Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world

Transcription

1

2 Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: Pearson Education Limited 2014 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6 10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. ISBN 10: ISBN 13: British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Printed in the United States of America

3 weight lifters or bodybuilders. The number of muscle fibers does not change, but the muscle as a whole gets larger because each muscle fiber increases in diameter. The BIG PICTURE What you don t use, you lose. Muscle tone is an indication of the background level of activity in the motor units in skeletal muscles. When inactive for days or weeks, muscles become fl accid, and the muscle fi bers break down their contractile proteins and grow smaller and weaker. If inactive for long periods, muscle fi bers may be replaced by fi brous tissue. Aerobic endurance is the length of time a muscle can continue to contract while supported by mitochondrial activities. Aerobic endurance is determined by the availability of substrates for aerobic metabolism from the breakdown of carbohydrates, lipids, or amino acids. Aerobic activities do not promote muscle hypertrophy. Training to improve aerobic endurance usually involves sustained low levels of muscular activity. Examples are jogging, distance swimming, and other exercises that do not require peak tension production. Because glucose is a preferred energy source, endurance athletes often load up on carbohydrates ( carboload ) for the three days before an event. They may also consume glucose-rich sports drinks during a competition. CHECKPOINT 16. Why would a sprinter experience muscle fatigue before a marathon runner would? 17. Which activity would be more likely to create an oxygen debt in an individual who regularly exercises: swimming laps or lifting weights? 18. Which type of muscle fibers would you expect to predominate in the large leg muscles of someone who excels at endurance activities such as cycling or longdistance running? See Answers to Checkpoints and Reviews at the end of the chapter. 7-8 Cardiac and smooth muscle tissues differ structurally and functionally from skeletal muscle tissue Here we consider the structural and functional properties of cardiac and smooth muscle tissue in detail. CARDIAC MUSCLE TISSUE Cardiac muscle tissue is found only in the heart. Cardiac muscle cells are relatively small and usually have a single, centrally placed nucleus. Like skeletal muscle fibers, cardiac muscle cells contain an orderly arrangement of myofibrils and are striated, but significant differences exist in their structures and functions. The most obvious structural difference is that cardiac muscle cells are branched, and each cardiac cell contacts several others at specialized sites called intercalated (in-ter-ka-la-ted) discs ( Figure 7-10a ). These cellular connections contain gap junctions that allow the movement of ions and small molecules and the rapid passage of action potentials from cell to cell, resulting in their simultaneous contraction. Because the myofibrils are also attached to the intercalated discs, the cells pull together quite efficiently. Cardiac muscle and skeletal muscle also have the following important functional differences: Cardiac muscle tissue contracts without neural stimulation, a property called automaticity. The timing of contractions is normally determined by specialized cardiac muscle cells called pacemaker cells. Cardiac muscle cell contractions last roughly 10 times longer than those of skeletal muscle fibers. Th e properties of cardiac muscle plasma membranes differ from those of skeletal muscle fibers. As a result, cardiac muscle tissue cannot undergo tetanus (sustained contraction). This property is important because a heart in tetany could not pump blood. An action potential not only triggers the release of calcium from the sarcoplasmic reticulum but also increases the permeability of the plasma membrane to extracellular calcium ions. Cardiac muscle cells rely on aerobic metabolism for the energy needed to continue contracting. The sarcoplasm contains large numbers of mitochondria and abundant reserves of myoglobin that store oxygen. SMOOTH MUSCLE TISSUE Smooth muscle cells are similar in size to cardiac muscle cells; they also contain a single, centrally located nucleus within each spindle-shaped cell ( Figure 7-10b ). Smooth muscle tissue is found within almost every organ, forming sheets, bundles, or sheaths around other tissues. In the skeletal, muscular, nervous, and endocrine systems, smooth 240

4 Actin and myosin are present in all three muscle types. However, the internal organization of a smooth muscle cell differs from that of skeletal or cardiac muscle cells in the following ways: FIGURE 7-10 Cardiac and Smooth Muscle Tissues. Smooth muscle tissue lacks myofibrils, sarcomeres, Cardiac muscle cell Robert B. Tallitsch Intercalated discs or striations. In smooth muscle cells, the thick filaments are scattered throughout the sarcoplasm. The thin filaments are anchored within the cytoplasm and to the sarcolemma. When a contraction occurs, the muscle cell twists like a corkscrew. Adjacent smooth muscle cells are bound together at these anchoring sites, thereby transmitting the contractile forces throughout the tissue. LM 575 Cardiac muscle tissue Functionally, smooth muscle tissue differs from other muscle types in several major ways: Calcium ions trigger contractions through a different a A light micrograph of cardiac muscle tissue. mechanism than that found in other muscle types. Additionally, most of the calcium ions that trigger contractions enter the cell from the extracellular fluid. Smooth muscle cells are able to contract over a greater Circular muscle layer T range of lengths than skeletal or cardiac muscle because the actin and myosin filaments are not rigidly organized. This property is important because layers of smooth muscle are found in the walls of organs that undergo large changes in volume, such as the urinary bladder and stomach. Many smooth muscle cells are not innervated by motor Frederic H. Martini Longitudinal muscle layer L neurons, and the muscle cells contract either automatically (in response to pacesetter cells) or in response to environmental or hormonal stimulation. When smooth muscle fibers are innervated by motor neurons, the neurons involved are not under voluntary control. Table 7-2 summarizes the structural and functional properties of skeletal, cardiac, and smooth muscle tissue. Smooth muscle tissue LM 100 b Many visceral organs contain several layers of smooth muscle tissue oriented in different directions. Here, a single sectional view shows smooth muscle cells in both longitudinal (L) and transverse (T) sections. muscles around blood vessels regulate blood flow through vital organs. In the digestive and urinary systems, rings of smooth muscles, called sphincters, regulate movement along internal passageways. CHECKPOINT 19. How do intercalated discs enhance the functioning of cardiac muscle tissue? 20. Extracellular calcium ions are important for the contraction of what type(s) of muscle tissue? 21. Why can smooth muscle contract over a wider range of resting lengths than skeletal muscle? See Answers to Checkpoints and Reviews at the end of the chapter. 241

