axon A long, thin fibre extending from the cell body (soma) of a neuron, conveying its output in the form of a spike (nerve impulse or action potential) and enabling communication with other neurons. Each neuron will have at most one axon. Axons typically split into many separate branches before connecting with the dendrites of other neurons.
brain stem A small stalk-like area at the bottom of the brain, lying in between the spinal cord and the rest of the brain. The brain stem controls many vital basic bodily functions, such as breathing, swallowing and blood pressure regulation. Because so many neural pathways pass through the brain stem, damage to this area can have profound effects.
cerebral cortex The deeply folded outer layers of the brain, which take up about two-thirds of its entire volume and are divided into left and right hemispheres that house the majority of the ‘grey matter’ (so called because of the lack of myelination that makes other parts of the brain seem white). The cerebral cortex is separated into lobes, each having different functions, including perception, thought, language, action and other ‘higher’ cognitive processes, such as decision making.
dendrites The short input fibres of a neuron that are organized into complicated tree-like patterns. Each neuron has many dendrites that make contact with axons from other neurons via synapses. Dendrites convey the incoming signals to the cell body (soma) of a neuron, which will then produce an output of its own.
frontal lobes One of the four main divisions of the cerebral cortex and the most highly developed in humans compared with other animals. The frontal lobes (one for each hemisphere) house areas associated with decision making, planning, memory, voluntary action and personality.
hippocampus A sea horse-shaped area found deep within the temporal lobes. The hippocampus is associated with the formation and consolidation of memories and also supports spatial navigation. Damage to this area can lead to severe amnesia, especially for episodic (autobiographical) memories.
myelination A process by which a neuron’s axons are coated with myelin, which both insulates the axon from other nearby axons and dramatically increases the speed of nerve impulses (spikes) travelling along it. Myelination, which relies on glial cells, is essential for efficient transmission of information in the brain.
occipital lobes Another of the four main divisions of the cerebral cortex, the occipital lobes are at the back of the brain and house regions mainly involved in vision. Damage to the occipital lobes can result in blindness or more selective deficits.
olfactory system One of the most evolutionarily ancient parts of the brain. The olfactory system underpins the sense of smell and is less well-developed in humans than in many other animals. Signals from olfactory sensory neurons in the nose are conveyed to the olfactory bulb deep inside the brain. Olfaction and taste are distinct from the other senses by responding to chemical stimulation.
parietal lobes The third major division of the cerebral cortex. The parietal lobes lie above the occipital lobes and behind the frontal lobes and are deeply involved in integrating information from the different senses. The parietal cortex is essential for organizing our experience of space and position and it is heavily involved in attentional processes.
Purkinje cells Found exclusively in the cerebellum, these neurons are among the largest in the brain and have elaborately branching dendritic structures. Purkinje cells provide long-range inhibitory control over output parts of the cerebellum, enabling fine motor co-ordination and error correction.
synapses The junctions between neurons, linking the axon of one to a dendrite of another. Synapses ensure that neurons are physically separate from each other so that the brain is not one continuous mesh. Communication across synapses can happen either chemically via neurotransmitters or electrically.
temporal lobes The last of the four main divisions of the cerebral cortex. These lobes are found low to the side of each hemisphere and are heavily involved in object recognition, memory formation and storage and language. The hippocampus is in the medial part of these lobes (the medial temporal lobe).
thalamus These are bundles (nuclei) of neurons that sit on top of the brain stem and are about the size and shape of a walnut. The thalamic nuclei are heavily interconnected with specific areas of the cerebral cortex and are thought to act as sensory relay areas, connecting sensory receptors (apart from olfaction) with the cortex.
