The Brain : A Very Short Introduction
Now, as you read these words, your brain is commanding your eyes to make small but very rapid (about 500° per second) left-to-right movements called saccades (right-to-left or up-and-down for some other written languages).
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Note: Til Saccades
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Note: SUMMARY CHAPTER 1
whole.
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Note: SUMMARY CHAPTER 1 Here we learn about some basic facts about brain .with the example of reading, how it works in the background
Galen’s authority dominated and therefore hampered medical science and practice for some 400 years. A particularly interesting example of his influence can be seen in the early anatomical drawings of Leonardo da Vinci (1452–1519). In one drawing of the head, the brain is depicted crudely consisting of three simple cavities labelled O, M, and N. Leonardo interpreted the anatomical division in functional terms in a way that would have been immediately recognizable to Galen in the 1st century.
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Note: How galen perpetuated hippocrates humorous theory
Brains of the most advanced insects (honey bees) have about one million neurons, snails about 20,000, and primitive worms (nematodes) about 300. Contrast these numbers with the hundred billion or so that are required for human levels of intelligence. But the individual neurons of simple organisms operate with the very same electrical and chemical signalling machinery found in today’s most advanced brains.
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Note: Numbe of neurons varies but not thier complexities
Chapter 4 From the Big Bang to the big brain
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Note: Chapter 4
This is reflected in the increase in the relative size of the forebrain, which in fish, amphibians, and reptiles is a very minor part of the brain, but which in mammals is much larger. It becomes so large in humans that it will not fit into the skull without being folded into itself, forming the characteristic convolutions of the cerebral cortex.
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Note: Word
brains. The truth is that we cannot yet explain why we enjoy exercising them in the creation and public display of music, art, poetry, and humour.
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Note: Chapter 4 summary
Chapter 5 Sensing, perceiving, and acting
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Note: Chapter 5
In fact the very idea that a cell can have an idea seems silly. A single cell after all is far too simple an entity. However, conscious awareness of one’s self comes from just that: nerve cells communicating with one another by a hundred trillion interconnections.
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Thinking about your brain is itself something of a conundrum because you can only think about your brain with your brain.
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Perhaps most remarkable of all however is the brain’s ability to generate conscious awareness, which convinces you that you are free to choose what you will do next.
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As the spinal cord, your brain extends the length of the backbone, periodically sprouting nerves that convey information to and from every part of you.
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About 20 per cent of the volume of the brain is occupied by blood vessels, which supply the oxygen and glucose for the brain’s exceptionally high energy demand.
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The blood supply provides an alternative communication channel between the brain and the body and between the body and brain. Endocrine glands throughout the body release hormones into the blood stream. These hormones inform the brain about the state of bodily functions, whilst the brain deposits hormonal instructions into its blood supply for distribution globally to the rest of the body.
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So when we say the brain does x or y, the word ‘brain’ is a shorthand for all of the interdependent interactive processes of a complex dynamical system consisting of the brain, the body, and the outside world.
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You are not aware of the blur and confusion during a saccade because fortunately there is a brain mechanism that suppresses vision and protects you from visual overload.
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The fovea is the only part of the retina specialized for high acuity vision (see Chapter 5), but it scrutinizes a very small area of our visual world. As a literal rule of thumb, foveal vision is restricted approximately to the area of your vision covered by your thumbnail held at arm’s length. It is a small window of clear vision within which you are able to decipher just 7 or 8 letters of normal print size at a time.
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Importantly, it is done automatically and on an unconscious level without you having to think through every step. If you had to consciously think about the mechanical process of reading, you would be illiterate!
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While we cannot read whole sentences at a glance, the brain does recognize each word as a whole. What is quite surprising however is that the order of the letters is not particularly important (good news for poor spellers). That is why you will be able to read the following passage without consciously having to decode it. I cdnuolt blveiee taht I cluod aulaclty
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While we cannot read whole sentences at a glance, the brain does recognize each word as a whole. What is quite surprising however is that the order of the letters is not particularly important (good news for poor spellers). That is why you will be able to read the following passage without consciously having to decode it.
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Light-sensitive cells called photoreceptors capture light focused as two slightly different images on the left and right retinae. The photoreceptors undertake a fundamental and remarkable transformation of energy, a transformation that must occur for all of our senses. This process is known as sensory transduction and always involves converting the energy in the sensory stimulus, in this case light energy, into an electrical signal. This is because the nervous system cannot use light or sound or touch or smell directly as a currency of information transmission. In the brain electricity is the critically important currency of information flow.
