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

contraction spreads from cell to cell across the intercalated discs, which means that cells do not need to be stimulated individually.

Figure 3.23  Cardiac muscle fibres.
    The heart has an intrinsic pacemaker system, which means that it beats in a coordinated manner without external nerve stimulation, although the rate at which it beats is influenced by autonomic nerve impulses, some hormones, local metabolites and other substances (see Ch. 5 ).

    Nervous tissue
    Two types of tissue are found in the nervous system:
    •excitable cells – these are called neurones and they initiate, receive, conduct and transmit information
    •non-excitable cells – also known as glial cells , these support the neurones.
    These are described in detail in Chapter 7 .

    Tissue regeneration
    The extent to which regeneration is possible depends on the normal rate of turnover of particular types of cell. Those with a rapid turnover regenerate most effectively. There are three general categories:
    •tissues in which cell replication is a continuous process regenerate quickly – these include epithelial cells of, for example, the skin, mucous membrane, secretory glands, uterine lining and lymphoid tissue
    •other tissues retain the ability to replicate, but do so infrequently; these include the liver, kidney, fibroblasts and smooth muscle cells. These tissues take longer to regenerate
    •some tissues are normally unable to replicate including nerve cells (neurones) and skeletal and cardiac muscle cells. Extensively damaged tissue is usually replaced by fibrous tissue, meaning that the functions carried out by the original tissue are lost.

    Membranes

    Epithelial membranes
    These membranes are sheets of epithelial tissue and supporting connective tissue that cover or line many internal structures or cavities. The main ones are mucous membrane, serous membrane and the skin (cutaneous membrane, see Ch. 14 ).
    Mucous membrane
    This is the moist lining of the alimentary, respiratory and genitourinary tracts which is sometimes referred to as the mucosa . The membrane surface consists of epithelial cells, some of which produce a secretion called mucus , a slimy tenacious fluid. As it accumulates the cells become distended and finally burst, discharging the mucus onto the free surface. As the cells fill up with mucus they have the appearance of a goblet or flask and are known as goblet cells (see Fig. 12.5, p. 282 ). Organs lined by mucous membrane have a moist slippery surface. Mucus protects the lining membrane from drying, and mechanical and chemical injury. In the respiratory tract it traps inhaled foreign particles, preventing them from entering the alveoli of the lungs.
    Serous membrane
    Serous membranes, or serosa , secrete serous watery fluid. They consist of a double layer of loose areolar connective tissue lined by simple squamous epithelium. The parietal layer lines a cavity and the visceral layer surrounds organs (the viscera) within the cavity. The two layers are separated by serous fluid secreted by the epithelium. There are three sites where serous membranes are found:
    •the pleura lining the thoracic cavity and surrounding the lungs ( p. 243 )
    •the pericardium lining the pericardial cavity and surrounding the heart ( p. 79 )
    •the peritoneum lining the abdominal cavity and surrounding abdominal organs ( p. 280 ).
    The serous fluid between the visceral and parietal layers enables an organ to glide freely within the cavity without being damaged by friction between it and adjacent organs. For example, the heart changes its shape and size during each beat and friction damage is prevented by the arrangement of pericardium and its serous fluid.
    Synovial membrane
    This membrane lines the cavities of moveable joints and surrounds tendons that could be injured by rubbing against bones, e.g. over the wrist joint. It is not an epithelial membrane, but instead consists of areolar connective tissue and elastic fibres.
    Synovial membrane