5 Table 7-2 Property Skeletal Muscle Fiber Cardiac Muscle Cell Smooth Muscle Cell Fiber dimensions (diameter length) 100 μm up to 60 cm μm μm 5 10 μm μm Nuclei Multiple, near sarcolemma Usually single, centrally located Single, centrally located Filament organization In sarcomeres along myofibrils In sarcomeres along myofibrils Scattered throughout sarcoplasm Control mechanism Neural, at single neuromuscular junction Automaticity (pacemaker cells) Automaticity (pacesetter cells), neural or hormonal control Ca 2 source Release from SR Extracellular fluid and release from SR Extracellular fluid and release from SR Contraction Energy source A Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues Rapid onset; tetanus can occur; rapid fatigue Aerobic metabolism at moderate levels of activity; glycolysis (anaerobic during peak activity) Slower onset; tetanus cannot occur; resistant to fatigue Aerobic metabolism, usually lipid or carbohydrate substrates Slow onset; tetanus can occur; resistant to fatigue Primarily aerobic metabolism 7-9 Descriptive terms are used to name skeletal muscles The muscular system includes all the skeletal muscles ( Figure 7-11 ). Th e general appearance of each of the nearly 700 skeletal muscles provides clues to its primary function. Muscles involved with locomotion and posture work across joints, producing movement of the skeleton. Those that support soft tissue form slings or sheets between relatively stable bony elements, whereas those that guard an entrance or exit completely encircle the opening. Here we consider several features of muscles that are used to name them: their attachment sites, known as origins and insertions, and their actions. ORIGINS, INSERTIONS, AND ACTIONS Each muscle begins at an origin, ends at an insertion, and contracts to produce a specific action. In general, a muscle s origin remains stationary while the insertion moves. For example, the gastrocnemius muscle (in the calf) has its origin on the distal portion of the femur and inserts on the calcaneus. Its contraction pulls the insertion closer to the origin, resulting in the action called plantar flexion. Th e determinations of origin and insertion are usually based on movement from the anatomical position. Almost all skeletal muscles either originate or insert on the skeleton. When they contract, they may produce flexion, extension, adduction, abduction, protraction, retraction, elevation, depression, rotation, circumduction, pronation, supination, inversion, or eversion. Actions of muscles may be described in two ways. The first describes muscle actions in terms of the bone affected. Accordingly, the biceps brachii muscle is said to perform flexion of the forearm. The second method, which is increasingly used by specialists of human motion (kinesiologists), describes muscle action in terms of the joint involved. Thus, the biceps brachii muscle is said to perform flexion at (or of) the elbow. We will primarily use the second method. Muscles can also be described by their primary actions: A prime mover, or agonist (AG-o-nist), is a muscle whose contraction is chiefly responsible for producing a particular movement. The biceps brachii muscle is a prime mover that flexes the elbow. Antagonists (an-tag-o-nists) are muscles whose actions oppose the movement produced by another muscle. An antagonist may also be a prime mover. For example, the triceps brachii muscle is a prime mover that extends the elbow. It is, therefore, an antagonist of the biceps brachii, and the biceps brachii is an antagonist of the triceps brachii. Agonists and antagonists are functional opposites if one produces flexion, the other s primary action is extension. A synergist (syn-, together ergon, work) is a muscle that helps a prime mover work efficiently. Synergists may either provide additional pull near the insertion or stabilize the point of origin. For example, the deltoid muscle acts to lift the arm away from the body (abduction). A smaller muscle, the supraspinatus muscle, assists the deltoid in starting this movement. Fixators are synergists that stabilize the origin of a prime mover by preventing movement at another joint. 242

6 FIGURE 7-11 An Overview of the Major Skeletal Muscles. Frontalis Temporalis Trapezius Clavicle Deltoid Masseter Sternocleidomastoid Pectoralis major Sternum Biceps brachii Triceps brachii Brachialis Pronator teres Palmaris longus Flexor carpi radialis Flexor digitorum Latissimus dorsi Serratus anterior External oblique Rectus abdominis Extensor carpi radialis Brachioradialis Flexor carpi ulnaris Tensor fasciae latae Vastus lateralis Rectus femoris Patella Gluteus medius Iliopsoas Adductor longus Gracilis Sartorius Vastus medialis Tibia Fibularis Gastrocnemius Tibialis anterior Extensor digitorum Soleus a Anterior view 243

7 FIGURE 7-11 An Overview of the Major Skeletal Muscles. (continued) Occipitalis Sternocleidomastoid Trapezius Deltoid Infraspinatus Teres minor Teres major Triceps brachii Rhomboid major Latissimus dorsi Brachioradialis Flexor carpi ulnaris Extensor carpi radialis External oblique Extensor digitorum Extensor carpi ulnaris Tensor fasciae latae Gluteus medius Gluteus maximus Semitendinosus Adductor magnus Biceps femoris Semimembranosus Gracilis Sartorius Gastrocnemius Soleus Calcaneal tendon Calcaneus b Posterior view 244