the spark of life: electricity in the human body. frances ashcroft

7/30/2019 The Spark of Life: Electricity in the Human Body. Frances Ashcroft

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The Spark of Life: Electricity in the Human Body. Frances Ashcroft

Were all familiar with the fact that machines are powered by electricity, but its perhaps not so widelyappreciated that the same is true of ourselves. Your ability to read and understand this page, to see andhear, to think and speak, to move your arms and legs even your sense of self is due to the electrical

events taking place in the nerve cells in your brain and the muscle cells in your limbs. And thatelectrical activity is initiated and regulated by your ion channels. These little-known but cruciallyimportant proteins are found in every cell of our body and in those of every organism on Earth, andthey regulate our lives from the moment of conception until we draw our last breath. Ion channels aretruly the spark of life for they govern every aspect of our behaviour. From the lashing of the spermstail to sexual attraction, the beating of our hearts, the craving for yet another chocolate, and the feel ofthe sun on your skin everything is underpinned by ion channel activity. Not surprisingly, given theirubiquity and functional importance, a multitude of medicinal drugs work by regulating the activity ofthese minute molecular machines, and impaired ion channel function is responsible for many humanand animal diseases. Pigs that shiver themselves to death, a herd of goats that falls over when startled,people with cystic fibrosis, epilepsy, heart arrhythmias or migraines all of us are victims of channeldysfunction.

The science of static electricity starts with the ancient Greeks fascination with amber. It is from theirword for amber, electrum, which derives from elector, meaning the shining one, that we get the wordelectron, and hence electricity.

But amber has another interesting and curious property. When rubbed with wool it generates staticelectricity, causing it to attract light, dry objects like small bits of tissue paper, feathers, specks of wheatchaff, and even your hair.

Amber generates a static charge because it attracts electrons from the atoms of the wool, becomingnegatively charged in the process and leaving the wool positively charged. The charge is transferred by

close contact between the amber and wool the friction produced by rubbing is not involved, it issimply that rubbing greatly increases the area of contact between the two surfaces.Parenthetically, there is nothing static about static electricity. The term refers only to the fact that thepositive and negative electric charges are physically separated. As soon as a positively charged materialcomes close enough to a negatively charged one, current will flow from one to the other as visiblydemonstrated by the leap of an electric spark.

It was William Gilbert, physician to Queen Elizabeth I, who first invented a sensitive instrument formeasuring static electricity (an early electroscope). He used it to compile a long list of materials thatcould be electrified by rubbing. He also distinguished the attractive power of amber from that ofmagnets, arguing that two different phenomena are involved.

There was no way to store static electricity until the invention of the Leyden jar in October 1745 by aGerman cleric, Ewald Jrgen von Kleist.Initially, it was believed that electricity was a fluid, so it seemed natural to use bottles and jars to storeit in, but it was later appreciated that this was not the case and today the Leyden jar has been replacedby the capacitor. This operates on the same principle. It consists of two parallel metal plates separatedby a thin layer of a non-conductive material such as mica, glass or air. The amount of charge acapacitor can store is determined by the area of the plates and the distance between them, and it can beconsiderable. The first atom smasher, built in the 1930s at Cambridge University by John Cockcroftand Ernest Walton, used banks of capacitors to generate and store up to almost a million volts.


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The electricity supplied to our homes is carried by electrons. These indivisible subatomic particlescarry a negative electric charge and because opposite charges attract one another (and similar chargesrepel) electrons always flow from a region of negative to positive charge. Confusingly, we definecurrent as the direction of flow of positive charges, which means that the current in a wire moves in theopposite direction to that in which the electrons flow! In contrast, almost all currents in the animalkingdom are carried by ions electrically charged atoms. There are five main ions that carry currents in