Your neurons (your marbles, if you prefer) are the information processing cells of your brain. You have between 90 and 100 billion of them, yet not one of them has any idea who you are. But somehow, by chattering among themselves across networks of billions of interconnections, neurons conjure up your self-awareness. Neurons receive messages from other neurons on their cell body and its short extensions – called dendrites – at specialized structures called synapses. Messages are sent to other neurons via long, slender fibres – called axons – in coded patterns of electrical spikes (nerve impulses). Each impulse is about 0.1 volt and lasts one- to two-thousandths of a second, hurtling along axons at up to 480 kph (300 mph). Arriving at a synapse, impulses trigger the release of signalling chemicals called neurotransmitters. These alter the pattern of spikes generated by the receiving neuron. And that is basically how the brain works. Well, not quite. Neurons work properly only if bathed in the right blend of chemicals. Glial cells, which outnumber neurons 50:1, maintain this condition. They help neurons wire together in the developing brain, nurture them in the adult brain, insulate axons, mop up dead cells, recycle used neurotransmitters and protect the brain from infection. They are the unsung heroes of the brain’s story.
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There are 4 km (2½ miles) of neuronal network interconnections packed into every cubic millimetre of grey matter.
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Could you think yourself thin? The brain is just two per cent of your body weight but consumes twenty per cent of your daily energy needs. Exercising the brain is energetically expensive. In spite of this, as humans evolved, the most thoughtful part of the cerebral cortex rapidly tripled in size beginning about two million years ago. Most of the additional cost of evolving our uniquely human cognitive abilities is consumed by a single enzyme that recharges the batteries that power electrical nerve impulses.
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3-SECOND BIOGRAPHIES
SANTIAGO RAMÓN Y CAJAL
1852–1934
Anatomist who defined the cellular components of mental activity
WALTHER NERNST
1864–1941
His theoretical work explained how voltages are generated by cells
BERNARD KATZ
1911–2003
Proposed the quantum/vesicular hypothesis of neurotransmitter release
30-SECOND TEXT
Michael O’Shea
For every networking executive neuron, there are 50 low-status but essential glial cells maintaining the neural environment.
Neurotransmitters convey signals between neurons, briefly exciting or inhibiting their electrical activity. They are released when nerve impulses arrive at synapses. They range from very small molecules, to medium compounds, to giant molecules called peptides. They are stored in tiny spheres called synaptic vesicles. Impulses cause the vesicles to release their contents into the synaptic gap between transmitting and receiving neurons. Released neurotransmitters act by binding to receptor proteins, each of which is tuned to just one neurotransmitter type. There are scores of neurotransmitters and even more receptors. Why so many? After all, if neurotransmitters mediate just two simple functions – excitation and inhibition – surely two transmitters and their receptors is enough? Things are not so simple. Many neurotransmitters do not trigger fast excitation or inhibition, but initiate quite slow metabolic processes in neurons, causing lasting changes in the strength of synaptic connections. Neurotransmitters can also initiate the switching ON and OFF of important genes, which can cause long-term change in neuronal and synaptic properties. Are these the changes in the brain on which memories depend? Probably, but we are far from a complete understanding of the brain’s complex chemical language.
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Active neurons release neurotransmitters that activate receptors in other neurons to change information flow in the brain in the short, medium and long term.
3-MINUTE BRAINSTORM
Nitric oxide (NO), a poisonous gas, is a most unlikely neurotransmitter. It cannot be stored in vesicles, so it is released as it is produced, within specialized active neurons. NO then spreads into swathes of the brain, where it can affect many receptive neurons without the transmitting neuron having to be directly connected to them. This ‘non-synaptic’ signalling is important in long-term memory formation.
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3-SECOND BIOGRAPHIES
OTTO LOEWI
1873–1961
First to show that a stimulated nerve releases a substance that has a physiological effect
HENRY DALE
1875–1968
Most famous for the so-called Dale’s Principle – that all synapses of a single neuron release the same neurotransmitter(s)
BERNARD KATZ
1911–2003
Proposed the quantum/vesicular hypothesis of neurotransmitter release
30-SECOND TEXT
Michael O’Shea
While you were pondering whether to order pizza or tacos, a sophisticated performance of chemical signalling just took place in your brain.
