6.3 Summary of Sections 4 to 6

Hair cells do not have axons and therefore do not generate action potentials.

The nerve that communicates with or innervates the hair cells along the basilar membrane is known as the vestibulocochlear nerve or VIIIth cranial nerve. The cochlear portion of the nerve contains afferent fibres that carry information in the form of action potentials from the organ of Corti to the brain, and efferent fibres that bring information from the cerebral cortex to the periphery.

Most of the af
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6.2 Number of neurons hypothesis

In addition to an increase in firing rate of neurons with differing dynamic ranges, the inclusion of discharges from many fibres whose CFs are different from those of the stimulus may also help to account for the wide dynamic range of the ear. You know from Section 3.3 that in response to a pure tone stimulus the basilar membrane vibrates maximally at a g
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5.1 Place code

We know that each hair cell occurs in a localised region of the cochlea, and that auditory nerve fibres contacting each hair cell fire action potentials in response to movement of the basilar membrane at that location. This means that the response of any given fibre should reflect the frequency selectivity of that location on the basilar membrane from which it comes. In other words, cochlear nerve fibres preserve the frequency selectivity found along the basilar membrane. Fibres on the outsid
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4 Neural processing of auditory information

In this section we will look at how the frequency selectivity found along the basilar membrane is preserved or modified by the auditory nerve and how information about the intensity of the signal is encoded in the response of the auditory nerve fibres.

The nerve that communicates with or innervates the hair cells along the basilar membrane is called the vestibulocochlear nerve or VIIIth cranial nerve. It enters the brainstem just under the cerebellum and conveys information from
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3.6 Synaptic transmission from hair cells

In addition to being sensory receptors, hair cells are also presynaptic terminals. The membrane at the base of each hair cell contains several presynaptic active zones, where chemical neurotransmitter is released. When the hair cells are depolarised, chemical transmitter is released from the hair cells to the cells of the auditory nerve fibres. Excited by this chemical transmitter, the afferent nerve fibres contacting the hair cells fire a pattern of action potentials that encode features of
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3.5.2 Mechanical force directly opens and closes transduction channels

It is believed that tip links aid in causing ‘channels’ to open and close near the top of the hair cell (Figure 16). Tip links are filamentous connections between two stereocilia. Each tip link is a fine fibre obliquely joining the distal end of one stereocilium to the side of the longest adjacent process. It is thought that each l
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3.3 The role of the basilar membrane in sound reception

So far we know that sound-induced increases and decreases in air pressure move the tympanum inwards and outwards. The movement of the tympanum displaces the malleus which is fixed to its inner surface. The motion of the malleus and hence the incus results in the stapes functioning like a piston – alternately pushing into the oval window and then retracting from it. Since the oval window communicates with the scala vestibuli, the action of the stapes pushes and pulls cyclically on the fluid
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3.2 The anatomy of the cochlea

The cochlea has a spiral shape resembling the shell of a snail (Figure 4a). You can approximate the structure of the cochlea by wrapping a drinking straw 2.5 times around the tip of a sharpened pencil. The hollow tube, represented by the straw, has walls made of bone and the central pillar of the cochlea, represented by the pencil, is a conical
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1 Sound reception: the ear

In order to hear a sound, the auditory system must accomplish three basic tasks. First it must deliver the acoustic stimulus to the receptors; second, it must transduce the stimulus from pressure changes into electrical signals; and third, it must process these electrical signals so that they can efficiently indicate the qualities of the sound source such as pitch, loudness and location. How the auditory system accomplishes these tasks is the subject of much of the rest of this block. We will
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2.5.2 Quantum fields and unification

From its inception, quantum physics was concerned not just with particles such as electrons, but also with light and other forms of electromagnetic radiation. In 1900 Planck discovered the quantum in the transfer of energy from matter to radiation, and in 1905, Einstein's explanation of the photoelectric effect assumed that the transfer of energy from radiation to matter occurred in a similarly quantised fashion. It is therefore hardly surprising that the development of quantum mechanics was
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6 Summary

There are two areas of general concern regarding the introduction of GM crops and food: the possible impacts on human health and on the environment. For some critics of GM technology, this reflects a feeling that GM technology is unnatural, as compared to conventional crop breeding. However, many techniques used in conventional crop development, for example, intergeneric and interspecific crossing, haploid breeding and mutation breeding, are highly technological and seem very far from being n
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5.2.2 The GM Economics Review

Scientists, as might be expected, played a limited role in this review, which was produced by a team dominated by economists. The review sought to evaluate the costs and benefits of commercial development of the GM crops available at that time in the UK. They also looked at possible developments over a 10–15-year period. The members of the team were obliged to recognise the limited ‘evidence-base’ available on the costs and benefits of GM crops, covering only a short period oftime.

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5.2.1 The GM Science Review

The review was undertaken by the GM Science Review Panel, chaired by the Government's Chief Scientific Adviser, Sir David King. Its role was to assess the evidence available in the peer-reviewed scientific literature. The panel produced two reports, the first in July 2003 and the second in January 2004. The main conclusions of these reports are listed below.

  • The risk to human health is very low.

  • There is little likelihood of such plan
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2.3 The effect of interstellar gas

You have seen that the ISM has been studied through the radiation that the gas and dust absorb, emit and scatter. Figure 15 summarizes the differences between these three phenomena.

Let's first consider the three phenomena in relation to the gas. The gas scatters very little light and so we need only consider absorption and emission of radiation. You have already met absorption and emission of photons by atoms (which we shall call photoexcitation and photoemissio
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2.2 Interstellar space is not empty

The difference between the apparent brightness of a star (as measured by its apparent magnitude), and its luminosity (represented by its absolute magnitude) is defined by the distance of the star. We can explicitly state this relationship as in Equations B and C:

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1.5 Star clusters and stellar evolution

Detailed observations of star clusters suggest that they occur because the stars in them form at about the same time. Moreover, the compositions of the stars are similar. Isolated stars (including isolated binary stars) result from the later partial or complete dispersal of a cluster.

The crucial points for us here are that all the stars in a cluster formed at about the same time, and all have similar compositions.

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1.2 The main classes of stars

The main classes of stars are shown in Figure 5.

The main sequence is ‘main’ in the sense that about 90% of stars fall into this class, and ‘sequence’ in the sense that it is a long, thin region that trails across the H–R diagram, covering a very wide range of temperatures and luminosities. The Sun i
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1.1 Constructing the H–R diagram

Three properties which are suitable for comparing stars are temperature, luminosity and radius. However, we don't need all three.

Question 1

Why not?


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Learning outcomes

By the end of this unit you should be able to:

  • define and use, or recognise definitions and applications of, each of the bold terms;

  • provide examples that show there is a continuum of desert climates and environments that link to diversity of flora and fauna;

  • explain, with examples, the thermoregulatory strategies of evaders, evaporators and endurers, and interpret relevant data;

  • describe the importance of integration of behavi
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Introduction

This unit is the first in a series of three on Animals at the extreme. It is concerned with the integration of behaviour anatomy, physiology and biochemistry in diverse vertebrates that live in deserts. Once you have completed this unit, you will be all the more able to appreciate the linked units that follow, Animals at the extreme: hibernation and torpor and Animals at the extreme: the polar environment. These units build on and develop some of the science you will stud
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