our bodies. Four are positively charged sodium, potassium, calcium and hydrogen (protons) andone, chloride, is negatively chacged. Because they are electrically charged, the movement of ionscreates an electric current. In the case of positively charged ions, the current flow is in the samedirection as the flow of ions, whereas for negatively charged ions (as for electrons) it is in the oppositedirection.The solutions inside our cells, and those of all other organisms on Earth, are high in potassium ions andlow in sodium ions. In contrast, blood and the extracellular fluids that bathe our cells are low inpotassium but high in sodium ions. These ionic differences are exploited to generate the electricalimpulses in our nerve and muscle cells.It is these ion movements that give rise to our nerve and muscle impulses.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 ionsthat leak into the cell and exchanges them for potassium ions. If the pump fails, the ion concentrationgradients gradually run down and when they have collapsed completely no electrical impulses can begenerated, 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. Aswe no longer have the energy to power the sodium pump and maintain the ion differences across ourcell membranes, our cells soon cease to function. And while externally applied electric shocks caninterfere with the electrical impulses in our nerve and muscle cells, they cannot restore the ionconcentration gradients across our cell membranes once they have collapsed. This, then, is why wecannot reanimate a corpse with electricity, and why the spark of life is different from the electricitysupplied to our homes.It is extraordinary to think that about a third of the oxygen we breathe and half of the food we eat isused to maintain the ion concentration gradients across our cell membranes. The brain alone uses about10 per cent of the oxygen you breathe to drive the sodium pump and keep your nerve cell batteriescharged.Ions take the path of least resistance and move down their concentration gradient from an area of highconcentration to one of low concentration. The number of sodium ions is much higher outside the cellthan inside, so that sodium ions flood into the cell when the sodium channel gates open. Conversely, asthere are many more potassium ions inside than out, potassium ions tend to leave the cell when thepotassium channels open. Because ions are charged, their flow produces an electric current. It is suchcurrents, 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 inour brains that give rise to our thoughts. This, in essence, is how the energy stored in the concentrationgradients is used to power the electrical impulses of our nerve and muscle fibres.Voltage-dependent gating requires that the channel is able to sense a change in the voltage field acrossthe membrane. All cells have a potential difference across their membranes, the inside of the cell beingabout 70 millivolts more negative than the outside. When a nerve fires an electrical impulse thispotential suddenly alters by about 100 millivolts, the insideof the cell briefly becoming positive withrespect to the outside.In resting nerve and muscle cells, the voltage-gated sodium and potassium channels are held firmlyshut by the negative membrane potential. They open only when the membrane potential becomes morepositive and when this happens it triggers an electrical impulse.


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Unlike nerve fibres, muscle fibres have a high density of chloride channels, and in normal muscle theflow of chloride ions across the membrane dampens down electrical excitability, ensuring that a singlenerve impulse produces only a single muscle twich.It has been known for centuries that the heart has an intrinsic rhythm and can continue to beat when itis removed from the living animal. One of the first to describe the phenomenon was the great Romanphysician Galen, and subsequently many others, including Leonardo da Vinci, reported that the heart

moves by itself. William Harvey even showed that when the heart of an eel was cut into ever-smallerparts each individual piece continued to pulsate.As in the case of nerve cells, ion channels are responsible for the electrical impulses of heart cells.However, more types of channel are involved in shaping the action potential of the heart. It is initiatedby the opening of sodium channels. These channels are similar, but not identical to those of nerve cells,which explains why fatal poisons like that of the puffer fish block electrical impulses in the nerves, butnot the heart.Opening of the sodium channel pores is quickly followed by the opening of calcium channels, whichenables calcium ions to flood into the cell, where they trigger the release of stored calcium and therebycontraction.The end of the cardiac action potential is produced by opening of potassium channels, and the resultingefflux of potassium ions returns the voltage gradient across the membrane to its resting value. As aconsequence, the calcium channels shut, preventing calcium influx, so that the heart relaxes. Unlikethose of nerve cells, many cardiac potassium channels take a long time to open, which helps ensure thatthe duration of the action potential in the heart is much longer.

All of us have experienced the speeding of the heart when we are excited or afraid, and the thumpingbeat that makes us feel as if our heart is about to burst. This is caused by the fight or flight hormoneadrenaline, which primes the body to cope with an adverse situation by increasing both the rate and theforce of contraction. It does so by opening additional calcium channels in heart cell membranes. Thisspeeds up the rate at which the sinus node cells fire, so that the heart rate is increased, and it also booststhe amount of calcium that is released from the intracellular stores and thereby enhances the strength ofcontraction. Adrenaline is made by the adrenal glands that lie just above the kidney, and is secreted into

the bloodstream in response to stress or exercise; a related substance with a similar action,noradrenaline, is released from nerves that innervate the heart.Long ago, before you were born, you had webbed hands and feet like those of a duck. As youdeveloped inside your mothers womb, the cells that made up the web of soft tissue between your digitswere killed off in a process known as programmed cell death (or apoptosis) so that you ended up withseparate fingers and toes. If this process of body sculpting fails, as occasionally happens, you end upborn with webbed fingers. Everyone who has kept tadpoles has seen such cell suicide in action for thegradual disappearance of the tadpoles tail as it develops into a baby frog occurs by apoptosis andreabsorption of the dying cells. Similarly, apoptosis is drawn to the attention of a woman every month,for the sloughing off of the lining of the womb that occurs at the start of her period is also the result ofprogrammed cell death. Perhaps most important of all, cell suicide plays a key role in the development

