The Spark of Life: Electricity in the Human Body (6 page)

The transmembrane sodium and potassium gradients are maintained by a minute molecular motor, known as the sodium pump, that spans the cell membrane. This protein pumps out excess sodium ions that leak into the cell and exchanges them for potassium ions. If the pump fails, the ion concentration gradients gradually run down and when they have collapsed completely no electrical impulses can be generated, in the same way that a flat battery cannot start your car. Consequently, your sense organs, nerves, muscles – indeed all your cells – simply grind to a halt. This is what happens when we die. As we no longer have the energy to power the sodium pump and maintain the ion differences across our cell membranes, our cells soon cease to function. And while externally applied electric shocks can interfere with the electrical impulses in our nerve and muscle cells, they cannot restore the ion concentration gradients across our cell membranes once they have collapsed. This, then, is why we cannot reanimate a corpse with electricity, and why the spark of life is different from the electricity supplied to our homes.

Maintaining the ion gradients is expensive, for electricity does not come cheap, even when we produce it ourselves. It is extraordinary to think that about a third of the oxygen we breathe and half of the food we eat is used to maintain the ion concentration gradients across our cell membranes. The brain alone uses about 10 per cent of the oxygen you breathe to drive the sodium pump and keep your nerve cell batteries charged. Perhaps surprisingly, it seems that merely thinking is energetically expensive.

The Precious Bodily Fluids

How our cells come to be filled with potassium ions is something of a puzzle. The simplest explanation is that the first cells evolved in a solution high in potassium. Left to themselves, lipids spontaneously organize into liposomes, tiny fluid-filled spheres enclosed by a single skin of lipids. Such lipid films may have been the origin of the first membranes and the liposomes they gave rise to may have formed the precursors to real cells. Over three and a half billion years ago, we may imagine, liposomes engulfed self-replicating molecules such as RNA or DNA
¹
and so gave rise to the very first cells.

The fluid enclosed within these first primitive cells would of necessity be the same as that which surrounded them. Thus the high internal potassium concentration characteristic of all cells – from the simplest bacterium to the most complex organism – may reflect the composition of the ancestral soup. This leaves a mystery. Where were those ancient waters rich in potassium? Currently, one popular view is that life evolved in the black smokers of the ocean floor, the hydrothermal vents that belch out superheated water rich in minerals. From a physiologist’s point of view, however, this seems rather unlikely, for the Precambrian seas were high in sodium, like those of today. Thus, I side with Charles Darwin, who suggested life evolved in a ‘warm little pond’; aeons ago, shallow puddles in which organic molecules could be concentrated, and into which potassium ions leached from the surrounding rocks or clays, may have been the birthplace of the first cells.

At some point in the far distant past, single cells discovered that living together gave them a selective advantage and the first multicellular organisms were born. Because the extracellular solution that bathes our cells is high in sodium, it is likely that such early multicellular organisms evolved in the sea, which is largely a solution of sodium chloride (common salt). It is a fascinating idea that the solutions inside our cells, and those that make up our extracellular fluids, provide a fingerprint to our past history and help chart where life first evolved.

Border Control

The presence of a cell membrane brought numerous advantages. Molecules no longer diffused away from each other at random, but could be retained in close proximity within the cell and, more importantly, interact with one another. Cells could become specialized for different functions – evolving into muscle, liver and nerve cells, to name but a few. Like the walls of a mediaeval city, the membrane also protected the cell from toxins in its immediate environment and restricted substances from entering and leaving, because the lipids of which it is composed are impermeable to most substances. As a consequence, tightly guarded gates that enabled vital nutrients and waste products to enter and leave the cell became a necessity.

These gates are highly specialized transport proteins. They come in many different varieties, but some of the most important are the ion channels. As Primo Levi once said, ‘everyone knows what a channel is: it forces water to flow from a source to an outlet between two basically insuperable banks.’ However, the term lends itself equally well to describing other types of conduits, including those which facilitate the flow of ions across the cell membrane. In essence, an ion channel is no more than a tiny protein pore. It has a central hole through which the ions move, and one or more gates that can be opened and closed as required to regulate ion movements. When the gate is open, ions such as sodium and potassium swarm through the pore, into or out of the cell, at a rate of over a million ions a second. Conversely, when the gate is closed, ion flux is prevented.

The very largest ion channels are simply giant holes, so big that many ions can go through at a time, and both negatively charged ions (anions) and positively charged ions (cations) can permeate, as well as quite large molecules. This type of channel is rather uncommon and it is easy to see why – all those ion concentration gradients that the cell sets up and protects so carefully would immediately be dissipated if such a channel were to open, causing the cell to die. Indeed, some bacterial toxins kill cells in exactly this way. Most channels, however, are choosy about the ions they allow to pass through their pores. Although some permit access to all cations (and others to all anions), the majority are far more discriminatory. A potassium channel, for example, will only let potassium ions through and excludes sodium and calcium ions, whereas a sodium channel allows sodium to permeate, but not potassium or calcium. As must by now be obvious, channels are generally called after the ion they most favour.

An Electrochemical Battle for Potassium

Under resting conditions all cells have a voltage difference across their membrane, the inside of the cell usually being between 60 and 90 millivolts more negative than the outside. This resting potential arises because of a tug of war between the concentration and electrical gradients across the cell membrane that the potassium ion experiences.