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In their turn, neurotransmitters convey signals
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They communicate with a network of retinal neurons through a mechanism that couples the fluctuating electrical signal in the photoreceptor to the release of a variety of chemicals known as neurotransmitters.
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In their turn, neurotransmitters convey signals from one neuron to another by generating or suppressing electrical signals in neighbouring neurons that are specifically sensitive to particular neurotransmitters. This transformation of an electrical into a chemical signal occurs mostly at specialized sites called chemical synapses. Electrical signals can also pass directly between neurons at sites known as electrical synapses.
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text. Memories must somehow be represented physically in the brain. Brain chemistry and structure is altered by experience and the stability of these physicochemical changes presumably corresponds to the retention duration of memory. So what exactly is a memory? What kind of physical trace is left in the brain after we have learnt some new skill or fact? What is forgetting and why are some memories quickly forgotten and others never? These are questions to which I shall return later. Finally we must consider one of the most elusive of problems. While accepting that everything that the brain does depends on lawful processes occurring within and between the brain’s cells, how can we explain how ‘meanings’ arise in our minds while reading words? How do marks on paper become images in the mind, how do they make you think? How can any of this be
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Memories must somehow be represented physically in the brain. Brain chemistry and structure is altered by experience and the stability of these physicochemical changes presumably corresponds to the retention duration of memory.
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While it is true that neurons can respond rather specifically to particular stimuli, most neuroscientists believe that there can be no one-to-one correspondence between the response of an individual neuron and a perception.
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Another way to think about the relationship between the activities of neurons and a perception is to consider how assemblies of nerve cells in different parts of the brain cooperate with one another in parallel.
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Chapter 2 From humours to cells: components of mind
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trepanation,
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trepanation, the removal of parts of skull to expose the brain,
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To the ancient Egyptians, it was the heart that was credited with intelligence and thought – probably for this reason it was carefully preserved when mummifying the deceased.
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In fact the long history of neuroscience prior to the scientific period suggests that it is not at all self-evident that mental functions must
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Hippocrates (460–370 bc) is usually accredited with being the first in the West to argue that the brain is the most important organ for sensation, thought, emotion, and cognition, he was not the first Greek to consider the question of physical embodiment of mind. Prior to the Hippocratic revolution, Pythagoras (582–500 bc) believed that matter and mind are connected somehow and that the mind is attuned to the laws of mathematics.
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according to Hippocrates, the four determinants of temperament were black bile (melancholy), yellow bile (irascibility), phlegm (equanimity and sluggishness), and sanguine (passion and cheerfulness).
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It seems to have been inspired, not by the evidence of observation, but by the need to conform with the equally unlikely postulates of contemporary Greek natural law, namely that there are four elements: earth, air, water, and fire.
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In one important respect Descartes was breaking new ground. By comparing the workings of the brain with that of complex hydraulic machines, he was regarding the most technologically advanced artefacts of his day as templates for understanding the brain. This is a tradition that persists today; when we refer to computers and computational operations as models of how the brain acquires, processes, and stores information, for example. So while Descartes was hopelessly wrong in detail, he was adopting a modern style of reasoning.
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the Englishman Thomas Willis (1621–75), who coined the term ‘neurology’, argued that solid cerebral tissue has important functions.
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He still held that fluid-flow was the key to understanding brain function, but his focus was on the solid cerebral tissues and he showed that nervous function depends on the flow of blood to them. Today’s functional brain imaging technique (fMRI) shows that small local increases in blood flow are associated with the activation of nerve cells
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Luigi Galvani (1737–98). In the late 18th century he discovered the importance of electricity to the operation of the nervous system.
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It was not until the middle of the 19th century that the ability of nerves and muscles to generate rapidly propagating electrical impulses was confirmed by the German physiologist Du Bois-Reymond (1818–96).
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The true cellular nature of the brain and of its mental functions was first recognized by the father of modern neuroscience, the Spanish neuroanatomist Santiago Ramon y Cajal (1852–1934). Although his proposition that the brain is a cellular machine may today seem commonplace,
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The true cellular nature of the brain and of its mental functions was first recognized by the father of modern neuroscience, the Spanish neuroanatomist Santiago Ramon y Cajal (1852–1934).
