I am a brain, Watson. The rest of me is a mere appendix.
SHERLOCK HOLMES 1
Brain against brute force – and brain came out on the top – as it’s bound to do.
TOAD OF TOAD HALL 2
One of the most profound questions in science is: why is the Universe constructed in such a way that it acquires the ability to become curious about itself? The question presupposes the existence of an objective Universe out there. Yet everything we know about reality, including our model of the Universe, is a construct of the human brain. ‘The brain’, as poet Emily Dickinson wrote, ‘is wider than the sky.’ Before we can truly address any of the really deep questions about the Universe, we first need to understand the filter through which we perceive that Universe.
Captain James T. Kirk of the starship Enterprise called space ‘the final frontier’. But he was mistaken. It is not space that is the final frontier. It is the human brain: the ultimate piece of ‘matter with curiosity’.
Our brain – ‘the apparatus with which we think that we think’3 – processes information from our senses, using it to update its internal model of the world. It then decides, on the basis of that information, what action to take. The brain is responsible for art and science and language and laughter and moral judgements and rational thought, not to mention personality, memories, movements and how we sense the world. ‘It is in the brain that the poppy is red, that the apple is odorous, that the skylark sings,’ wrote Oscar Wilde.4
Not bad for a chunk of unprepossessing matter with the consistency of cold porridge. The question is: how did something as complex and amazing come about? The answer is inextricably bound up with the origin of the nervous system – and with the harnessing of lightning.
In the beginning, there were simple bacteria – microscopic bags of gloop with the complexity of small cities. They faced a serious problem: how to orchestrate their internal ‘factories’ to make the micro-machinery of life – the Swiss-army-knife molecules known as proteins. The solution they hit on was to release molecules such as glutamate, which diffused throughout their liquid interiors. When such a chemical messenger docked with a molecular receptor – fitting into a cavity like a key into a lock – it triggered the cascade of chemical reactions needed to make a protein.
After almost 3 billion years stalled at the single-cell stage, life made the giant leap to multicellular organisms. But it continued to use its ancient, tried-and-tested system of internal communication. Take sponges, for instance. These colonies of cells pulse in synchrony in order to pump food-laden water through channels in their bodies. Sponge cells achieve this feat of coordination by detecting chemical messengers such as glutamate, which are released by other sponge cells. It is nothing more than what happens inside a single bacterium writ large. If it ain’t broke, don’t change it, as far as nature is concerned.5
The chemical messengers of a sponge take many seconds to diffuse to all of its cells and trigger a response. This is acceptable for a creature living in surroundings that are constant and predictable. However, in a rapidly changing environment, where a quick response to threats is essential for survival, a faster method of internal communication is imperative. Such a means is provided by electricity.
Remarkably, electricity is as ancient a feature of cells as chemical messengers. Cellular membranes are leaky and prone to let through dangerous charged atoms such as the sodium in salt.6 In order to survive, bacteria needed a way to pump out such ions. They solved the problem with the aid of tunnel-like proteins called ion channels, which span the cell membrane and can open and shut to expel ions. But, inevitably, pumping ions through such a channel creates an imbalance of electric charge between the inside and the outside of the cell. It is this voltage difference that provides a cell with a nifty communication opportunity.7
To send a super-fast signal, a cell needs only to manipulate the voltage across its membrane, which it can do simply by pumping ions rapidly through an ion channel. This causes an abrupt change in the voltage across the membrane, which has a knock-on effect on the next ion channel, and the next, and so on. Like a microscopic Mexican wave, an electrical signal propagates along the membrane, thousands of times faster than any chemical messenger, literally at lightning speed.
Of course, a communication system based on electricity – a true cellular telephone system – needs a means not only of transmitting a signal but a means of detecting it at its destination and doing something useful with it. Cells have this covered too. A type of channel known as a voltage-gated ion channel can open in response to an electrical signal, allowing ions such as calcium to pass through the membrane. These then trigger a cascade of cellular processes, effectively turning the incoming electrical signal back into a bog-standard chemical messenger, which can do something useful such as trigger the building of a protein.