A gene is a set of instructions in DNA for making a protein. There are about 22,000 genes in the human genome. Although proteins are the essential cogs and levers in the functioning of all neurons, no cell needs all 22,000 genes. So neurons, along with other cells, turn on only the genes required for their own needs. As needs change, different genes are turned on or off. This changing pattern of active genes is particularly notable in the functioning of synapses. This is important because changing the connections in neural circuits allows us to learn from experience. Consider a neural circuit that detects a potentially threatening sensory stimulus. If the threat persists, strengthened circuit connections will be required to sustain and enhance vigilance. To achieve this, signals are dispatched from the sharp end of neurons – the synapses – to their central nuclei and there the DNA is ordered to turn on the required genes. Freshly made synapse-reinforcing proteins are then rushed back to the same synapses that ordered them. So while genes certainly affect brain function, the fact that they can be influenced by their environment frees our behaviour from a rigid genetic determinism as the brain’s genetic machinery responds adaptively to changing circumstances.
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The brain uses 70 per cent of our 22,000 genes. Those affecting synaptic function are particularly important because their activity can be regulated by experience.
3-MINUTE BRAINSTORM
Genes have an important role in disorders of the mind and behaviour, such as ADHD, autism, bipolar disorder, depression and schizophrenia. In fact, while these are considered as clinically different, recent research suggests that they share genetic risk factors. The identification of shared genetic causes of a range of psychiatric disorders may lead to the discovery of an underlying molecular mechanism for mental illness. This would represent a major advance in the development of preventative medicines.
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3-SECOND BIOGRAPHY
FRANCIS CRICK & JAMES WATSON
1916–2004 & 1928–
Awarded the Nobel Prize in 1962 for determining the structure of DNA and suggesting how DNA encodes and replicates genetic information
30-SECOND TEXT
Michael O’Shea
Neurons turn genes on or off to order protein material when needed; it’s a little like just-in-time delivery systems, but it works.
Acknowledged by many to be the architect of modern neurobiology, as a young man Cajal tried very hard to keep out of medicine altogether. He wanted to be an artist, but his father (a professor of dissection) was equally adamant that he should be a doctor. After miserable but educational apprenticeships with a cobbler and a barber, Cajal gained his medical licence and buckled down to join the family business, eventually being appointed Professor of Anatomy at Valencia.
He kept up with his drawing, making many anatomical studies, and it is possible that it was his artistic eye that led him to his greatest discovery. When in 1887, now at the University of Barcelona, he looked at the Italian physician Camillo Golgi’s immaculately stained slides of brain cells, he saw what others had not. Until this time, the prevailing orthodoxy was that the nervous system was a single reticular (mesh-like) construct without discrete cellular components (neurons). Cajal realized however that what Golgi’s images clearly showed was that the nervous system was a network of discrete cellular components. This was a correct interpretation that crucially allowed neurons to be regarded as the functional units of the brain – free agents that could form many synaptic connections, each capable of being modified to allow for growth and adaptation. Cajal studied this newly revealed phenomenon for four years, identifying as well so-called ‘dendritic spines’ – small membranous protrusions from a neuron’s input fibres that typically receive input from a single synapse. He used his artistic skills to make meticulous drawings and then published his findings in his magnum opus Revista Trimestral de Histología Normal y Patológica, which had an impact on the scientific community similar to Darwin’s breakthrough On the Origin of Species.
By providing the most accurate description of the neuron’s function and mechanism, it changed the way neuroscience worked and cleared the path for the formulation of neuron doctrine proposed by German anatomist Heinrich von Waldeyer-Hartz. Cajal was a prodigious publisher and contributor to medical journals. He was greatly feted and heaped with awards, including the 1906 Nobel Prize for Physiology or Medicine, which he shared with Golgi. Cajal also found time to work in other areas of medicine, notably cancer, and to set up his own research institute in Madrid. For all the breadth of his achievements, he will always be best remembered for disentangling the neuron from its imagined network.
