The Spark of Life: Electricity in the Human Body (11 page)

The War of Soups and Sparks

While Dale favoured chemical transmission, the flamboyant Australian neurophysiologist John Carew Eccles was equally certain that communication was electrical, believing that transmission at nerve–nerve synapses was too fast to be chemical. A long-running debate known as the ‘war of the soups and sparks’ began that greatly enlivened the previously rather staid scientific meetings of the Physiological Society. Young, excitable, domineering and extremely energetic, Eccles presented his views with characteristic forcefulness. Dale, a member of the Establishment, and by now both a Fellow of the Royal Society and a Nobel Laureate, adopted a calm magisterial air. Yet Dale and Eccles were essentially shadow boxing. Although their public debates could be tense, highly charged, and, to some observers, quite astonishingly adversarial, their personal relationship was far from acrimonious, as they exchanged friendly letters and shared their results prior to publication. Furthermore, their scientific disagreement provided a valuable incentive for them to seek far more evidence to support their ideas than might otherwise have been the case.

Eccles was impressed by the long time it took for the heart to slow down when the vagal nerve was stimulated. Since this was known from Loewi’s work to involve a chemical transmitter, he inferred that the much faster transmission that took place at the junction between nerve and skeletal muscle could not be chemical and thus must necessarily be electrical. He was outraged at Dale and Feldberg’s suggestion that acetylcholine mediated transmission at the nerve–muscle junction. By 1949, however, sufficient evidence had accumulated for Eccles to concede that transmission at the neuromuscular junction was indeed chemical.

He reserved judgement, however, about what happened at nerve–nerve synapses in the spinal cord and brain, remaining convinced that here, at least, electrical transmission might prevail. The crucial experiment that resolved the debate was carried out late one night in mid-August 1951 in Dunedin, New Zealand, by Eccles and his colleagues Jack Coombs and Lawrence Brock. Eccles claimed it was inspired by his conversations with the philosopher Karl Popper who argued that nothing could be proved in science, only disproved. So Eccles set out to prove that neurotransmission in the central nervous system was not electrical and – to his immense surprise – he succeeded. The breakthrough came because the team used fine glass micropipettes, which they inserted into neurones of the spinal cord to pick up the electrical signals in the post-synaptic cells when the nerve was stimulated. Coombs, an electrical engineer, designed, built and operated the specialist apparatus needed to stimulate and record from the neurones. Eccles had arranged the experiment so that the potential trace would go down if transmission were chemical and would go up if it were electrical. It went down, and Eccles was momentarily stunned – the electrical transmission hypothesis was thereby falsified. It was a dramatic night in other ways too. Coombs’s wife gave birth, her baby girl being delivered by his co-worker Brock (a medic) while Eccles continued to experiment into the early hours of the morning.

Thomas Huxley once described ‘the slaying of a beautiful hypothesis by an ugly fact’ as the great tragedy of science. But Eccles did not mourn the loss of his idea – he wrote immediately to Dale informing him he was now convinced that neurotransmission must be chemical. Dale replied, congratulating Eccles on the beauty of his observations and wryly commenting that ‘your new-found enthusiasm is certainly not going to cause any of us embarrassment’. He later wrote that Eccles’s conversion to the chemical hypothesis was like that of Saul on the road to Damascus, ‘when the sudden light shone and the scales fell from his eyes’. It is one of the great strengths of the scientific method, and a measure of the quality of a scientist, that when the data show conclusively that a favoured hypothesis is wrong, it is quickly abandoned.

Mind the Gap

When the nerve impulse arrives at the end of the axon, it must somehow cause the release of transmitter from the tiny vesicles in which it is stored. Calcium ions play a crucial role in this process. The concentration of calcium ions is more than ten thousand times less inside our cells than outside it, and it is held at this level by molecular pumps that quickly remove any calcium that enters, either by ejecting it from the cell or by storing it in intracellular compartments. One reason calcium is kept so low is that it functions as an intracellular messenger, conveying information about events at the cell membrane to intracellular proteins and organelles. At the nerve terminal, for example, calcium triggers the synaptic vesicles to release the acetylcholine they contain into the gap between the nerve and the muscle.

When a nerve impulse arrives at the nerve terminal, it causes calcium channels to open, allowing calcium ions to flood into the cell. This triggers synaptic vesicles filled with the neurotransmitter acetylcholine to move to, and fuse with, the cell membrane, releasing their contents into the synaptic gap. Acetylcholine then diffuses across the gap and binds to its receptors in the muscle fibre membrane. Binding of the neurotransmitter opens an intrinisc ion channel in the acetylcholine receptor, enabling sodium ions to enter the cell. The flow of sodium current triggers an electrical impulse in the muscle. In this way, the electrical signal passes from nerve to muscle via a chemical intermediary.

Calcium enters the cell via calcium channels in the pre-synaptic nerve membrane that open in response to the voltage change produced by the arrival of the nerve impulse. It is crucial that these channels open only when an impulse arrives and that they only remain open for a brief time; uncontrolled calcium influx can be dangerous as it triggers prolonged release of transmitter. One of the many toxic ingredients of the venom of the deadly black widow spider is alpha-latrotoxin, which inserts itself into the cell membrane, forming calcium-permeable pores that allow calcium ions to flood into the cell in an unregulated fashion and cause massive transmitter release and muscle spasms.

Similarly, increased transmitter release is produced by genetic mutations that prolong the duration of the nerve impulse and so increase calcium influx. People with such mutations may experience periodic attacks of dizziness, uncontrollable muscle shakes and uncoordinated movements so that they find it difficult to walk and lose their balance. They may also vomit. Attacks are often brought on by emotional stress, such as the excitement of watching your favourite football team play. Given the symptoms, it is not surprising that people with this condition are sometimes castigated for being drunk, a fact which the affected individual may find particularly galling if, as has been known, they happen to be teetotal.