Voltage-gated ion channels, just like regular ion channels, are present in bacteria. Cells that use them for internal communication simply borrowed them and adapted them to the new and specialised task.
An internal cellular telephone system was in existence even before the first multicellular animals. In fact, it can be seen in action in a water-living, single-celled creature called Paramecium. When Paramecium is swimming along and bumps into an obstacle, a voltage is created across its membrane. This causes a Mexican wave of ions to pulse around its body. Lightning fast, the wave reaches hair-like extensions on the surface of the cell, which, when they ripple in synchrony, can propel the cell. Instantly, these cilia reverse their beating, causing Paramecium to back away from the obstacle.
A useful trick for a single-celled creature such as Paramecium turns out to be indispensable for a multicellular organism. After all, as creatures grew ever larger, it became likely that the place on their bodies where they sensed a dangerous touch was a long way from the place where a muscle had to be contracted in response. Sending a signal via a chemical messenger was far too slow. Long before an animal could take evasive action, it might be eaten. Electricity was the only solution. And nature responded by creating a specialised electrical cell – the nerve cell.
A nerve cell has a cell body with a nucleus like a normal cell. But there the similarity ends. One side of the cell is extended like a long, thin wire, while the other side sports a number of finger-like extensions. The long, thin wire, known as an axon, transmits an electrical pulse to another nerve cell, whereas the finger-like extensions, known as a dendrites, receive electrical signals from the axons of other nerve cells.
Crucially, the axon of one nerve cell does not touch the dendrite of another. There is a gap – known as the synapse. Here, the electrical signal from the axon is converted into chemical messengers.8 These diffuse across the gap and dock with receptors, which open ion channels and thus trigger a new electrical signal. Sound familiar? It is the very same molecular lock-and-key system that bacteria came up with almost 4 billion years ago. Life, far from discarding its ancient and sluggish communication system, mediated by chemical messengers, integrates it into its super-fast and modern communication system, mediated by electricity.
The mediation of the electrical signal by chemical messengers is not just an unfortunate hangover from the beginning of life. It makes it possible for an almost infinite array of responses from a nerve cell. This is because there are a host of different chemical messengers, or neurotransmitters, each of which has an effect on a dendrite if and only if the dendrite possesses a receptor for it. Some trigger, or excite, an electrical current in the dendrite whereas others prevent, or inhibit, a current.
The two most important neurotransmitters in the human brain are glutamate – the fossil relic of the system of chemical messengers used by bacteria billions of years ago – and gamma-aminobutyric acid, or GABA. Virtually all communication between nerve cells, or neurons, in the brain is mediated by these two simple amino acids. Other neurotransmitters such as dopamine and acetylcholine merely moderate their action. Most drugs that affect behaviour work by blocking or mimicking a particular neurotransmitter, thus stimulating a receptor site and generating the same effect as the neurotransmitter. For example, lysergic acid diethylamide, or LSD, a mere speck of which causes dreamlike psychedelic hallucinations, has a chemical structure very similar to the neurotransmitter serotonin.
Because a nerve cell has extensions capable of both sending and receiving electrical signals, it can join together with others in a network, with each nerve cell connected via its dendrites to the axons of many other nerve cells. Such a network can behave in a complex way.
Even a single nerve cell can exhibit memory. Say an electrical signal from a sense – perhaps touch – comes in along a dendrite and triggers the nerve cell to send a signal along its axon to contract a muscle. If, in addition to going to the muscle, the axon splits and part of its signal feeds back into a dendrite of the nerve cell, it triggers contraction again. And again. And again. A nerve cell can refire about every hundredth of a second. In this way, the nerve cell remembers the stimulus. If four nerve cells are connected together, they can exhibit complicated behaviour such as contracting a muscle to move away from either a stimulus on the left side or on the right side of an animal. This gives some hint of the complex behaviour possible if nerve cells connect together not in quartets but hundreds or thousands or even hundreds of billions.