Born in Petilla de Aragón, Spain
1873
Graduated from the medical school of the University of Zaragoza
1874–75
Served as an army doctor, accompanied expedition to Cuba
1883
Appointed Chair of Anatomy at the University of Valencia
1888–1894
Published Revista Trimestral de Histología Normal y Patológica, results of his systematic histological study of the nervous system
1888
Discovered that axons terminate freely and the existence of dendritic spines on neural dendrites
1891
Promulgated his theory of the individuality of the nerve cell
1892
Published his Law of Dynamic Polarization
1901
Appointed director of the Biological Research Laboratory, which would become the Cajal Institute in 1922
1906
Shared Nobel Prize for Physiology or Medicine with Camillo Golgi for their work on the structure of the nervous system
17 October 1934
Died in Madrid
Imagine you’ve just picked up a typical 1.36-kg (3-lb) adult human brain. The outer spongy tissue that you grip in each hand is the cortex. Look at the apparently random pattern of grooves on the surface – the sulci – and you’ll see some deeper lines. These landmarks show the divisions between the main lobes of the cortex: the frontal lobes, the temporal lobes near the ears, the parietal lobe at the crown of the head and the occipital lobe at the rear. Lift the brain above your head and sprouting underneath you’ll see the brain stem, responsible for regulating the most basic life-sustaining functions, including breathing and heart rate. Also note the cauliflower-like cerebellum nestled next door. In a living person, the brain stem would connect to the spinal cord, thereby linking the brain with the rest of the body. Now place the brain back down and gently pry apart the two hemispheres so that you reveal the inner structures, including the top of the brain stem, known as the midbrain. Above this is the egg-shaped thalamus – the brain’s relay station. Nearly all incoming sensory information is sent here before being passed on to the cortex. In the traditional language of anatomy, the brain stem is the metencephalon, the thalamus is part of the diencephalon and the outer cortex is the telencephalon.
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The brain can be divided crudely into three basic parts: the outer cortex; the diencephalon, including the thalamus; and the brain stem.
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We now take it for granted that the brain is the seat of thought, but this wasn’t always the case. Even after the importance of the brain was demonstrated by Galen in the second century ad, it would take more than a millennium for this view to be universally accepted. Writing as late as the 17th century, the English philosopher Henry More argued the human brain has as much potential for thought as ‘a bowl of curds’.
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3-SECOND BIOGRAPHY
GALEN
129–ca 210/216 AD
The ‘prince of physicians’, credited with providing the first demonstration of the importance of the brain to behaviour
30-SECOND TEXT
Christian Jarrett
Blueprint for the brain: A compact space-saving multi-functional unit built to respond instantly to environmental changes; organically fuelled.
Hanging off the back of the brain is a second ‘little brain’ (the literal translation of ‘cerebellum’), resembling a fist-sized cauliflower. Densely packed with cells, it accounts for ten per cent of the brain’s volume and yet contains around half the neurons found in the entire central nervous system. Like the big brain, the cerebellum is comprised of two hemispheres, except here they are joined by a narrow structure known as the vermis (literally ‘worm’). Further, in common with the cerebral cortex, the highly convoluted cerebellar cortex is made up of white matter in its deeper parts, with grey matter nearer the surface. The cerebellum contains many intricately branched Purkinje cells, which are found only in this brain structure. Since at least the early 19th century, neuroscientists have recognized the important role played by the cerebellum in the control of movement and posture. Abnormalities in its function, whether caused by inherited disease, brain damage or the effect of alcohol, lead to difficulties in walking and a general clumsiness of movement. In recent years, our understanding of the cerebellum has undergone a revolution and it is now thought to be involved not just in motor control but in memory, mood, language and attention.
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The cerebellum, or ‘little brain’, is densely packed with neurons and its primary function is in motor control and co-ordination, although we now know it does much more.
3-MINUTE BRAINSTORM
The cerebellum is responsible for the fact that we are unable to tickle ourselves. As well as calculating the movements necessary to achieve a desired action (known as an ‘inverse model’), another of the cerebellum’s roles is to form predictions (‘forward models’) of the probable sensory consequences of our own actions and to cancel them out. Self-tickling doesn’t work because of this process (see here).
