Muscular tissue

Within the body there are three varietes of muscle: 1) smooth muscle, also referred to as involuntary or non-striated muscle; 2) cardiac muscle, and 3) skeletal muscle, also known as voluntary or striated muscle. Smooth muscle forms the muscular layer of the walls of blood vessels and of hollow organs such as the stomach. It is not under voluntary control, contracting slower and less powerfully than skeletal muscle. However, it is able to maintain its contraction longer. Cardiac muscle is also not under voluntary control, but although it exhibits striations it is considered to be different from skeletal muscle.

Skeletal muscle

Skeletal muscle constitutes over one-third of the total human body mass. It consists of non-branching striated muscle fibers, bound together by loose areolar tissue. Muscles have various forms; some are flat and sheet-like, some are short and thick, while others are long and slender. The length of a muscle, exclusive of tendons, is closely related to the distance through which it is required to contract. Experiment has shown that muscle fibres have the ability to shorten to almost half their resting length. Consequently, the arrangement of fibres within a muscle determines how much it can shorten when it contracts. Irrespective of muscle fibre arrangement, it has to be remembered that all movement is brought about by muscle shortening, with the consequent action of pulling across joints changing the relative positions of the bones involved.

Muscle forms

The arrangement of the individual fibres within a muscle can be in one of two ways only; either parallel or oblique to the line of pull of the whole muscle. When the fibres are parallel to the line of pull they are grouped in a discrete bundle giving a fusiform muscle(figure a, example biceps brachii) or spread as a broad, thin sheet(figure b, example external oblique of the abdomen). When contraction occurs it does so through the maximal distance allowed by the length of the muscle fibres. However, it is of limited power.

Muscles whose fibres are oblique to the line of pull cannot shorten to the same degree, but because of the increased number of fibres packed into the same unit area they are much more powerful. Such arrangements of fibres are known as pennate, of which there are three main patterns(figure c). In unipennate muscles the fibres attach to one side of the tendon only(e.g. flexor pollicis longus). Bipennate muscles have a central septum with the muscle fibres attaching to both sides of the septum and to its continuous central tendon(e.g. rectus femoris). Finally, some muscles possess several intermediate septa, each of which has associated with it a bipennate arrangement of fibres. The whole is known as a multipennate muscle(e.g. deltoid).

Muscle structure

Muscle consists of many individual fibres, each being a long, cylindrical, multinucleated cell of varying length and width. Each fibre has a delicate connective tissue covering(endomysium), separating it from its neighbours, yet connecting them together. Bundles of parallel fibres(fasciculi) are bound together by a more dense connective tissue covering(perimysium). It is groups of fasciculi which are bound together to form whole muscles, and are enclosed in a fibrous covering(epimysium), which may be thick and strong or thin and relatively weak.

Muscle attachments

The attachment of muscle to bone or some other tissue is always via the connective tissue elements of the muscle. Sometimes the perimysium and epimysium unite directly with the periosteum of bone or with joint capsules. Where this connective tissue element cannot readily be seen, the muscle has a fleshy attachment and leaves no mark on the bone, although the area is often flattened or depressed. In many instances the connective tissue elements of the muscle fuse together to form a tendon, consisting of bundles of collagen fibres. There is, however, no direct continuity between the fibres of the muscle and those of the tendon. Tendons can take various forms, all of which are generally strong. They can be round cords, flattened bands or thin sheets, the latter being an aponeurosis. Attachments of tendon to bone nearly always leave a smooth mark; it is only when the attachment is by a mixture of fleshy and tendinous fibres, or when the attachment is via a long aponeurosis, that the bone surface is roughened.

Where a muscle or tendon passes over or around the edge of a bone it is usually separated from the bone by a bursa, which serves to reduce friction during movement. Bursae are sac-like dilatations which may communicate directly with an adjacent joint cavity or exist independently, and contain a fluid similar to synovial fluid.

When a tendon is subjected to friction it may develop a sesamoid bone. Once formed thse have the general effect of increasing the lever arm of the muscle, and act as pulleys enabling a slight change in the direction of pull of the muscle, e.g. the patella and the quadriceps tendon(figure a).

Because each end of a muscle attaches to different bones, observation of its principal action led to the designation of tone end being the origin and the other the insertion; the insertion being to the bone which showed the freest movement. Such a designation is however misleading, since the muscle can cause either of the two attachments to move relatively freely. The term attachment is therefore the preferred one to use.

Muscle action

When stimulated, a muscle contracts so as to bring its two ends closer together. If this is allowed to happen the muscle length obviously changes, although the tension generated remains more or less constant; such contractions are termed isotonic. If however, the length of the muscle remains unaltered(due to some externally applied force) then the tension it develops usually increases in an attempt to overcome the resistance; such contractions are termed isometric.

Isotonic contractions can be of two types, concentric, in which the muscle shortens, or eccentric, in which the muscle lengthens. Eccentric contraction occurs when the muscle is being used to control the movement of a body segment against an applied force.

When a muscle, or group of muscles, contracts to produce a specific movement, it is termed a prime mover. Muscles which directly oppose this action are called antagonists. Muscles which prevent unwanted movements associated with the action of the prime movers are known as synergists.

In all actions, part(often the larger) of the muscle activity is directed across the joint, thereby stabilizing it by pulling the two articular surfaces together.

When testing the action of a muscle to determine whether it is weakened or paralysed, the subject is usually asked to perform the principal action of the muscle against resistance. This may be insufficient to confirm the integrity of the muscle. The only infallible guide is to palpate over the muscle belly or its tendons to determine whether it is contracting during the manoeuvre.