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All of this contributed to a rather confusing picture which anatomists found difficult to reconcile with a simple cellular model of brain structure.
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It was therefore not surprising perhaps that cell theory, the idea that tissues are composed of cells, was thought not to apply to the brain and a radical alternative model was proposed. This came to be called the ‘reticular theory’ of brain anatomy – a surprisingly resilient interpretation that persisted well into the 20th century.
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Towards the end of the 19th century, the Italian anatomist Camillo Golgi (1843–1926) developed a way of highlighting the morphology of very few neurons in any particular region of the brain. It was a staining method that fitted the bill because it allowed individual neurons to be viewed unobstructed by the tangled mass of branched processes of neighbouring cells.
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Immediately it was apparent that there are discrete cells in the brain, but they are astonishing cells – unlike any others. They differed markedly from one another, in particular with respect to the complex patterns of their numerous branched processes.
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the reticular theory, which held that the brain contains no discrete components, was actually obstructive to scientific progress. Progress was hindered by the concept of a machine without discrete functional components because without them it is impossible to formulate a plausible mechanism to explain how the brain might work. Scientists were sure the brain machine must have components and, given the complexity of what the brain does, lots of them. But what were they, what did they look like, and what did they do?
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The neuron doctrine is rightly attributed to Ramon y Cajal who, with the help of Golgi’s new staining method, made two profound propositions. The first quite simply is that the neuron is a cell. You might think that this must have been self-evident to anyone who bothered to view a brain treated with Golgi’s method. After all, cells in the brain would be clearly visible and thus by the evidence of one’s own eyes the reticular theory must be wrong. Somewhat astonishingly, however, in spite of the images provided by his own technique, even Camillo Golgi remained a convinced reticularist.
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The neuron doctrine is rightly attributed to Ramon y Cajal who, with the help of Golgi’s new staining method, made two profound propositions. The first quite simply is that the neuron is a cell. You might think that this must have been self-evident to anyone who bothered to view a brain treated with Golgi’s method. After all, cells in the brain would be clearly visible and thus by the evidence of one’s own eyes the reticular theory must be wrong. Somewhat astonishingly, however, in spite of the images provided by his own technique, even Camillo Golgi remained a convinced reticularist. The second of Cajal’s
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The second of Cajal’s propositions was brilliantly insightful: neurons are structurally polarized with respect to function. For the first time, the workings of the brain were explicitly associated with the functions of physical structures at a microscopic level. Cajal concluded that a neuron’s function must be concerned with the movement and processing of information in the brain. He could only guess about the form in which information might be encoded or how it might move from place to place. In a stroke of genius, however, he postulated that it would be sensible for the components of function to impose directionality on information flow (or streaming as he called it). So he proposed that information flows in one direction, from an input region to an output region. The neuron’s cell body and its shorter processes, known as dendrites, perform input functions. Information then travels along the longest extension from the cell body, known as the axon, to the output region – the terminals of the axon and its branches that contact the input dendrites and cell body of another neuron.
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The basic cellular components of mental functions are pretty much the same in all animals, the humble and the human.
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1906 Cajal shared the Nobel Prize for Physiology and Medicine with Golgi, ‘in recognition of their work on the structure of the nervous system’. This was the first time that the Nobel Prize had been shared between two laureates. The award was controversial because the two disagreed on a crucially important matter – Golgi remained convinced that Cajal was wrong to reject the reticular theory. It was of course Golgi who was wrong and fundamentally
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In 1906 Cajal shared the Nobel Prize for Physiology and Medicine with Golgi, ‘in recognition of their work on the structure of the nervous system’. This was the first time that the Nobel Prize had been shared between two laureates. The award was controversial because the two disagreed on a crucially important matter – Golgi remained convinced that Cajal was wrong to reject the reticular theory. It was of course Golgi who was wrong and fundamentally so.
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By the middle of the 20th century, neuroscience had become the fastest growing discipline in the history of scientific endeavour and by the end of that century a more or less complete understanding, in exquisite molecular detail, of how neurons generate electrical and chemical signals would be achieved.
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discredited pseudo-science of phrenology, a theory developed by the idiosyncratic Viennese physician Franz Joseph Gall (1758–1828). Gall believed that the brain is the organ of the mind but he went much further and postulated that different distinct faculties of the mind, innate attributes of personality, and intellectual ability, are located in different sites in the brain. Gall reasoned that different individuals will have these innate faculties and that the degree of their development would be reflected in the size of the surface region of the brain that housed that particular faculty.
