7.2 Selected to survive: studies of the PNS

Viktor Hamburger carried out a series of classic embryologieal experiments over a period of about 30 years. He investigated the relationship between the size of target tissue in chick embryos and the size of the pool of neurons that innervated it. His technique was to remove or add target tissue to the tissue which would eventually form a limb, usually the hind limb, and is called the limb bud. A few days later he observed the effect of the tissue addition or removal on the pool of neurons de
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7.1 Neuron proliferation

There is a huge proliferation of neurons in early life. Even whilst that proliferation continues, some cells, e.g. neuroblasts, stop being able to divide. At some later stage the proliferation itself virtually ceases. It follows that cells switch from being able to divide, to being unable to divide, and that they switch at the appropriate time: the process of cell proliferation is controlled. The details of the control of proliferation are not yet understood and are not considered here. But o
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3.4 Sensitive periods

The steroid hormone testosterone plays a major role in the development of mammals. In particular it is instrumental in causing differences between males and females. One well explored difference concerns play-fighting in young rodents. In the rat, play-fighting is a sequence which begins when one animal pounces on another. The pounce is followed by wrestling and/or boxing and the play-fight usually finishes with one animal on top of the other. A similar sequence of play-fighting is seen in yo
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Preamble

The two main types of superconducting materials are known as type-I and type-II superconductors, and their properties will be discussed in the remainder of this unit. All of the pure elemental superconductors are type-I, with the exception of niobium, vanadium and technetium. The discussion of the effects of magnetic fields and currents on superconductors earlier in this unit has been confined to thin cylinders of type-I materials like lead or tin in a parallel magnetic f
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2.5 Critical current

The current density for a steady current flowing along a wire in its normal state is essentially uniform over its cross-section. A consequence of this is that the magnetic field strength B within a wire of radius a, carrying current I, increases linearly with distance from the centre of the wire, and reaches a maximum value of μ0I / 2Author(s): The Open University

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2.4 Critical magnetic field

An important characteristic of a superconductor is that its normal resistance is restored if a sufficiently large magnetic field is applied. The nature of this transition to the normal state depends on the shape of the superconductor and the orientation of the magnetic field, and it is also different for pure elements and for alloys. In this subsection we describe the behaviour in the simplest situation; we shall discuss other more complex behaviour in Section 4.

If an increasing magnet
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2.3.1 Perfect diamagnetism

Diamagnetism is due to currents induced in atomic orbitals by an applied magnetic field. The induced currents produce a magnetisation within the diamagnetic material that opposes the applied field, and the magnetisation disappears when the applied field is removed. However, this effect is very small: the magnetisation generally reduces the applied field by less than one part in 105 within the material. In diamagnetic material, B = μμ0H
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2.2 Persistent currents lead to constant magnetic flux

An important consequence of the persistent currents that flow in materials with zero resistance is that the magnetic flux that passes through a continuous loop of such a material remains constant. To see how this comes about, consider a ring of metal, enclosing a fixed area A, as shown in Figure 6a. An initial magnetic field B0 is applied perpendicular to the plane of the ring when the temperature is above the critical temperature of the material from which the rin
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2.1 Zero electrical resistance

In this section we shall discuss some of the most important electrical properties of superconductors, with discussion of magnetic properties to follow in the next section.

The most obvious characteristic of a superconductor is the complete disappearance of its electrical resistance below a temperature that is known as its critical temperature. Experiments have been carried out to attempt to detect whether there is any small residual resistance in the superconducting state. A sensitive t
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1 Superconductivity

Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes (Figure 1) as he studied the properties of metals at low temperatures. A few years earlier he had become the first person to liquefy helium, which has a boiling point of 4.2 K at atmospheric pressure, and this had opened up a new range of temperature to experimental investigation. On measuring the resistance of a small tube filled with mercury, he was astonished to observe that its resistance fell from ~0.1 Ω at a temper
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Learning outcomes

After studying this unit you should be able to do the following:

  • explain the meanings of the newly defined (emboldened) terms and symbols, and use them appropriately;

  • distinguish between perfect conduction and perfect diamagnetism, and give a qualitative description of the Meissner effect;

  • explain how observation of a persistent current can be used to estimate an upper limit on the resistivity of a superconductor, and perform calculations related to
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References

Castells, M. (1997) The Information Age: Economy, Society and Culture, vol. 2, The Power of Identity, Oxford, Blackwell.
Fiske, J. (1993) Introduction to Communication Theory, London and New York, Routledge.
Fuller, S. (1997) Science, Buckingham, Open University Press.
Gibbons, M. (1999) ‘Science's ne
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3.1 Introduction

In 2000, the House of Lords Select Committee on Science and Technology produced an influential report that highlighted the complex and increasingly problematic relationship between contemporary science and society, particularly in the field of biotechnology (House of Lords Select Committee on Science and Technology, 2000). The report argued that many of these concerns were seen by the public to be the result of a perceived lack of transparency in the relationship between science, industry, pu
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1.9 The rock cycle

As you are reading this, rocks are being formed and destroyed on the Earth. Rocks are being heated and squeezed to form new metamorphic rocks; other rocks are melting to form magmas, which eventually cool and solidify as new igneous rocks; and the processes of weathering, erosion, transport and deposition are generating new sediments. The continuous action of rock-forming processes means that (given time) any rock in the Earth's crust will become transformed into new types of rock and that th
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6.9 Summary of Part E


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15 Appendix: multiplication tables

If you want to be able to do division without using a calculator, you need to know by heart what you get if you multiply any two numbers up to 10. All the possible combinations can be shown in a multiplication table (also called a times table), like the one below.

Figure 37
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7.4 Elixirs of the nervous system: neurotrophins

According to Section 7.2 axons obtain an elixir from targets at their synapses.

Confirmation that there is indeed an elixir came from a series of events that reveals how much of science really works. Elmer Bucker, working with Hamburger in the mid-1940s, had removed a limb bud from a chick and replaced it with a tumour from
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2.1 Zero electrical resistance

In this section we shall discuss some of the most important electrical properties of superconductors, with discussion of magnetic properties to follow in the next section.

The most obvious characteristic of a superconductor is the complete disappearance of its electrical resistance below a temperature that is known as its critical temperature. Experiments have been carried out to attempt to detect whether there is any small residual resistance in the superconducting state. A sensitive t
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Learning outcomes

After studying this course, you should be able to:

  • develop an appreciation of the huge variety of different mammals that exist on Earth today

  • see how fossil evidence can help us to understand evolutionary history

  • understand how the structure of DNA can help us to detect differences between different species

  • apply the techniques of DNA analysis to work out which mammals are most closely related to each other

  • appreciate t
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Darwin Now Awards
Janice Ansine is a recipient of a Darwin Now Award. She is currently working on a variety of projects in Jamaica, looking at contemporary responses to Darwin and Into the historic role of Jamaica in providing material for his research.
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Rule, approach or aid Comment Cases
The literal rule Uses plain ordinary grammatical meaning of words and avoids judicial law making, but can lead to absurd decisions and injustices and assumes unattainable perfection in draftsmanship Fisher v Bell (1960)