of the nervous system and in how your brain is wired up. Early in development, many nerve cells areborn and send forth their axons towards their destination in an exploratory manner. If they find theircorrect targets, a tentative connection is established, impulses speed excitedly down the lines, chemicalkisses are exchanged, and the link is cemented. Nerve cells whose axons fail to find their correct targetsproduce more feeble impulse activity and simply wither away through lack of use. Many die duringbrain development and without such cell suicide the brain could not function correctly. Apoptosis isalso a way to ensure that damaged cells that might threaten an organisms survival are eliminated. Yourimmune system can kill cells infected with viruses this way, and cells whose DNA is damaged areencouraged to commit suicide to prevent cancers forming.


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There are several ways in which a cell can self-destruct but, as you have probably guessed, one of themis mediated by an ion channel. It also involves the mitochondria, tiny intracellular organelles, about thesize of a bacterium, that are found in almost every cell of your body. The ancestors of mitochondriawere once free-living entities, rather similar to the blue-green algae (the cyanobacteria) that form thefamiliar green scum on lakes in hot summers, but around two billion years ago these ancestralmitochondria gave up the solitary life and became incorporated within early cells. Thus like the Star

Trek aliens known as the Trill, we live our lives in partnership with another organism but this is noscience fiction and our symbionts are microscopic. Almost all plant and animal cells containmitochondria and they are essential for life: without them, multicellular organisms could not function,as mitochondria act as molecular furnaces where fuels such as sugar and fats are burned with oxygen toproduce chemical energy. Cells that require a lot of energy, like muscle cells, have large numbers ofmitochondria.Unlike animal cells, the cells of most terrestrial plants are not bathed in a salty extracellular fluid. Ionsare present at very low levels in plant cell walls and thus an influx of sodium ions would not be a viablemeans of producing an action potential. Instead, plants must rely on chloride efflux.So the reason chilli peppers taste so hot is that they open the same ion channel as high temperature andbecause the brain cannot tell the difference between the two stimuli it interprets them both as heat.These channels are not just found in the tongue, they are also present in the skin of your fingertips, faceand other sensitive parts of the body.The minty, fresh taste of menthol, found in peppermint oil, arises from the fact it activates an ionchannel that detects cold temperatures. This channel is structurally very similar to the capsaicinreceptor and in fact we now know that there is a whole family of such channels, called TRP channels,each of which detects a different shade of temperature. Many of these channels are also sensitive to arange of pungent or painful chemicals not just capsaicin, but substances such as wasabi (the hotJapanese horseradish), mustard, garlic and camphor.A common theme in the nervous system is that the response to a continuous stimulus graduallyweakens. We are pre-programmed to respond most strongly to changes in our environment and cease topay attention if nothing new happens, a phenomenon that has a clear evolutionary advantage.Some unfortunate children have intractable epilepsy that is unresponsive to drug therapy and involvesparts of the brain inaccessible to surgery. An old treatment that is surprisingly effective in some of thesepatients is to severely restrict their consumption of carbohydrates. Known as the ketogenic diet becauseit leads to the rise of metabolic by-products known as ketone bodies in the blood, it stops most seizuresin about a third of patients and reduces their frequency in a further third.Mammals that live in the sea would drown if they fell sleep underwater, so they rest half of their brainat a time, with one side remaining awake while the other is deeply asleep. So too do many birds, whichoften sleep away the night with one eye open, keeping watch for predators.Quite why we sleep is still something of a mystery, but there is evidence that one reason is that it isimportant for memory consolidation. As you will no doubt already know from experience, withoutadequate sleep our ability to remember things diminishes. Strikingly, even a short nap can help withlearning a new task.It is not impossible to control another creatures behaviour simply by stimulating the correct bit of thebrain. Jos Manuel Rodriguez Delgado was sufficiently confident of this idea that in 1963 he steppedinto a bullring in Cordoba in front of an aggressive fighting bull. As it charged towards him, Delgadostood his ground and calmly twiddled a button on a remote control device that sent a signal to atransceiver connected to an electrode implanted in the animals brain. Electrical stimulation of thecaudate nucleus stopped the bull in its tracks: it skidded to a halt within a few feet of the scientist.

the spark of life: electricity in the human body. frances ashcroft

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