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thought that the skull would take the shape of the brain’s relief and therefore that the bumps on the surface of the skull could be ‘read’ as an index of various psychological aptitudes.
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Gall thought that the skull would take the shape of the brain’s relief and therefore that the bumps on the surface of the skull could be ‘read’ as an index of various psychological aptitudes.
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British Phrenological Society was not disbanded until 1967).
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Chapter 3 Signalling in the brain: getting connected
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vindicated.
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So the basic requirements of signalling coded information in the nervous system are that the signals have to be routed correctly and sent reliably over long distances as rapidly as possible.
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Brief electrical pulses (lasting a few thousandths of a second), known as action potentials or nerve impulses, travel along biological cables (axons) that extend from the cell bodies of neurons to connect their input to their outputs with other neurons.
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Compared to the speed of electrical information traffic along the wires in a computer (close to the speed of light), conduction velocities of impulses in the brain are slow, about 120 metres per second in the fastest conducting axons.
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When they reach the terminals of axons, impulses trigger the release of chemical signals that are able to initiate or suppress electrical signals in other neurons.
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In this way neurons transmit information from one to another by an alternating chain of electrical and chemical signals. The chemical signals are released at specialized sites called synapses, at which the chemical signals (neurotransmitters) pass across a very narrow gap separating two neurons. Released neurotransmitter molecules work by binding to and thereby activating specialized receptor molecules located on the surface of the receiving
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In this way neurons transmit information from one to another by an alternating chain of electrical and chemical signals. The chemical signals are released at specialized sites called synapses, at which the chemical signals (neurotransmitters) pass across a very narrow gap separating two neurons. Released neurotransmitter molecules work by binding to and thereby activating specialized receptor molecules located on the surface of the receiving neuron on the other side of the synapse.
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An activated receptor causes a brief electrical response, called a synaptic potential, in the receiving neuron. These potentials may be either inhibitory or excitatory depending on whether the voltage in the receiving neuron becomes more negative (inhibitory or hyperpolarizing) or less negative (excitatory or depolarizing). Inhibitory potentials make the receiving neuron less likely to fire a nerve impulse. Excitatory potentials increase that probability. A ‘decision’ to produce nerve impulses is therefore made through the summation of all of the inhibitory and excitatory potentials impinging on a neuron. Once a critical threshold voltage is reached by this summation, nerve impulses will be generated. The more the excitation, the higher will be the frequency of the impulse train.
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An important way that information is coded in the brain is by the impulse frequency (number of impulses or action potentials per second) and by the pattern of impulses. Nerve impulses travel rapidly along the axon, feeding information to many other neurons where the process of neurotransmitter release and chemical communication is repeated. Neurons may receive chemical signals from hundreds of other neurons through a thousand or more synapses on their surfaces, each having some influence on the ‘decision’ to fire a nerve impulse and on the firing rate. The complexity of the resulting signalling network in the brain is almost unimaginable: one hundred billion neurons each with one thousand synapses,
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An important way that information is coded in the brain is by the impulse frequency (number of impulses or action potentials per second) and by the pattern of impulses. Nerve impulses travel rapidly along the axon, feeding information to many other neurons where the process of neurotransmitter release and chemical communication is repeated.
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Neurons may receive chemical signals from hundreds of other neurons through a thousand or more synapses on their surfaces, each having some influence on the ‘decision’ to fire a nerve impulse and on the firing rate
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The complexity of the resulting signalling network in the brain is almost unimaginable: one hundred billion neurons each with one thousand synapses, producing a machine with one hundred trillion interconnections!
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When a neuron is inactive or at rest there exists a stable negative voltage across the membrane of about −70mv, known as the resting potential.
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Nerve impulses attain a positive voltage of about +50mv before returning to the resting potential. So the total voltage excursion of a nerve impulse is about 120mv or 0.12 volts.
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Axons are very poor conductors of electricity, so bad in fact that over relatively short distances, far less than a typical axon’s length, most of the original signal will leak away into the salty surroundings. This inescapable problem is a consequence of the way the laws of physics apply to the flow of electricity in electrical cables immersed in salty water.
