6.1 Firing-rate hypothesis

Information about stimulus intensity is encoded in two ways: the firing rates of neurons and the number of active neurons.

Intensity is assumed to be encoded by an increase in discharge rate of action potentials within the auditory system. As the stimulus gets more intense, the basilar membrane vibrates at a greater amplitude causing the membrane potential of activated hair cells to be more depolarised and this causes the nerve fibres that synapse onto the hair cells to fire at a greate
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5.2 Frequency code

Although the evidence for the place theory of frequency coding is compelling, there is some question as to whether the tuning curves obtained from neurons in the auditory nerve provide a mechanism for frequency discrimination that is fine enough to account for behavioural data. People can detect remarkably small differences in frequency – in some cases as small as 3 Hz (for a 1000 Hz signal at moderate intensity). What accounts for this ability? As early as 1930, the American experimental p
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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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3.7.1 Summary of Sections 3.4 to 3.7

Hair cells are found in the organ of Corti and run the length of the basilar membrane. They transform mechanical energy into neural signals.

When the basilar membrane vibrates in response to sound, hair cells located at the site of maximal vibration on the basilar membrane are stimulated. This means that the mechanical properties of the membrane allow the auditory system to distinguish one frequency from another by the location on the membrane that is maximally excited by a particular f
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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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3.5.1 Hair cells transform mechanical energy into neural signals

The tectorial membrane runs parallel to the basilar membrane, so when the basilar membrane vibrates up and down in response to motion at the stapes, so does the tectorial membrane. However, as shown in Figure 14, the displacement of the membranes causes them to pivot about different hinging points and this creates a shearing force betw
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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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2.1 Structure and function of the outer and middle ear

Figure 1 is a diagram of the human ear. The outer ear consists of the visible part of the ear or pinna, the external auditory canal (meatus), and the tympanic membrane (tympanum) or eardrum. The human pinna is formed primarily of cartilage and is attached to the head by muscles and ligaments. The deep central portion of the
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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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Learning outcomes

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

  • distinguish between the major anatomical components of the outer, middle and inner ear;

  • describe the function of the outer, middle and inner ear;

  • describe the structure of the cochlea;

  • describe the structural arrangements of the organ of Corti and the function of the basilar membrane;

  • explain the difference between the four coding mechanisms used in order to transmit inform
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Introduction

This unit examines the basic mechanisms responsible for our ability to hear. Humans are able to distinguish a remarkable range of sounds and hearing provides us with a unique source of information about what is occurring in our immediate surroundings. Our sense of hearing depends entirely on the sensory receptors of the inner ear known as hair cells. Hair cells are extremely vulnerable and can be affected by disease, ageing and over-exposure to loud noise. Once destroyed, they do not regenera
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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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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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4.2 Scientific risk analysis

In the context of national and international legislation on the safety of food and animal feed, much of the thinking about assessing risk has come from the experience of developing legislation to cover potentially toxic chemicals. In this regard, the terms ‘risk’ and ‘hazard’ are particularly important. ENTRANSFOOD (European network safety assessment of genetically modified food crops) has defined the terms as follows:


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4.1 Introduction

Pusztai and his team were attempting to develop suitable tests to assess the safety of GM potatoes. Typically, testing the safety of GM food involves comparing its composition and/or its effects with that of the conventionally produced food it most closely resembles. We have seen that such comparisons were at the heart of Pusztai's work.

The comparison of GM and conventional crops and food has led to the so-called principle of substantial equivalence, which has been used extensiv
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3.3 Drawing conclusions

Sections 2.1 and 2.2 have summarised some of the major aspects of the Pusztai affair, but it should be said that almost every detail has been the subject of prolonged and heated dispute. Our purpose is not to attempt to denigrate individuals or institutions. Rather, the hope is that the tale carries some general messages of value about how science is undertaken and communicated, which can sometimes become clearer when things go wrong.

Communicating preliminary scientific information via
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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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1.1 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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Unit questions

Question 1

Which of the factors (a)-(g) is/are a valid reason(s) for the fact that penguins are numerous and diverse around Antarctica but absent from the Arctic?

  • (a) The climate in the Arctic
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