Free Ross & Wilson Anatomy and Physiology in Health and Illness by Anne Waugh, Allison Grant

walls.

    Bone
    Bone cells (osteocytes) are surrounded by a matrix of collagen fibres strengthened by inorganic salts, especially calcium and phosphate. This provides bones with their characteristic strength and rigidity. Bone also has considerable capacity for growth in the first two decades of life, and for regeneration throughout life. Two types of bone can be identified by the naked eye:
    • compact bone – solid or dense appearance
    • spongy or cancellous bone – ‘spongy’ or fine honeycomb appearance.
    These are described in detail in Chapter 16 .

    Muscle tissue
    Muscle tissue is able to contract and relax, providing movement within the body and of the body itself. Muscle contraction requires an adequate blood supply to provide sufficient oxygen, calcium and nutrients and to remove waste products. There are three types of specialised contractile cells, also known as fibres : skeletal muscle, smooth muscle and cardiac muscle.

    Skeletal muscle tissue ( Fig. 3.21 )
    This type is described as skeletal because it forms those muscles that move the bones [of the skeleton], striated because striations (stripes) can be seen on microscopic examination and voluntary as it is under conscious control. In reality, movements can be finely coordinated, e.g. writing, but may also be controlled subconsciously. For example, maintaining an upright posture does not normally require thought unless a new locomotor skill is being learned, e.g. skating or cycling, and the diaphragm maintains breathing while asleep.

Figure 3.21  Skeletal muscle fibres. A. Diagram. B. Coloured scanning electron micrograph of skeletal muscle fibres and connective tissue fibres (bottom right).
    Fibres are cylindrical, contain several nuclei and can be up to 35 cm long. Skeletal muscle contraction is stimulated by motor nerve impulses originating in the brain or spinal cord and ending at the neuromuscular junction (see p. 411 ). The properties and functions of skeletal muscle are explained in detail in Chapter 16 .

    Smooth muscle tissue ( Fig. 3.22 )
    Smooth muscle may also be described as non-striated, visceral or involuntary . It does not have striations and is not under conscious control. Smooth muscle has the intrinsic ability to contract and relax. Additionally, autonomic nerve impulses, some hormones and local metabolites stimulate contraction. A degree of muscle tone is always present, meaning that smooth muscle is completely relaxed for only short periods. Contraction of smooth muscle is slower and more sustained than skeletal muscle. It is found in the walls of hollow organs:
    •regulating the diameter of blood vessels and parts of the respiratory tract
    •propelling contents of the ureters, ducts of glands and alimentary tract
    •expelling contents of the urinary bladder and uterus.

Figure 3.22.  Smooth muscle. A. Diagram. B. Fluorescent light micrograph showing actin, a contractile muscle protein (green), nuclei (blue) and capillaries (red).
    When examined under a microscope, the cells are seen to be spindle shaped with only one central nucleus. Bundles of fibres form sheets of muscle, such as those found in the walls of the above structures.

    Cardiac muscle tissue ( Fig. 3.23 )
    This type of muscle tissue is found only in the heart wall. It is not under conscious control but, when viewed under a microscope, cross-stripes (striations) characteristic of skeletal muscle can be seen. Each fibre (cell) has a nucleus and one or more branches. The ends of the cells and their branches are in very close contact with the ends and branches of adjacent cells. Microscopically these ‘joints’, or intercalated discs , can be seen as lines that are thicker and darker than the ordinary cross-stripes. This arrangement gives cardiac muscle the appearance of a sheet of muscle rather than a very large number of individual fibres. The end-to-end continuity of cardiac muscle cells has significance in relation to the way the heart contracts. A wave of