Contractile cells

Introduction

Several cell types are specialized to generate motile forces through contraction. This chapter presents an overview of the main types of contractile cells. The general structure of each type of cell is described, together with details of their fine structure and how this relates to the molecular basis of contraction.

Contractile cells are specially adapted to generate motile forces by the interaction of the proteins actin and myosin (contractile proteins).

There are four groups of contractile cells: muscle cells, myofibroblasts, pericytes and myoepithelial calls. Muscle cells are the main type and comprise striated (voluntary) muscle, cardiac muscle and smooth (involuntary) muscle. Myofibroblasts have a contractile role in addition to being able to secrete collagen. Pericytes are smooth, muscle-like cells that surround blood vessels. Myoepithelial cells are an important component of certain secretory glands.

Different arrangements of actin and myosin in each type of contractile cell, together with important structural adaptations, modulate and control contraction.

Skeletal muscle

Skeletal muscle cells form the structural basis of muscles (see Chapter 13 ), which are responsible for voluntary movement under the influence of the nervous system and for maintenance of posture. This section presents the characteristics of muscle cells related to contraction. The innervation, specialisations and attachment of muscle to the skeletal system are discussed in Chapter 13 .

In embryogenesis, each skeletal muscle cell forms from the fusion of many hundreds of precursor cells (myoblasts), so in the adult each is a syncytium containing hundreds of nuclei, which are located just beneath the cell membrane. Each skeletal muscle cell is a long, thin, cylindrical structure, typically 50–60 μm in diameter in an adult and up to 10 cm long, depending on its location ( Fig. 5.1 ).

In adult muscle, there is a resident population of muscle precursor cells (satellite cells), which can divide to form new muscle cells after tissue damage.

In addition to contractile proteins, skeletal muscle cell cytoplasm contains numerous mitochondria, along with abundant glycogen to provide energy. Each muscle cell is surrounded by an external lamina (see p. 69).

Because of long usage, special terms are often used to describe skeletal muscle cell components. These are sarcolemma (cell membrane), sarcoplasm (cell cytoplasm) and sarcoplasmic reticulum (endoplasmic reticulum).

The contractile elements of skeletal muscle cells (myofibrils) are thin, cylindrical structures 1–2 μm in diameter. They are composed of overlapping, repeating assemblies of thick (mainly myosin) and thin (mainly actin) filaments.

Any one muscle fibre has hundreds of myofibrils running parallel along its length, the alternating zones of thick and thin filaments giving rise to the descriptive term striated muscle ( Fig. 5.2 a).

Ultrastructurally, the thick and thin filaments are held in place by plates of accessory proteins, visible as lines, which divide the myofibrils into functional units called sarcomeres (see Fig. 5.2 b). Sarcoplasm, mitochondria and other cellular elements are packed between the myofibrils.

There is a regular arrangement of contractile proteins within each sarcomere, with each thick filament being surrounded by six thin filaments (see Fig. 5.2 c and d). Contraction of muscle occurs as thick and thin filaments slide past each other ( Fig. 5.3 ) and is therefore accompanied by a decrease in the width of the light bands. The width of the dark bands remains unchanged.

Skeletal muscle function depends on a precise alignment of actin and myosin filaments within each myofibril. This is achieved by accessory proteins, which link the different components and hold them in register with each other. These proteins can only be visualized using immunohistochemical techniques.

ADVANCED CONCEPT

Myofibrils

Thin filaments are mainly formed from actin

Thin filaments are 8 nm in diameter and composed mainly of the protein actin. Each thin filament (F-actin) is formed by the polymerization of many single molecules of globular actin (G-actin). These actin filaments are polar, with all G-actin molecules pointing in the same direction.

To form a complete thin filament, two actin filaments become attached by their tail ends to α actinin in the Z line so that they face in opposite directions (i.e. away from the Z line).

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