The earliest nerve cells, though connected to each other, were also connected to the external world – receiving an input signal directly from senses or providing an output signal to, for instance, contract a muscle. There was no computation in between. However, at some stage in the history of life, nerve cells began to connect only to other nerve cells. This enabled such neurons to process the input information from the environment in new and complex ways in order to decide on an appropriate response. It was an epochal moment in the history of life. It marked the birth of the brain.
‘Basically there are two types of animals,’ says Columbian neuroscientist Rodolfo R. Llinás. ‘Animals, and animals that have no brains; they are called plants. They don’t need a nervous system because they don’t move actively, they don’t pull up their roots and run in a forest fire! Anything that moves actively requires a nervous system; otherwise it would come to a quick death.’9
A neuron is often likened to a logic gate of a computer.10 A logic gate, built from transistors, can be wired together with other logic gates to create a circuit that, for instance, adds together two numbers. But, whereas a logic gate has only two electrical inputs and spits out a signal that depends on the current flowing in those two inputs, a neuron can have 10,000 or more dendritic inputs, and spit out a signal that depends on the complex interplay of all those electrical inputs on numerous neurotransmitters and receptors at the nerve cell’s synapse. So, although it is true that a neuron is the fundamental building block of a biological computer, just as a logic gate is the basic building block of a silicon computer, it is more than this. A neuron is a computer in its own right.
Brains, built of neurons, are expensive to run. The human brain accounts for a mere 2–3 per cent of the mass of an adult yet guzzles about a fifth of the body’s energy when resting.11 Having said this, the brain does all of its mega-computation on roughly 20 watts of power, the equivalent of a very dim light bulb. By comparison, a supercomputer capable of an analogous rate of computation requires 200,000 watts – it is 10,000 times less energy-efficient than the brain.
For some creatures, however, the energy expense of running a brain is simply too great. The juvenile sea squirt has a rudimentary nervous system that enables it to wander through the sea searching for a suitable rock or hunk of coral to cling to and to make its home. ‘When it finds its spot and takes root, it doesn’t need its brain any more,’ says American cognitive scientist Daniel Dennett. ‘So it eats it!’12
Despite this rather disturbing example of autocannibalism, the benefits of having even a simple brain usually appear to outweigh the costs. For instance, the nematode worm, Caenorhabditis elegans, has a brain with a mere 302 neurons – so few that its brain is completely encoded in its DNA. The nematode worm, unlike the sea squirt, does not eat its brain. It must therefore provide the worm with an important competitive advantage.13
The human brain weighs about three pounds and has about 100 billion neurons – by sheer coincidence, roughly the same number of stars in our Galaxy, galaxies in our Universe, and people who have ever lived. ‘The human brain is the most complex object known in the Universe,’ says Edward O. Wilson, ‘known, that is, to itself.’14 According to a theory developed by American neuroscientist Paul MacLean, in the course of evolution three distinct brains have emerged, accreting one on the other. ‘With modern parts atop old ones, the brain is like an iPod built around an eight-track cassette player,’ says American journalist Sharon Begley.15
The oldest and most primitive part of our three-pound universe includes the brainstem and the cerebellum, which turn out to be the main structures in the brain of a reptile. Our ‘reptilian brain’ controls vital automatic functions such as body temperature, breathing, heart rate and balance. Wrapped around the reptilian brain is a structure that developed in the first mammals about 200 million years ago. The main parts of this limbic system are the hippocampus, amygdala and hypothalamus. They record memories of good and bad experiences and so are responsible for emotions. Wrapped around the limbic brain is the largest structure of all, which first became important in primates. This cerebrum, or neocortex, can overrule the knee-jerk responses of the more primitive parts of the brain. It is responsible for language, abstract thought, imagination and consciousness. It has an almost boundless ability to learn new things and it is the seat of our personality. In short, the neocortex is what makes us human.
Actually, there is one more layer wrapped around the reptilian brain, limbic system and neocortex – and that is, of course, the hard bony shell of the skull. ‘Because important things go in a case, you got a plastic sleeve for your comb, a wallet for your money and a skull for your brain,’ observed George Costanza in Seinfeld.16 The skull is actually reinforced by three layers of protective tissue known as the meninges, in between which is a special shock-proof liquid known as cerebrovascular fluid. An infection here causes the potentially fatal inflammation known as meningitis.
