on his honeymoon, so Hodgkin enlisted the help of Bernard Katz, a young refugee from Nazi Germany.
During the war years, Hodgkin and Huxley had become convinced that the action potential must be caused by a transient increase in the permeability of the nerve membrane to sodium ions, and Hodgkin was eager to test this idea. To his satisfaction, he found he could record impulses when the axon was bathed in normal seawater but not when the sodium ions in the seawater were replaced with another ion. Thus it seemed that a current carried by sodium ions moving from the external solution into the axon might underlie the overshooting nature of the action potential. Parenthetically, this current is caused by the opening of sodium channels in the axon membrane but at the time no one, including Hodgkin and Huxley, knew that ion channels even existed.
The big problem in understanding precisely how nerves work was that the action potential was all-or-none. Nothing happened until the electrical stimulus exceeded a certain threshold and then everything happened all at once, with the membrane potential suddenly and explosively switching from its resting level to one some 100 millivolts more positive and then rapidly switching back again. What was needed was some way to prevent the stimulus-induced change in membrane potential and make the voltage stand still, so that the currents associated with a potential change imposed by the experimenter could be measured in isolation. This was achieved with the help of an ingenious device known as a voltage clamp. The way it worked was to inject a current that was equal in amplitude but opposite in direction to that which flowed across the membrane. That way the transmembrane potential did not alter, as the membrane current was cancelled out. Furthermore, the magnitude of the current flowing across the membrane was directly proportional to that injected by the voltage clamp, enabling precise measurements of the currents underlying the action potential. It was a brilliant solution to the problem.
The voltage clamp was invented independently by Hodgkin and Katz in Plymouth, and by Cole and George Marmont in Woods Hole. The Americans were more advanced in its development and were the first to conduct experiments with the voltage clamp (a term that Cole disliked), in 1947. Cole informed Hodgkin of their experiments and when Hodgkin visited Cole at Woods Hole in March 1948 they swapped experimental details. Hodgkin quickly realized that Cole’s apparatus was rather better than his own. On his return to England, Hodgkin and Huxley modified their equipment, incorporating improvements on Cole’s method, and in one short month in August 1949 they obtained all the results that were needed to show how nerves work. The secret to their success lay in the sophisticated design of their experiments and an approach quite distinct from that of Cole.
Cole was astonished at the speed of their progress, commenting, ‘Hodgkin and Huxley went ahead with amazing speed [. . .] I had occasional reports from them. But again I did not appreciate the beautiful simplicity of the fundamental concepts and the spectacular detail and successes of [their analyses. . . .] It was only after Hodgkin had sent me drafts of their manuscripts [. . .] that I began to understand what had grown from my simple idea to tame the squid axon.’ The phrasing of the latter sentence, together with his statement that through ‘free exchange of methods and results, they [i.e. his rivals] were able within a year to repeat all my work with very considerable improvements’, contains hints of the inner turmoil that the Cambridge scientists’ success must have generated.
Hodgkin and Huxley’s elegant experiments revealed precisely how the nerve generates an electrical impulse. The action potential is caused by an initial increase in the permeability of the membrane to sodium ions. This is produced by the opening of sodium channels, which allows positively charged sodium ions to rush into the nerve cell and drive the membrane potential positive (depolarization). Less than a millisecond later, the potassium channels open, permitting potassium ions to exit the nerve and return the membrane potential to its resting level (repolarization). Together, these opposing ion fluxes generate a transient change in voltage that constitutes the nerve impulse.
Calculated Progress
Having measured the amplitude and time course of the sodium and potassium currents, Hodgkin and Huxley needed to show that they were sufficient to generate the nerve impulse. They decided to do so by theoretically calculating the expected time course of the action potential, surmising that if it were possible to mathematically simulate the nerve impulse it was a fair bet that only the currents they had recorded were involved. Huxley had to solve the complex mathematical equations involved using a hand-cranked calculator because the Cambridge University computer was ‘off the air’ for six months. Strange as it now seems, the university had only one computer at that time (indeed it was the first electronic one Cambridge had). It took Huxley about three weeks to compute an action potential: times have moved on – it takes my current computer just a few seconds to run the same simulation. What is perhaps equally remarkable is that we often still use the equations Hodgkin and Huxley formulated to describe the nerve impulse.
Three years after finishing their experiments, in 1952, Hodgkin and Huxley published their studies in a landmark series of five papers that transformed forever our ideas about how nerves work. The long time between completing their experiments and publication seems extraordinary to present-day scientists, who would be terrified of being scooped by their rivals. Not so in the 1950s – Huxley told me, ‘It never even entered my head.’ In 1963, Hodgkin and Huxley were awarded the Nobel Prize. Deservedly so, for they got such beautiful results and analysed them so precisely that they revolutionized the field and provided the foundations for modern neuroscience.
The Scramble for Squid
Hodgkin and Huxley’s experiments generated much excitement and led to an annual migration of scientists to the marine laboratories at Plymouth and Woods Hole. As squid are migratory and the scientists had teaching duties this inevitably turned into ‘summer camp for scientists’, and – particularly at Woods Hole – led to a hothouse of experiments and ideas. Squid were in short supply and the best squid were keenly fought for so that a pecking order quickly developed. By the mid-1960s the hectic scramble for squid was so considerable it motivated some scientists to find a place to work in the winter, and Montemar, near Valparaiso in Chile, provided the perfect place. There was an added bonus, as the Chilean squid – and their axons – are much larger.
Although many other cell types are used to investigate the mechanism of the nerve impulse today, including mammalian brain cells, the squid axon remains a valuable preparation for scientific study. In Plymouth during the 1940s the few squid caught were so mangled by the trawlers’ nets that they did not survive long once they arrived back at the lab, which meant that experiments had to be conducted immediately. Because the boats did not return until late afternoon, this usually meant working throughout the night. Consequently, Hodgkin and Huxley spent their mornings catching up on sleep and planning experiments. This was also true when I visited Woods Hole in the 1980s, when many scientists crawled into bed around 4 a.m. after a hard night at the bench. In Chile today, many squid are caught by rod and line and consequently suffer less damage. But their huge size means they are less easy to keep in holding tanks, so scientists still face the night shift.
I vividly recall from the time I spe
nt at Woods Hole that the axons which generated the best results were commemorated in a most singular fashion. At the end of the experiment, they were flicked onto the ceiling of the laboratory, which eventually acquired a pattern of dried-out squiggles, somewhat reminiscent of a Jackson Pollock painting. Only the very best axons, however, were ‘sent to Heaven’.
Fire!
Sodium and potassium channels that open in response to changes in the voltage gradient across the cell membrane are the keystone of electrical signalling in our brain, heart and muscle. In resting nerve cells, both kinds of channel are tightly shut. When the nerve is stimulated, first the sodium channels and then, with a short delay, the potassium channels swing into action producing a transient change in membrane potential – the nerve impulse. But what triggers the whole thing off?
Crucially, the sodium and potassium channels that are involved in the action potential are sensitive to voltage and they open if the membrane potential is made more positive (depolarized). This is exactly what happens when a nerve cell is excited by an incoming signal from another nerve cell, or by an externally applied electric shock. The larger the change in membrane potential this produces, the more sodium channels open and the more sodium ions flood into the cell. You may recall that Ohm’s law dictates that a change in current will produce a concomitant change in voltage. In a nerve cell the sodium current drives the voltage more positive, which opens more sodium channels, which makes the membrane more positive, which opens more channels, and so on and so on in a positive feedback cycle. This explains the explosive, all-or-none nature of the action potential.
The Spark of Life: Electricity in the Human Body Page 7