At rest, many potassium channels are open in the cell membrane. As potassium ions are high inside the cell but low outside, they rush out of the cell down their concentration gradient and, because potassium ions carry a positive charge, their exodus leads to a loss of positive charge – or, to put it another way, the inside of the cell becomes gradually more negative. At some point, the exit of potassium ions is impeded by the increasing negative charge within the cell, which exerts an attractive force on the potassium ion that counteracts further movement. The membrane potential at which the chemical force driving the potassium ions out of the cell and the electrical force holding them back exactly balance one another is known as the equilibrium potential.

If the membrane were only permeable to potassium ions, the resting membrane potential would be exactly the same as the potassium equilibrium potential. However, the real world is not so simple and in most cells a few other types of ion channel are open that allow positively charged ions to sneak into the cell, pushing the resting potential to a more positive level.

The importance of the resting potential is that it acts like a tiny battery in which electric charge (in the form of ion gradients) is separated by the insulating properties of the lipid membrane. This stored energy is used to power the electrical impulses of our nerve and muscle fibres.

Ions take the path of least resistance and move down their concentration gradient from an area of high concentration to one of low concentration. The number of sodium ions is much higher outside the cell than inside, so that sodium ions flood into the cell when the sodium channel gates open. Conversely, as there are many more potassium ions inside than out, potassium ions tend to leave the cell when the potassium channels open. Because ions are charged, their flow produces an electric current. It is such currents, carried by ions surging through ion channels, that underlie all our nerve and muscle impulses, and that regulate the beating of our hearts, the movement of our muscles and the electrical signals in our brains that give rise to our thoughts. This, in essence, is how the energy stored in the concentration gradients is used to power the electrical impulses of our nerve and muscle fibres.

Suck it and See

Given the importance of ion channels, it may seem surprising that their very existence was not dreamt of until the middle of the last century and that even by the early 1970s the idea that ions crossed the membrane via specialized protein pores was still a matter of speculation. To demonstrate their existence directly it was necessary to measure the current that flows through a single channel when it opens. This was far from easy, because the current is extremely tiny and can only be measured with highly specialized electronic equipment. If you consider that the currents flowing through a single ion channel when it is open are about a million millionth of the current needed to power your kettle – a few picoamps only – you will get some idea of just how infinitesimally small they are.

The problem was solved using an elegant technique invented by two German scientists, Erwin Neher and Bert Sakmann, which won them a Nobel Prize. Truly innovative science often arises at the interface of different disciplines and their prize-winning combination of talent illustrates this perfectly. Neher was trained as a physicist and Sakmann in medicine, so they brought complementary skills to the problem; together they provided the breadth of vision to see where this new technology could take them and the attention to detail needed to perfect the method. As their colleague David Colquhoun once said, they are ‘scientist’s scientists’ – modest, unassuming, courageous – and inspirational.

Neher and Sakmann reasoned that if ion channels actually existed, it must be possible to record the tiny currents that flow through them, and in the early 1970s they set out to try. Their idea was to use a fine fluid-filled glass tube as a recording electrode. Because the tip of the tube was very small, when it was gently placed on the surface of the cell it was possible to isolate just one ion channel in the piece of membrane under the electrode tip. The tiny currents that flowed through the channel when it was open could then be detected. The technique is known as the ‘patch-clamp’ method because it records the current flowing though a minute patch of the cell membrane.

Left
. The patch-clamp method showing how the glass electrode can isolate a single channel in a patch of the cell membrane, enabling the tiny currents that flow through the channel when it is open to be detected.
Right
. Single-channel current recording (above). When the channel flickers open (below), the brief current carried by the ions moving through it appears as a downward deflection of the trace. The channel shown below is closed when it binds the intracellular chemical ATP and opens when ATP unbinds.

It was some years before Neher and Sakmann succeeded, however. The difficulty was that they needed highly specialized equipment to amplify the very small signals and these were not only commercially unavailable, they had not even been invented. Thus they had to build the amplifiers themselves. Every time there was a new advance in technology they rebuilt their equipment and tried again. A crucial problem was that the tiny signal they were looking for was buried in the noise. Like the hiss you hear on an old radio, electric circuits (including biological ones) generate electrical noise. Neher and Sakmann tried many tricks to reduce the background noise and finally their perseverance paid off. Around 1974 they started to see single-channel currents emerge from the noise – these blips in the recordings appeared as tiny square pulses of current, generated as ions flowed through the pore each time the channel opened. At first they did not dare to report their results as they only saw currents under the most favourable conditions, but eventually, after a lot of work, they were sufficiently convinced to publish.

Their paper created quite a stir, but it was clear that this was not an easy technique to master, and few people tried to replicate their results immediately. The background noise also continued to be a problem, confounding measurements of small currents. For the next two years the scientists tried to improve the quality of the recordings and they became increasingly frustrated because nothing worked. And then, right out of the blue, they had a tantalizing glimpse of how good their recordings potentially might be. In previous experiments they had occasionally observed a dramatic reduction in noise – so much so that the current trace became a flat line – but, thinking that the tip of the electrode had become clogged up with debris, they immediately terminated the experiment (and inadvertently threw out the baby with the bathwater). However, on very rare occasions, a similar thing occurred when they were recording the usual little noisy blips and they were astonished to see the currents suddenly appear in startling clarity. What had happened, but they did not know it at the time, was that the cell membrane had sealed itself very tightly to the glass electrode, so eliminating almost all background noise. It afforded a step change in the resolution of the recording.