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These laws were first formulated by the British scientist Lord Kelvin (1824–1907) who figured out how to send telegraphic information across the Atlantic Ocean through a submarine cable. Lord Kelvin defined a parameter called the ‘length constant’, which allows us to compare how good different types of cable are at transmitting electrical signals over a distance. A length constant is the distance over which about two-thirds of the electrical signal’s amplitude will be lost and its value can vary enormously.
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The brain is a major consumer of bodily energy. While it is only 2 per cent of our body weight, it consumes 20 per cent of our energy and moreover 80 per cent of the brain’s energy consumption is devoted to a single task: producing biological batteries, the power source of the amplifiers of electrical signals in axons.
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For a submarine cable, the length constant is a small fraction of the distance over which information must be sent and the same is true for biological cables, axons. So in a similarly salty environment both submarine cables and axons must detect a failing electrical signal and boost it back to its original strength before sending it on its way again.
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The pumping creates an imbalance between the inside and outside concentrations of the two ions. Sodium ions are maintained at about tenfold higher concentration outside than inside the neuron and approximately the reverse situation exists for potassium. These concentration gradients, in the absence of barriers, would result in sodium entering and potassium leaving the neuron.
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Biological cables are inherently slow conductors of signals because the nerve impulse depends on the movement of ions across a membrane rather than the displacement of electrons along a wire. Higher transmission speed can be achieved by improving the insulation of the axon membrane and by increasing the electrical conductivity of the axon’s core.
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So to obtain significant gains in speed we would have to produce axons of gigantic girth. A related drawback is that a brain would contain fast components, but inevitably there would be fewer of them. Evolving high performance brains depended in part on the miniaturization of the brain’s components and on getting as many fast neurons as possible packed into a small volume. For this, evolutionary selection pressure did not favour giant neurons.
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Nonetheless, giant axons certainly do exist in brains. There are many examples in the nervous systems of invertebrate and lower vertebrates, where they tend to be involved in initiating very rapid responses such as escape reactions.
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Their experiments led to the discovery of voltagesensitive sodium and potassium flow and to the ionic theory of the action potential described above. The Hodgkin and Huxley account of the action potential in the squid axon was to earn them a share of the Nobel Prize for Physiology and Medicine in 1963, not least because the principles and mechanisms uncovered in the squid were universal – explaining even how our axons transmit electrical signals.
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Transmission speeds in excess of 100 metres per second are possible by improving the axon’s insulation with a multilayered Swiss-roll-like wrapping called myelin.
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The autoimmune disease known as multiple sclerosis (MS) cruelly highlights the importance of myelin in normal brain and bodily function. In MS the body’s immune system damages the myelin and the ability of axons to conduct action potentials is disrupted. This produces various symptoms including unsteady movements of the limbs, blurred vision, abnormal eye movements, loss of coordination, slow word recall, and forgetfulness.
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Myelin is produced by glia, cells in the nervous system that outnumber neurons at least tenfold. There
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Myelin is produced by glia, cells in the nervous system that outnumber neurons at least tenfold.
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Neuron to neuron communication occurs at specialized points of contact between nerve cells called synapses and there is perhaps in excess of 100 trillion of them in the human brain. Synaptic communications are essentially private, in the sense that a single synapse allows one neuron to speak to just one other.
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apposition
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The next time you unsuccessfully try to swat a fly your failure will be due to the speed of transmission of visual information (about you) passing through electrical synapses in the escaping fly’s brain.
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At a typical chemical synapse a narrow gap or cleft between two communicating neurons makes the direct exchange of electrical or chemical signals impossible. For information to be transmitted across the gap the electrical activity of a neuron must cause the release of a chemical message that diffuses across the synaptic cleft to the receiving neuron. The synaptic machinery allowing electrical activity in the signalling neuron to be coupled to the release of neurotransmitter is highly complex and specialized, as are the mechanisms that capture the chemical message and initiate responses in the receiving neuron.
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The pre-synaptic side is specialized for the synthesis, storage, and release of a neurotransmitter. On the post-synaptic side the chemical message is recognized and converted into an electrical signal.
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In this chapter we have examined in some detail the cellular and molecular mechanisms of the most basic of brain functions – the ability of the brain’s component cells to communicate with one another.
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If the connections in the whole brain were unravelled, the strand would be long enough to encircle the earth twice – such is the phenomenal interconnectivity of the brain.
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curiosities.