The neocortex is divided into two hemispheres, connected by a bundle of nerve fibres called the corpus calossum. In effect, therefore, we have two brains. Usually, the left side is better at problem solving, maths and writing while the right side is creative and better at art or music. For reasons that are not completely understood, the left side of the brain controls the movement of the right side of the body and vice versa. This is why people who suffer a stroke in the left side of their brain lose movement on the right side of their body and vice versa. A stroke is usually caused by a blood clot in the brain that blocks the local blood supply, damaging or destroying nearby brain tissue.
But the wonder of the brain is not in its gross structure but in its microstructure – in its 100 billion or so neurons and 1,000 billion other support cells, which surround the neurons and their axons, providing them with energy and generally keeping them healthy.17 However, the sheer number of neurons reveals little about the operation of the brain. ‘The liver probably contains 100 million cells,’ says American neuroscientist Gerald D. Fischbach. ‘But 1,000 livers do not add up to a rich inner life.’18
The key to the brain’s amazing capabilities are the connections between its neurons. ‘All that we know, all that we are, comes from the way our neurons are connected,’ says Tim Berners-Lee, inventor of the World Wide Web.19 A single neuron may posses 10,000 or so dendrites through which it can interact with 10,000 or so other neurons. In total, the brain may contain something like 1,000 trillion connections.
The big question is: how does all this mind-bogglingly complex neuronal circuitry allow us to remember things and to learn things?
The common experience of memory is that we remember things that are important to us and forget things that are no longer important to us. Of course, we all forget the occasional important thing, like where we put down a book we were reading or a shopping list scrawled on a scrap of paper. But, by and large, we remember and learn things if they are significant to us – that is, connected to things we already know. If you hear a new word in French and you already speak French, you are far more likely to remember it than if you do not speak French. If you know how to balance on a skateboard, you will learn how to balance on a surfboard more easily than someone who has never used a skateboard.
In addition to this, repetition seems to be important to remember and learn things. Babies learning to speak repeat the same words over and over. Children learn times tables by reciting them over and over again until they are finally drummed into their skulls. People learning the guitar strum the same sequence of chords, hour after hour.
None of this, of course, tells us how the brain’s neuronal circuitry enables us to remember things and learn new skills. But it does hint that two crucial processes in the brain are making connections with things we already know and repetition.
The things we already know are encoded in the pattern of connections between the brain’s 100 billion neurons, just as the knowledge of how to contract a muscle to move away from a stimulus in the four-neuron network mentioned earlier was encoded in the connection between the quartet of neurons. Nobody knows exactly how the pattern encodes complex information. Although it is perfectly possible to point to a bunch of magnetic memory domains in a computer and say, ‘That is storing a 6 or the letter P’, it is not yet possible to point to a bunch of interconnected neurons in the brain and say they are storing the smell of newly baked bread or the knowledge of how to balance on one leg. Nevertheless, all the evidence points to the pattern of connections between neurons being key to what we know.
The connections between neurons are made by dendrites. Dendrites are therefore synonymous with what we know. To remember something or learn a new skill, therefore, something must happen to the dendritic connections between neurons.
Imagine two neurons that are connected – the axon of the first attached to a dendrite of the second. Now imagine that the first neuron starts firing because it is receiving some stimulus – perhaps some sensory information from the outside world. Remember, the dendritic connection between the two neurons represents something we already know.
Now, if the stimulus is repetitive and related to what we know – and the neurotransmitters in the synaptic gap between the axon and the dendrite are primed to amplify the electrical signal if it is related – the dendrite strengthens its connection. This can happen in many ways, but one way is for the dendrite to grow a large number of spines that multiply its connection points.