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THE BASIC ARCHITECTURE OF THE BRAIN
HOW WE PICK UP A CUP OF COFFEE
3-SECOND BIOGRAPHIES
JAN EVANGELISTA PURKINJE
1787–1869
Described the intricately branching Purkinje cells that now bear his name
SANTIAGO RAMÓN Y CAJAL
1852–1934
Used revolutionary techniques to reveal the underlying cellular structure of the cerebellum
MASAO ITO
1928–
Pioneer in characterizing the functional circuits of the cerebellum
30-SECOND TEXT
Christian Jarrett
The brain develops from a hollow tube formed from the skin of the very early embryo. Cells multiply more rapidly at the front of the tube, which enlarges, becoming the embryonic brain. Newly produced cells transform into immature neurons. By the time the embryo is about four weeks old, these migrate to their destinations, growing dendrites and axons and forming the first of what will be trillions of synaptic connections. There is no blueprint for these interconnections – the embryonic brain generates an excess of neurons and synapses, allowing competition and interactions with the environment to sculpt functional circuits. About half the embryonic neurons are killed off, having failed to form useful connections. Some surviving neurons – involved in transmitting information over long distances – have their axons insulated by glial cells, a process called myelination, which increases the speed and quality of information transmission. Until recently, brain development was thought to be completed in early childhood. In fact, grey matter volume increases gradually through childhood, peaks in early adolescence and shrinks as an adolescent becomes an adult. This reduction in brain volume seems odd, but reflects the brain’s ability to adapt to the environment by pruning unused synapses and strengthening useful ones (or so we may tell ourselves!).
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The properties of the brain are maintained by dynamic, plastic mechanisms that originate in the embryo but continue to operate after birth and on into adulthood.
3-MINUTE BRAINSTORM
Brain development is characterized by a convergence of genetic (nature) and environmental (nurture) influences. A misunderstanding of the interaction between them lies behind questions such as: ‘Is this or that trait due to nature or nurture?’ But it is wrong to present the question as an ‘either/or’ proposition. The genome does not contain enough information to make a brain on its own, so genes have evolved to exploit information coming from the environment; information essential for fine-tuning developing neuronal networks.
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NEUROGENESIS & NEUROPLASTICITY
3-SECOND BIOGRAPHIES
RITA LEVI-MONTALCINI
1909–2012
Won Nobel Prize in 1986 for discovering ‘nerve growth factor’ (with Stanley Cohen), a key chemical shaping neural development
ROGER SPERRY
1913–94
Won Nobel Prize in 1981. He showed that chemical signals provide the basic mechanisms for wiring the brain
30-SECOND TEXT
Michael O’Shea
You’re not losing brain cells, you are culling the slackers to reveal a diamond-precision, lean, mean thinking machine.
The origins of brains can be found about a billion years ago with the appearance of the first multicellular organisms. Their cells needed to communicate with each other, so they evolved neural nets – a kind of diffuse proto-brain still found in some creatures today, such as jellyfish. Later geological and climatic events provided new environments and challenges that spurred further brain evolution, including the emergence of groups of neurons specialized for specific tasks. It’s tricky to pin down when these neuronal hubs connected together to form the first brain, but we know that around half a billion years ago the fish-like ancestors of modern-day vertebrates had brain-like structures. Looking at the animal kingdom today, we can see how evolutionary pressures shaped the emergence of different types of brain. The fruit fly, for instance, lacks a cortex but has large antennal lobes and ‘mushroom bodies’ dedicated to processing smell. The rat has large areas of cortex devoted to processing information from its whiskers. Fish have an enlarged cerebellum, a structure involved in movement. There are many theories for what caused the massive expansion of the human brain, including bipedalism (which freed up the hands for tool use), larger social groups and the emergence of language.
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Brains began evolving millions of years ago, allowing organisms to search and respond to the external world in ever more sophisticated and flexible ways.
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A recurring debate is whether the human brain is continuing to change. Genetic evidence published in 2005 suggested that it is. A team at the University of Chicago identified two versions of genes involved in brain development that had appeared relatively recently in human history – microcephalin and ASPM. The first appeared around 37,000 years ago, the other approximately 5,800 years ago. Their rapid and continuing spread through the population suggests they confer some kind of advantage.
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30-SECOND TEXT
Christian Jarrett