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These are the most basic functions of the nervous system – providing animals with the ability to detect salient features of their changing surroundings and to respond appropriately.
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When the ability to move was coupled with the ability to sense, movement could be directed to resource-enriched regions and individuals doing this most effectively reproduced more and prospered.
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it made sense to concentrate the senses and the central nervous system at the head end, a phenomenon known as ‘cephalization’.
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Much of the brain’s structural complexity arises because its evolution has been less of an ‘out with the old, in with the new’ process, and more a case of ‘on with the old, in with the new’. This has resulted in new structures being layered upon more primitive ones, which may retain their original functions albeit in the context of the new opportunities provided by the newly acquired ones.
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The spinal cord has a relatively simple structural organization: a central region of ‘grey matter’ containing synapses and the cell bodies of neurons and a surrounding ‘white matter’ consisting of the axons transmitting information up and down the cord.
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The three divisions of the brain can be thought of as a hierarchy in which the forebrain controls the midbrain, which controls the hindbrain. The brainstem (mid- and hindbrain) is concerned with essential but non-cognitive bodily functions such as breathing, the regulation of blood flow, and the coordination of locomotion. Basic processing of sensory information is also performed by the midbrain structures, but more complex processing occurs when this partly processed information is distributed up the hierarchy to the forebrain. The forebrain can be regarded as the executive centre, which considers sensory information of all kinds and formulates commands, decisions, and judgements based on the sensory information and on experience.
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The substantia nigra is intimately involved in the initiation and maintenance of voluntary movements. Damage to this part of the midbrain is linked with the symptoms of Parkinson’s disease, a condition characterized by muscle tremor and a difficulty in initiating actions.
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More than any other brain structure, it is the cerebral cortex that makes us human. Within the cortex plans are made, volitional behaviour is initiated, the neural machinery of language is located, and conscious perceptions are assembled from sensory information. It is the locus of all of our creative intelligence and imagination. If indeed we have free will, then it is in the cortex that its secret will be found.
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The frontal lobes are the largest, accounting for about 40 per cent of the cortex; the temporal, parietal, and occipital lobes account for about 20 per cent each. The ridges of the surface convolutions are called gyri and the valleys that separate them are called sulci, the deeper of which are called fissures.
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These observations confirmed what had been assumed, namely that while both hemispheres have a conscious awareness of things, it is the left that expresses its awareness in words.
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For instance information from the eyes projects to the most posterior part of the occipital lobe known as the primary visual cortex. Information coming from the body and skin, called somatosensory information, projects to a strip of the parietal lobe known as the somatosensory cortex.
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If we trace the evolution of the human brain, the greatest and most rapid growth has occurred in the frontal lobes of the cortex, which accounts for some 40 per cent of the structure. In our nearest living relatives, the chimpanzees, the frontal cortex accounts for about 17 per cent.
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The evolutionary lines leading to modern humans and other living primates, including chimpanzees, diverged about 14 million years ago.
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Deficiencies and damage in the frontal cortex compromise the ability of an individual to make sensible predictions about the future consequences of events and actions in the present. Although intellectual power may be left unaltered, normal restraints on interpersonal discourse are seriously affected by frontal damage and one cannot help but conclude that much of what makes us human, civilized, and creative resides in the part of the brain that has grown so disproportionately in the course of our evolution.
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Although there is no watertight explanation for the runaway pace of evolutionary change that human brain development would seem to require, one of the more imaginative ideas is that our frontal brain is an ornament required for courtship display. According to this idea, the human brain is the product of the mutual preference of men and women for mating with partners who display unusually creative intelligence in the rituals of courtship. This can result in a form of natural selection called sexual selection. It depends on creativity in courtship and the large brain that it requires being heritable traits.
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Although we generally refer only to the five traditional senses – sight, touch, audition, taste, and smell – there are in fact many more. Other sensations include those of heat and cold, gravitation, acceleration, pain, etc.
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In the visual modality for example there is the ability to sense the motion, colour, form, brightness, texture, and contrast of objects.
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Sensations however are just one contributory component to perception. It is possible to perceive what is not sensed, not to perceive what is sensed, and to construct more than one perception from the same sensations.
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Perceptions are the brain’s educated guesses about what the combined senses are telling it, and as such they will almost always depend on interactions between different modalities.
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In effect our brains impose different conscious perceptions on the same information registered by our sensory systems.
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