muscle tissue
Specific terms have been introduced to identify particular parts of muscle cells (which are also known as muscle fibres). The muscle cell membrane is known as the sarcolemma, the cytoplasm as the sarcoplasm and the smooth endoplasmic reticulum as the sarcoplasmic reticulum.
Types of muscle tissue
There are three types of muscle tissue classified by location and the arrangement of the contractile myofilaments in the muscle cells:
■ skeletal muscle contraction is involved, mainly, in moving the skeleton
■ cardiac muscle contraction moves blood through the heart
■ smooth muscle in the walls of many tubes and hollow organs (e.g. in some blood vessels and the urinary bladder) controls the flow of substances along or out of such structures.
Skeletal and cardiac muscle cells are referred to as striped or striated muscle as the contractile myofilaments are arranged in a repeating pattern in the sarcoplasm and this is manifested as transverse dark and light striations across the length of the muscle cells (Figs 5.1A,B and 5.2). The precise arrangement of the contractile myofilaments are revealed only by using an electron microscope (see below). Although smooth muscle cells contain actin and myosin myofilaments, they are not arranged in a regular pattern and smooth muscle cells do not display striations (Fig. 5.1C).
Other cell types with the ability to contract have also been detected using techniques which identify contractile proteins in their cytoplasm. These types include:
■ myoepithelial cells, which surround acini and ducts of some exocrine glands; their contraction aids the expulsion of secretions
■ myofibroblasts, which occur in many regions of connective tissue; they aid repair by secreting collagen and undergoing contraction during scar formation
■ pericytes, which surround capillaries; after injury they undergo mitosis and replace damaged fibroblasts and smooth muscle cells.
Muscle cells and connective tissue
Confusingly, muscle cells are also referred to as muscle fibres, probably because some are long, thin structures which measure several centimetres in length. (In contrast, connective tissue fibres, although long and thin, are protein molecules, not cells.)
The connective tissue associated with muscle cells consists of three covering layers (Figs 5.1 and 5.3). Each muscle cell is covered by, and bound to, fine connective tissue fibres which form an external (basal) lamina known as an endomysium. Several muscle cells are held together as bundles by connective tissue fibres known as the perimysium. The connective tissue covering a muscle itself, e.g. the biceps brachii, is called the epimysium; this layer is also known as deep fascia.
Contraction of a muscle, such as biceps brachii, involves the contractile proteins in individual muscle cells moving and shortening their overall length. This movement generates a force of contraction and it is transmitted to the sarcolemma and the external lamina. The laminae around adjacent muscle cells are connected to each other and to the other layers of connective tissue. In this way the force of muscle cell contraction is transmitted to all the connective tissue layers. Such contractile forces from muscle cells, applied to connective tissue fibres, is then transferred to other structures such as bone e.g. the humerus or soft tissue, e.g. the eyeball.
It is important to appreciate that when muscles stop contracting they passively relax and the contractile proteins resume their original position but this does not exert a force. Muscles can only contract (pull), never push!
Skeletal muscle
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.
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