The Comparative Structure and Function of Muscle

Henry Huddart, G. A. Kerkut

• Publisher: Pergamon
• Year: 2013
• Print ISBN:
  9780080178455
• eBook ISBN:
  9781483280455
• Topic: Biological Sciences
• Subtopic: Zoology
• Index: Biological Sciences

The Comparative Structure and Function of Muscle is based upon a series of lectures given at the University of Lancaster over the last seven years, and it follows a natural division into structure, electrophysiology and excitation and mechanical activity. Within each section, an attempt is made to cover all muscle types in as wide a range of animals as the literature will allow. This book comprises 10 chapters, with the first one focusing on the fine structure of skeletal muscle. The following chapters then discuss the fine structure of cardiac and visceral muscle; the innervation of muscle; the ionic basis of the resting potential; the action potential and the activation of muscle; electrical activity and electrochemistry of invertebrate skeletal muscle; electrical activity of invertebrate and vertebrate cardiac muscle; the electrical activity and electrochemistry of visceral muscle; the mechanics of muscle; and excitation-contraction coupling and relaxation. This book will be of interest to practitioners in the fields of anatomy and the health sciences.

SECTION 1

THE STRUCTURE OF MUSCLE

Introduction

THE widespread application of electron miscroscopic techniques to muscle tissue during the last twenty years has revealed just how diverse in detail but uniform in fundamental characteristics muscle is. Fine structural studies, of course, are not an end in themselves. Of great importance, however, is the attempt to correlate differences in fine structure with differences in function. In this approach, the electron microscope is simply used as another analytical tool, no less important than the oscilloscope or the intracellular electrode.

Fortunately, muscle has attracted many microscopists/physiologists who have used this structure/function approach, and as a result, much is now known about how variations in structures such as myofibrils, the T system and the sarcoplasmic reticulum are correlated with variations in contractility, speed of response and excitation–contraction coupling mechanisms.

Fine structural studies have shown that the convenient physiological classification of muscle into skeletal (or striated), cardiac and visceral (or smooth) has some basis in structural reality, and so this division of muscle has been retained for both vertebrates and invertebrates throughout this book. In a work of this nature it would be somewhat pointless to engage in a detailed description of the fine structure of all muscle types since only the basic concepts need be known to clarify the analysis of muscle functioning. Much more detail of muscle fine structure will be found in specialist works such as Bourne (1960, second edition 1972–) and Huxley (1960) and in volumes on certain animal groups such as molluscs (Hoyle, 1964) and insects (Hoyle, 1965). Many of the recent excellent ultrastructural atlases also deal with muscle and are well worth examining (Fawcett, 1966; Toner and Carr, 1968; Smith, 1968; Sandborn, 1970).

CHAPTER 1

The Fine Structure of Skeletal Muscle

Publisher Summary

The morphology of skeletal muscle varies enormously in different animals and at different positions in the same animal body, dependent upon the function the muscle executes. Not only is the obvious feature of size variation seen, but also variations are seen in the arrangement of the individual fibers within the muscle. Even among a relatively uniform group of animals, such as the vertebrates, muscles may be either long-fibred and strap-like or short-fibred and highly divided, the latter being often called multipennate muscles indicating multiple tendon insertions. Among the invertebrates, skeletal muscle is even more varied in its gross morphology. It may be tubular, sheet-like or highly spongy, and diffuses in animals relying upon a hydrostatic skeleton, but it is usually more orientated in the discrete muscle systems associated with the external skeletons of arthropods. Even within the arthropods, however, the arrangement of the individual fibers within a muscle can be variable. Muscles may be basically strap-like in fiber orientation as in many coxal, flight, and intersegmental muscles or pinnate in form as in many femoral muscles, but all of them show the advantages of first-order lever systems.

THE gross morphology of skeletal muscle varies enormously in different animals, and at different positions in the same animal body, dependent upon the function the muscle executes. Not only is the obvious feature of size variation seen, but, more significantly, variations are seen in the arrangement of the individual fibres within the muscle. Even among a relatively uniform group of animals such as the vertebrates, muscles may be either long-fibred and strap-like (such as the sartorius and the gastrocnemius), or short-fibred and highly divided (such as the deltoid), the latter being often called multipennate muscles, indicating multiple tendon insertions. A very readable account of the gross morphology of skeletal muscle and its relation to lever factors and the skeleton can be found in Young

Among the invertebrates, skeletal muscle is even more varied in its gross morphology. It may be tubular, sheet-like or highly spongy and diffuse in animals relying upon a hydrostatic skeleton (such as annelids and molluscs), but it is usually more orientated in the discrete muscle systems associated with the external skeletons of arthropods. Even within the arthropods, however, the arrangement of the individual fibres within a muscle can be very variable. Muscles may be basically strap-like in fibre orientation, as in many coxal, flight and intersegmental muscles or pinnate in form (with radiating fibres) as in many femoral muscles, but all seem to show the advantages of first order lever systems (see Fig. 1.1). Chapter 9 gives a fuller treatment of this topic.