Of course, two neurons connected by a single dendrite can encode only a ridiculously minimal grain of information. However, since all you know is encoded in the totality of dendritic connections in your brain, by strengthening the connections not just between pairs of neurons but the connections between large numbers of neurons, new knowledge is permanently connected to something you already know and a memory is laid down. ‘That is what learning is,’ wrote novelist Doris Lessing. ‘You suddenly understand something you’ve understood all your life, but in a new way.’20
‘Whenever you read a book or have a conversation, the experience causes physical changes in your brain,’ says American science writer George Johnson. ‘It’s a little frightening to think that every time you walk away from an encounter, your brain has been altered, sometimes permanently.’21
By this process of strengthening connections between neurons, the network that encodes all you know continually changes. But it not only strengthens connections, it makes new connections and it loses some as well. Think of the neural network of the brain as a vast thicket. In places it is growing and in other places it is being pruned back, as connections are lost between neurons that share nothing in common. This is the process of you forgetting.
What the brain can do that nothing else in the known Universe can do is constantly rebuild and rewire itself. ‘The principal activities of brains are making changes in themselves,’ according to Marvin Minsky.22
As for learning a new skill, it is a very similar process to laying down a memory. Say riding a bike requires using certain muscles. Strengthening of the dendrites that connect to neurons that control such muscles makes it easier and faster to control them. Thus, just as a memory is encoded in a network of neurons, a skill such as riding a bike or reading a book is encoded in a network of neurons. It becomes hard-wired, automatic.
This strengthening and weakening of connections between neurons or the creation of new connections to modify the network is known as neuroplasticity. Even for me to concoct this explanation, neuroplasticity had to occur in my brain. And neuroplasticity had to occur in your brain for you to understand my explanation. (If you did not understand it, no new permanent connections were made and I have left your brain just the way it was before!)
The brain is a computer but it is a remarkable kind of computer. Whereas a silicon-based computer carries out a task according to the program fed to it by a human being, the brain has no external programmer. It is a self-programming computer. A baby is born with a network of neurons and the potential to connect them in a bewilderingly large number of possible ways. The programming of the baby’s brain – the growing of new connections, the strengthening of some connections and the pruning back of many more – is done by its experience of the world, the information flooding in, hour by hour, day by day, through its eyes, ears, nose and skin.
Although it is very hard to see individual neurons forging links with neighbouring neurons, it is perfectly possible to see the brain programming itself at a much coarser level. The technique of functional magnetic resonance imaging (fMRI) reveals areas of the brain that are working when a person is performing a particular task. For instance, when people have been taught to meditate, it has been possible to see new areas of their brains light up in fMRI scans – new programming. Perhaps one of the most famous examples of fMRI research is a study of London taxi drivers. Eleanor Maguire of University College, London, showed how a region of the drivers’ brains – that associated with spatial awareness – was actually larger than in non-taxi drivers.
‘The brain is a muscle. Use it or lose it,’ seems a facile statement. But – apart from the small matter of the brain not being a muscle – the ‘use it or lose it’ mantra encapsulates a deep truth about the brain. Just as exercising with weights encourages physiological processes that grow more muscle cells, the processes of remembering things, learning things and so on, encourages the brain to grow more neuronal connections. And, just as not exercising causes muscles to atrophy, not exercising the brain causes it to weaken or lose altogether many of its existing neuronal connections. Even Charles Darwin, who knew nothing of neurons, realised the truth of ‘use it or lose it’. ‘If I had to live my life over again, I would have made a rule to read some poetry and listen to some music at least once every week,’ he wrote in his autobiography. ‘For perhaps the parts of my brain now atrophied would thus have been kept active through use.’
Neuroplasticity is the brain’s big secret. Like natural selection in evolution and DNA in genetics, it is an idea so central to understanding the brain that, without it, nothing makes any sense. Neuroplasticity explains how new experiences constantly rewire the brain – the ultimate lump of programmable matter. It explains how the blank slate of a baby’s brain becomes an adult brain. It explains how a stroke victim may recover lost faculties when the task of the afflicted neurons is taken over by neurons in an adjacent area of the brain. Rehabilitation is long and hard because the process of reprogramming is analogous to a child learning skills for the first time.
