Contraction

Muscle is stimulated to contract when it receives a nerve impulse from the central nervous system. Each striated muscle fibre is composed of myofibrils made of thin actin filaments and thick myosin filaments. These fibres overlap in such a way that under the microscope muscle has the appearance of alternating light and dark bands or striations. These bands are separated into units called sarcomeres, which are the units of contraction.

During contraction, the actin and myosin filaments slide over one another and cross-bridges form between the heads of the myosin filaments and the heads of the actin filaments. The cross-bridges swing through an arc, pulling the thin filaments past the thick ones, and the sarcomere shortens. Once this movement is completed the cross-bridge detaches itself from the thin filament and reattaches itself further away – in other words, the cross-bridges between the myosin and actin filaments act as a ratchet mechanism, thus shortening the muscle (Fig. 4.1). This process requires energy input, which is provided by adenosine triphosphate (ATP) molecules; calcium ions are also essential to the process of muscle contraction.

Fig. 4.1 The ‘ratchet mechanism’ involving actin and myosin within striated muscle fibres. A Muscle before contraction (shortening). B When the muscle shortens, the thick and thin filaments slide in between one another. The dark bands remain the same width in the shortened (contracted) muscle but the light bands and ‘H zones’ get narrower. C Close-up view of the ‘ratchet mechanism’, showing cross-bridges between the thin actin and thick myosin filaments.

The nerve that stimulates the muscle to contract enters the muscle and then splits up into many fibres to innervate the bundles of muscle fibres. The number of muscle fibres supplied by one nerve fibre will vary depending on the type of movement for which the muscle is responsible. If it is a delicate movement then a nerve fibre may only innervate a small number of muscle fibres. However, in larger movements, such as those made by the muscles of the limbs, one nerve fibre may supply 200 or more muscle fibres. A single nerve together with the muscle fibres that it supplies is called a motor unit.

Muscle tone

Many of the skeletal muscles in the body are always in a slight state of tension, known as muscle tone. Even when an animal is at rest, muscles, such as those responsible for maintaining posture, will not be truly relaxed. Muscle tone is achieved by a proportion of the motor units within the muscle being activated so that some of the muscle fibres are contracting while others are relaxed. The nervous system can adjust this, and the number of motor units stimulated will increase when the animal is in an anxious state, i.e. the muscles become ‘twitchy’ or ‘jumpy’. Thus, muscles undergo two types of contraction:

The more a muscle is used or exercised, the larger it will become – it is said to be hypertrophied. However, if a muscle is not used for some reason, e.g. due to injury or illness when the animal may be recumbent, or if a limb is in a cast, it will wither or shrink in size – it is said to be atrophied.

Anatomy of a muscle

A ‘classically’ shaped muscle (Fig. 4.2) has a thick, fleshy central part called the belly and tapers at each end – the head. Here, the connective tissue muscle sheath is continuous with the dense fibrous connective tissue of the tendon that attaches the muscle to a bone. A muscle is attached to a bone at two points: its starting point is called its origin; this moves least during contraction. The opposite end, where the muscle inserts on the bone, is called the insertion. However, a muscle can have more than one belly, all inserting at one point, in which case the muscle is said to have a number of heads, e.g. the biceps muscle has two heads. The length of the tendon attaching the muscle to a bone will vary, and in some cases tendons are far longer than the muscle itself, e.g. flexor and extensor tendons running over the digits.

Fig. 4.2 Basic structure of a muscle. The connective tissue between the muscle fibres is continuous with that of the tendon.

Not all muscles take the ‘classical’ shape described above. Sometimes they are present in flat sheets, in which case the tendon is also drawn out into a flat sheet of connective tissue – this type of arrangement is called an aponeurosis, e.g. the muscles of the abdominal wall. Some muscles form a circular ring and serve to control the entrance or exit to a structure, e.g. the stomach and the bladder. These are called sphincter muscles.

A bursa is a connective tissue sac lined with synovial membrane and filled with synovial fluid. These typically develop between a bony prominence and a tendon, ligament or muscle and their function is to reduce friction between the associated structure and the bone. Sometimes a bursa completely wraps around a tendon forming a synovial or tendon sheath (Fig. 4.3).

Fig. 4.3 A Structure of a bursa. B Structure of a synovial sheath.

The skeletal muscles of the body can be classed as either intrinsic or extrinsic:

Muscles of the head

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