12 Muscle

Structure of muscle

The muscular system consists of many muscles through which the movements of the body are carried out. Voluntary muscles are attached to bones, cartilages, ligaments, skin or to other muscles by fibrous structures called tendons and aponeuroses. The individual fibres of voluntary muscle with their sheaths of sarcolemma are bound together into bundles by the endomysium and are covered by the perimysium. The bundles, or fasciculi, are bound together by a denser covering called the epimysium and these groups form the individual voluntary muscles of the body (see Fig. 3.8). All muscles have a good blood supply from nearby arteries. Arterioles in the perimysium give off capillaries which run in the endomysium and across the fibres. Blood vessels and nerves enter the muscle together at the hilum.

Most muscles have tendons at one or both ends. Tendons are made of fibrous tissue and are usually cord-like in appearance, though in some flat or sheet-like muscles the cord is replaced by a thin, strong fibrous sheet called an aponeurosis. Fibrous tissue also forms a protective covering or muscle sheath, known as fascia.

Where one muscle is attached to another the fibres may interlace, the perimysium of one fusing with the perimysium of the other, or the two muscles may share a common tendon. A third type of connection occurs in the muscles of the abdominal wall where the fibres of the aponeuroses interlace, forming the linea alba, which can be seen as a shallow groove above the umbilicus.

Action of muscle

When a muscle contracts, one end normally remains stationary while the other end is drawn towards it (Figs 12.1 and 12.2). The end that remains stationary is called the origin and that which moves is called the insertion. It is not uncommon, however, for a muscle to be used, as it were, the wrong way round so that the insertion remains fixed and the origin moves towards it. The gluteus maximus provides an illustration. Its origin is in the sacrum and it is inserted into the femur. When the insertion moves towards the origin the flexed thigh is extended; when the body is bent forward at the hips the standing position is regained by movement of the origin towards the insertion. This arrangement economizes on the number of muscles required and further economy is achieved by the placing of muscles so that they can carry out more than one action. Muscles must cross the joint they move; some cross two joints producing movement in both, e.g. the biceps crosses both elbow and shoulder, causing flexion of both.

Fig. 12.1 Bone movement as muscle contracts.

Fig. 12.2 A typical voluntary muscle showing contraction.

Muscles only act by contracting and pulling; they cannot push, though they can contract without shortening and so hold a joint firm and fixed in a certain position. When the contraction passes off, the muscles become soft but do not lengthen until stretched by the contraction of opposing muscles, known as antagonists.

Muscles never work alone – even the simplest movement requires the action of many muscles. Picking up a pencil requires the movement of fingers and thumb, wrist and elbow and possibly of the shoulder and trunk as the body leans forward. Each muscle must contract just sufficiently and each antagonist relax equally to allow the movement to take place smoothly without jerking. This concerted action of many muscles is termed muscle co-ordination. Any new action involving co-ordination requires time and practice until the new combination of muscle movement has been acquired and only then can it be carried out without great mental effort and concentration.

The sensory nerve gives ‘muscle sense’, which is not a very acute sensation but is sufficient to allow awareness of contraction and relaxation in the muscle. There is no awareness of this sensation until a conscious effort is made to relax or contract a muscle, at which time the previous degree of concentration becomes obvious. Under normal circumstances the muscles are in a state of partial contraction known as muscle tone; it is because of muscle tone that a position can be maintained for long periods without exhaustion. This is dependent on a mechanism whereby different groups of muscle fibres contract and relax in turn, giving periods of rest and activity to each group. The muscles having the highest degree of tonicity in humans are those of the neck and back.

Contraction of muscle

The composition of muscle is as follows:

• 75% water

• 20% protein

• 5% mineral salts, glycogen and fat.

Muscle contraction occurs as a result of nerve impulses. The nerve impulses, which are electrical, are transmitted to the muscle cells by chemical means and this is accomplished by the neuromuscular junction (Fig. 12.3). Nerve impulses arrive at the neuromuscular junction, which contains small packages of acetylcholine. This is released into the space between the nerve and the muscle, the synaptic cleft. When the acetylcholine attaches to the muscle cell it causes depolarization (see Chapter 4), and hence electrical activity spreads over the muscle cell leading to contraction.

Fig. 12.3 The neuromuscular junction.

In the disease myasthenia gravis, individuals release normal amounts of acetylcholine but the acetylcholine cannot attach to the muscle cells because of changes in the area of the muscle cell adjacent to the synaptic cleft. Individuals with myasthenia gravis have skeletal muscle weakness.

Energy is required for muscle fibres to contract; this is obtained from the oxidation of food, particularly carbohydrates. During digestion, carbohydrates are broken down to a simple sugar called glucose. The glucose that is not required immediately by the body is converted to glycogen and is stored in the liver and muscles. Muscle glycogen constitutes the source of heat and energy for muscular activity (Fig. 12.4). During the oxidation of glycogen to carbon dioxide and water, a compound is formed which is rich in energy. This compound is called adenosine triphosphate (ATP). When it is necessary for muscular contraction, the energy from ATP can be released as it changes to adenosine diphosphate (ADP). During the oxidation of glycogen, pyruvic acid is formed. If oxygen is plentiful, as it usually is during ordinary movement, pyruvic acid is broken down to carbon dioxide and water and, during the process, energy is released, which is used to make more ATP. If insufficient oxygen is available, the pyruvic acid is converted to lactic acid, which accumulates and produces muscle fatigue.

Fig. 12.4 Changes during muscle contraction. ATP, adenosine triphosphate; ADP, adenosine diphosphate.

Skeletal muscle is also known as striated (striped) muscle (see Chapter 3) as a result of its microscopic appearance. The striations are due to the structure of the proteins of which the muscle is composed. These proteins are called actin and myosin. When muscle is contracted the striations are narrower – this is thought to result from the movement of one protein relative to the other in what is described as the sliding filament theory (Fig. 12.5). One of the proteins (myosin) has projections which, with the expenditure of energy, allow it to ‘walk’ along the other protein, thus leading to contraction when the muscle is stimulated by an electrical impulse.

Fig. 12.5 The sliding filament theory.

During violent exercise more oxygen is brought to the muscles but, even so, not enough oxygen reaches the muscle cells, particularly at the beginning of the effort. Lactic acid accumulates and diffuses into the tissue fluid and blood. The presence of lactic acid in the blood stimulates the respiratory centre, and the rate and depth of respiration are increased. This continues, even after the exercise is over, until sufficient oxygen has been taken in to allow the cells of the muscles and the liver to oxidize the lactic acid completely, or convert it to glycogen. The extra oxygen needed to remove the accumulated lactic acid is called the ‘oxygen debt’, which must be repaid after the exercise is completed.

Chief muscles of the body

Muscles of the head

The muscles of the head (Fig. 12.6) are divided into two groups according to their function: the muscles of expression and the muscles of mastication.