And neoplasticity persists as long as you live. Your brain will still be able to make new connections even when you are a hundred years old. A centenarian can learn to use a computer – they might not learn as fast as a child but they can do it.
‘The brain boggles the mind,’ says James Watson, co-discoverer of DNA.23 It remains the last and grandest frontier in biology, the most complex thing we have yet discovered in our Universe. But we have taken the first tentative steps along the road to understanding it. Nevertheless, there is still a long way to go. But is the destination even reachable? ‘If the human brain were so simple that we could understand it, we would be so simple that we couldn’t,’ wrote the American biologist Emerson M. Pugh.24
Logically, Pugh is correct. The human brain can never completely understand the human brain. It would be like suspending yourself in mid-air by yanking upwards on your shoe laces. However, the brain is not trying to understand the brain. Many brains are trying to understand the brain: the combined minds of international scientific community. ‘All the brains are not in one head’, as an Italian proverb puts it.
We are still no closer to answering the question posed at the beginning of this chapter: why is the Universe constructed in such a way that it acquires the ability to become curious about itself? But, if we understand the brain, we shall finally be able to address it. ‘As long as our brain is a mystery,’ said Santiago Ramón y Cajal, the father of neuroscience, ‘the Universe, the reflection of the structure of the brain, will also be a mystery.’
1 In Arthur Conan Doyle, ‘The Adventure of the Mazarin Stone’.
2 In Kenneth Grahame, The Wind in the Willows.
3 Ambrose Bierce, The Devil’s Dictionary.
4 Oscar Wilde, De Profundis.
5 Remarkably, if a sponge is minced up and its cells put in water, the cells will reconstitute themselves as a sponge once more.
6 If a charged atom or molecule is common in one location, such an ion will tend to move, or diffuse, to an area of lower concentration.
7 ‘The Origin of the Brain’, http://tinyurl.com/d7sbhpk.
8 To be precise, the chemical messengers are contained in structures at the end of an axon known as terminal buttons. It is these that release them into the synaptic gap.
9 Interview with PBS, USA.
10 See Chapter 9, ‘Programmable matter: Computers’.
11 Peter Norvig, ‘Brainy Machines’.
12 Daniel Dennett, Consciousness Explained.
13 David Dalrymple, on leave from Harvard University, is aiming to build a complete simulation of the C. elegans nervous system. This will require first determining the function, behaviour and biophysics of each of the 302 neurons (Randal A. Koene, ‘How to Copy a Brain’, New Scientist, 27 October 20 12, p. 26). It is the first small step on the road towards a daring goal: the copying of a human brain into another material – for instance, the silicon of computers.
14 Edward O. Wilson, Consilience.
15 Sharon Begley, ‘In Our Messy, Reptilian Brains’.
16 Spike Feresten, ‘The Reverse Peephole’, Seinfeld season 9 episode 12, 15 January 1998.
17 An outgrowth of a support cell known as a glial cell sheaths some neurons. The myelin sheath stops the electrical current of the axon leaking out into the surroundings just as plastic insulation stops electricity leaking out of the wires in your home. This is important if the current has to travel a long way – for instance, down the spine to the muscles of a limb. Myelin is white so neurons encased in it are called white matter in contrast to the grey matter of the rest of the brain. People with multiple sclerosis, or MS, progressively lose the myelin sheaths around their white matter and so gradually lose the use of their limbs. Their thought processes, which are carried out in the grey matter, however, remain unaffected.
18 Gerald D. Fischbach, ‘Mind and Brain’, Scientific American, vol. 267 no. 3 (September 1992), p. 49.
19 Tim Berners-Lee, Weaving the Web: The Past, Present and Future of the World Wide Web by its Inventor.
20 Doris Lessing, The Four-Gated City.
21 George Johnson, In the Palaces of Memory: How We Build the Worlds Inside Our Heads.
22 Marvin Minsky, The Society of Mind.
23 James Watson, Discovering the Brain.
24 George E. Pugh (son of Emerson Pugh), The Biological Origin of Human Values.