On the other hand, inadequate calcium influx means that too few vesicles are encouraged to liberate their contents so that transmitter release is insufficient to trigger muscle contraction. This happens in Lambert Eaton myasthenic syndrome (LEMS), a condition in which the body produces antibodies against the calcium channels at the neuromuscular junction. These antibodies bind to the calcium channels and cause them to be removed from the nerve membrane so that nerve impulses fail to release any transmitter. The result is muscle weakness or paralysis. Most cases of LEMS are actually due to a tumour (usually a lung cancer) elsewhere in the body that possesses a similar type of calcium channel. The immune system’s response to this cancer is to attack it by producing antibodies and those directed against the cancer cells’ calcium channels cross-react with calcium channels present in the nerve terminals. LEMS is thus a red flag, warning the clinician to search for a possible tumour. It can be a valuable sign, for the sooner the lung cancer is treated the better the outcome for the patient.

All Docked Up and Ready to Go

It is sometimes envisaged that the interior of a cell resembles a pea soup, in which chemicals and organelles mill around in a random fashion. This is very far from the truth. Inside the cell everything has its place and is anchored in its correct position by a highly structured protein network called the cytoskeleton. This is particularly evident at the nerve terminal, where vesicles packed with transmitter are released only at specialized sites known as ‘active zones’. Here several vesicles sit docked with the membrane, pretriggered for immediate release as soon as they get the signal to go. The calcium channels sit adjacent to the docking sites, reducing the distance that calcium has to travel once it enters the cell. This helps make transmission very fast. Within a millisecond (a thousandth of a second) of an electrical impulse reaching a nerve terminal, about 30 million molecules of acetylcholine are released. These quickly diffuse across the gap to their receptors on the muscle membrane, where they only remain bound for a couple of milliseconds, so that everything is over in about 20 milliseconds.

A small number of docked vesicles are ‘trigger happy’ and do not wait for a calcium signal: they are spontaneously released at a low frequency (they are too few to cause muscle contraction). Having a system that is all geared up and ready to go ensures that the arrival of a nerve impulse results in very rapid transmitter release – something you may be grateful for when your brain tells your hand to withdraw from a scalding-hot pan handle.

Complex molecular machinery is needed to overcome the enormous energy barrier that normally prevents the membrane of the synaptic vesicle fusing with that of the cell. This includes the numerous proteins that make up the docking and release complex, which act as molecular midwives, facilitating vesicle docking and membrane fusion. Precisely how binding of calcium ions to these proteins triggers the cascade of conformational changes that causes the vesicle and surface membranes to fuse is still unclear. However, inhibiting midwife protein function blocks neuromuscular transmission. Botulinum toxin, for example, prevents transmitter release and muscle contraction by destroying a specific set of these proteins.

Not all synaptic vesicles are primed for release. Most are stored at some distance from the release sites and must move to the membrane before they can be released; they must also undergo maturation processes that ready them for docking and release. Calcium also serves as a signal for mobilizing these troops of vesicles.

Poison Darts

Once liberated from the nerve terminal, acetylcholine diffuses across the tiny gap to the post-synaptic membrane of the muscle fibre, where it interacts with its receptors. Binding of the transmitter causes a conformational change in the acetylcholine receptor that opens an intrinsic ion channel, allowing a simultaneous influx of sodium and efflux of potassium ions. This decreases the voltage difference across the muscle membrane and (if it is sufficiently large) triggers an electrical impulse in the muscle fibre. In this way, acetylcholine serves to link the action potential in the nerve to one in the muscle, and ultimately to muscle contraction.

A large number of drugs and poisons work by interfering with the action of acetylcholine at its muscle receptor. The most famous is curare – the poison used by South American Indians to tip their arrows and the darts used in their blowpipes. Curare blocks binding of acetylcholine to its receptors in the muscle membrane and so prevents the nerve from stimulating the muscle fibre. Consequently, an animal hit by a dart is completely paralysed and falls out of the tree to the ground, where it is either slaughtered or dies from respiratory failure. Fortunately, curare is poorly absorbed by the digestive system, so animals killed in this way are safe to eat.

Curare was once also used in warfare and even the slightest nick with a poisoned arrow could be fatal. It was much feared. In his account of the discovery of Guiana (Guyana), Sir Walter Raleigh wrote that ‘the party shot endureth the most insufferable torment in the world, and abideth a most ugly and lamentable death’. Depending on the dose, the victim can be awake, aware and sensitive to pain, but unable to move or breathe: unless given artificial respiration they will eventually die of respiratory failure. Toxins like curare have been used to tip arrows and spears for hundreds of years and the ancient Greek word toxicon, from which the name toxin derives, meant ‘bow’ or ‘arrow poison’.

Curare can be extracted from many different South American plants, but the best known is the climbing pareira vine
Chondrodendron tomentosum
. The great Prussian explorer Alexander von Humboldt was the first European to describe how it is prepared, in 1800. He noted that the juice from the vine was extracted and mixed with a sticky preparation from another plant to make a thick treacly substance that could be glued to an arrowhead. Some years ago, I asked to view the curare-tipped blow darts owned by the Pitt Rivers Museum in Oxford. I was allowed to see them, but not to handle them or borrow them for a public lecture, for health and safety reasons. I was somewhat indignant, as I was certain the drug must have long since deteriorated. I was wrong: recently, curare that was 112 years old was shown to still be effective. Interestingly, the pure toxin was first isolated from a native preparation of curare held by the British Museum; it had been stored in a bamboo tube and the active alkaloid was hence named tubocurarine (tube-curare).