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Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
ISBN: 9781305389892

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BuyFindarrow_forward

Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
ISBN: 9781305389892
Textbook Problem

Which description is characteristic of a motor unit?

a. A single motor unit’s muscle fibers vary among the slow/fastaerobic and slow/fast anaerobic forms.

b. When receiving an action potential, a motor unit is controlledby a single efferent axon that causes all its fibers to contract.

c. When a motor unit contracts, certain sections of the muscleas a whole remain relaxed.

d. If a motor unit controls walking, it is found in large numbersin the same volume of muscle.

e. If a motor unit controls finger movement, it contains a largenumber of muscle fibers that are stimulated over a large area.

Summary Introduction

Introduction:

The control of muscle contraction does not depend upon its ability to turn the cross-bridge cycle on and off, rather it depends on motor units to carry out the advanced control of each contraction. Each motor unit consists of muscle fibers that are innervated by branches of the axon coming out of a single efferent neuron.

Explanation

Justification for the correct answer:

Option (b) is that when receiving an action potential, a motor unit is controlled by a single efferent axon that causes all of its fibers to contract. A single efferent neuron consists of branches of the axon that control the muscle fibers in a motor unit, causing contraction of the muscle fibers every time when neuron releases an action potential. Hence, option (b) is correct.

Justification for the incorrect answers:

Option (a) states that a single motor unit’s muscle fibers vary among the slow/fast aerobic and slow/fast anaerobic forms. This is a wrong statement as all the muscle fibers in a motor unit are the same, whether slow/fast aerobic and slow/fast anaerobic forms. So, it is an incorrect option.

Option (c) states that when motor unit contracts, certain sections of the muscle as a whole remain relaxed. This is a wrong statement because during the contraction of a motor unit, the force generated is distributed throughout the muscle fiber, not just in certain parts of it...

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