As its name suggests this type of muscle is associated with the skeletal system, and it is responsible for movements of bones and joints. Skeletal muscle is also known as voluntary muscle as its contraction is usually under conscious control via its nerve supply. Skeletal muscle gives characteristic form to the body’s contour (along with adipose tissue). Although gross anatomy is beyond the scope of this book, it is important to appreciate that the attachment sites of a skeletal muscle to the skeleton, and the position of the muscle in relation to a joint, will determine the line of pull of the muscle and the type of movement that occurs at the joint as the muscle contracts.

Skeletal muscle cells are multinucleate, long and cylindrical in shape. They form by fusion of several cells (myoblasts) during development in utero. The nuclei of skeletal muscle cells are at the periphery of the sarcoplasm (Figs 5.1A and 5.2), a feature which allows them to be distinguished from other types of muscle cell. The contractile myofilaments (actin and myosin) within muscle cells are arranged as bundles (myofibrils) with their long axes along the length of the cells (Fig. 5.3) and each cell contains hundreds of myofibrils. The contractile proteins in the myofibrils overlap each other (Fig. 5.4A) and form a functional unit (a sarcomere). Aligned in this way the myofibrils refract light differently, hence the striped appearance (Fig. 5.2). The dark bands are known as A bands and the I bands are light.

In histological sections of skeletal muscle cells which have been sectioned along their length, transverse striations, reflecting the arrangement of the contractile proteins, are apparent (Fig. 5.2). If skeletal muscle cells are not sectioned along their length the striations are not apparent, but it is possible to recognise the type of muscle cell by the peripheral position of the nuclei (Figs 5.1A and 5.5). Numerous blood capillaries (Fig. 5.6) can be demonstrated in the connective tissue layers around muscle cells. The capillaries ensure a good blood supply, which provides the oxygen and nutrients necessary for muscle contraction.

Many skeletal muscles are anchored to bone by tendons (Chapter 4). The collagen fibres covering the muscle cells are continuous with the collagen of the tendon. In turn, bundles of collagen, known as Sharpey’s fibres, pass into the bone and anchor the tendon. Tendons take on a variety of forms, such that the attachment may be focal or broad. In some muscles the connective tissue surrounding the muscle cells attaches to a collagen (fascial) layer (Fig. 5.7), rather than to bone.

Variation occurs in the size of muscle cells, the number of mitochondria they contain and the enzymes present, as well as their content of myoglobin. Myoglobin, an oxygen-carrying protein, resembles haemoglobin and gives muscle a reddish colour. Such red muscle cells are predominant in muscles in which sustained contraction is required, e.g. to maintain posture. In contrast, in muscles where fast contraction and spurts of activity occur myoglobin is sparse and the muscles appear whitish to the naked eye.

This type of muscle, like skeletal muscle, appears striated (Figs 5.1B and 5.10) owing to the arrangement of the actin and myosin proteins in the sarcoplasm. Unlike skeletal muscle, cardiac muscle contraction is not under conscious control. Cardiac muscle cells are cylindrical and much shorter than skeletal muscle cells. Each cardiac muscle cell has a single, centrally placed nucleus and some cells branch (Fig. 5.1B). Specialised cell junctions (Chapter 2) link cardiac muscle cells end to end and these regions, called intercalated discs, are visible in histological sections (Fig. 5.10). Desmosomes and zonulae adherens anchor the muscle cells together at intercalated discs. In addition, there are gap junctions at these discs which ensure the excitation stimulating contraction spreads rapidly between muscle cells and results in their synchronised contraction. The mechanism of contraction is similar to that of skeletal muscle. The connective tissue associated with cardiac muscle (Fig. 5.10) is arranged in layers similar to those of skeletal muscle and it transmits the forces of the muscle contraction. The arrangement of connective tissue fibres forms a fibrous ‘skeleton’ for the heart. This arrangement helps to ensure that the contraction of heart muscle cells constricts the chambers of the heart in synchrony and it also anchors the heart valves and aids their function (Chapter 10).

Smooth muscle cells are fusiform in shape and tapered at each end with a centrally placed nucleus (Fig. 5.1C). Smooth muscle cells are usually grouped together and form sheets or bundles, and their appearance in histological sections depends on the angle at which they are sectioned (Figs 5.1C and 5.11) and their state of contraction (Fig. 5.12). Contraction of smooth muscle cells is not under conscious control and they are described as involuntary muscle cells. The contractile proteins in smooth muscle cells are arranged irregularly and some are linked with adherens-type junctions at intervals on the inner surface of the cell membrane. Each muscle cell is attached to an external lamina of connective tissue (endomysium) (Fig. 5.1C) and muscle cells are bundled together by layers of connective tissue (Fig. 5.11). Between some muscle cells the external lamina is incomplete and gap junctions link adjacent smooth muscle cells. The presence of gap junctions aids the spread of the excitatory stimulus for contraction and helps groups of cells to contract as a single unit. When smooth muscle cells contract they take on a corkscrew-like shape, which may be apparent as a change in shape of the nucleus (Fig. 5.12), and the force is transmitted via their external laminae to other surrounding connective tissue.