FIG. 1.1 The structure and arrangement of some insect skeletal muscles. (a) Main femoral musculature of a typical insect. The apodemes are inserted above and below the tibial articulation, which thus restricts movement into the vertical plane in relation to the femur. (b) The femur/tibia dicondylic joint. The cross-hatched area is flexible cuticle known as the articular corium. (c) Isolated single unit insect muscle (e.g. a coxal muscle). (d) Multi-unit muscle with separate muscle units (e.g. stick insect flexor tibialis). (c) Undivided multi-unit muscle (e.g. flexor tibialis of cockroach and Lcpidoptcra. From Huddart (1971b).

No matter how varied the fibre arrangement within muscles, each individual fibre has a basically similar ultrastructure. In physiological terms, the ideal physical model of a muscle cell consists of a contractile component and a non-contractile component. This physiological description is mirrored in the cell’s fine structure, where the contractile component is seen to consist of a series of rod-like elements orientated in the longitudinal axis of the cell, the myofibrils, and the non-contractile component consists of ground sarcoplasm containing nuclei, mitochondria, glycogen deposits and the longitudinal tubules of the sarcoplasmic reticulum. The relative balance between these two major cellular components varies in different muscle fibres and this has important consequences in terms of fibre power output. Variation in the myofibril fraction of the cell is strongly correlated with variations both in the speed of contraction and tension exerted. Since the non-contractile fraction of the cell represents a considerable scries and parallel elastic component, causing great viscous damping and drag on the myofibrils, it will be obvious that cells with a large non-contractile component will be inherently inefficient. In terms of maximizing the tension output per unit cross-sectional area, the greater the myofibril proportion of cell volume, the greater the contractile efficiency of the cell. The sketch in Fig. 1.2 shows just how variable the myofibril density can be in various skeletal muscle fibres.

FIG. 1.2 The distribution of myofibrils in some skeletal muscle fibres. (a) Anodonta (Mollusca); (b) Octopus (Mollusca); (c) hirudinean (Annelida); (d) insect leg muscle and vertebrate skeletal muscle; (e) cockroach flight muscle; (f) Thyone (Echinodermata). (a), (b), (c) and (f) redrawn from Hoyle (1957) and Florey (1966), (d) and (c) drawn from fresh material (frozen sections examined with phase contrast).

The greatest overall myofibril density seems to be that in insect flight muscles (Fig. 1.3) where the myofibrils may account for anything from 70 to 80% of fibre cross-sectional area. In these fibres the ground sarcoplasm is reduced to an absolute minimum, the only major non-contractile inclusions being the mitochondria and the rows of sarcoplasmic reticular tubules (Fig. 1.4). This situation is very different from that seen in many skeletal muscles such as the stick insect leg muscles (Huddart and Oates, 1970a; Fig. 1.5), crustacean phasic and tonic fibres (Jahromi and Atwood, 1967), and in visceral muscle (see Chapter 2), where a considerable ground sarcoplasm is present and where contraction speed and mechanical output are considerably lower than that seen in flight muscle.

FIG. 1.3 Low power survey electron photomicrograph of cockroach flight muscle. Portions of three muscle fibres are visible in the field and a single axon near its terminal with various tracheal profiles. Note that there is little wastage of fibre area on non-contractile inclusions, and the periphery of the fibre has little sarcoplasm. Obvious structures visible arc columnar myofibrils (M), darker-staining dense mitochondria (Mi) and sarcoplasmic reticulum (S)....

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Introduction
• SECTION 1: THE STRUCTURE OF MUSCLE
• SECTION 2: THE ELECTRICAL ACTIVITY OF MUSCLE
• SECTION 3: THE MECHANICAL ACTIVITY OF MUSCLE
• A Short Glossary of Fine Structural and Physiological Terminology
• References
• Author Index
• Subject Index
• OTHER TITLES IN THE ZOOLOGY DIVISION