Concepts: Disorders of muscles and bones, and exercise practice

5.1 Disorders of the muscular system

Tetany is a condition of rapid, wild contractions of the muscles caused by a low level of calcium ions in the blood. When the calcium falls, the muscle fibres become unusually easy to excite and they fire in spasms. Myasthenia gravis is an autoimmune disorder in which the body's own antibodies attack the junction between the motor nerve and the muscle, so the muscle receives a weaker signal and the person develops fatigue and weakness, especially of the eyelids and the limbs. Muscular dystrophy is a group of inherited disorders in which the skeletal muscle progressively degenerates and weakens; the commonest form appears in childhood and affects boys more often than girls. In all three, the problem lies in the muscle or in the signal that reaches it, not in the bone.

5.2 Disorders of the skeleton

Osteoporosis is an age related disorder in which the bone mass decreases and the bones become brittle, so a minor fall can break a bone, especially the hip, the wrist or the spine. The disorder is common in women after menopause because the fall in oestrogen speeds up the loss of bone. Gout is an inflammation of the joints caused by the accumulation of crystals of uric acid in the joint, and it is very painful; the big toe is a common site. Arthritis is inflammation of one or more joints, with pain, swelling and stiffness, and it has several forms including the wear and tear of the cartilage in old age. A person with any of these disorders is asked to keep the joints moving within limits, to maintain a healthy weight and calcium intake, and to follow the treatment advised by a doctor.

The contrasts that matter are simple: osteoporosis is a loss of bone mass, gout is a deposit of uric acid, arthritis is inflammation of the joint, and tetany is a calcium problem in the blood. Myasthenia gravis affects the neuromuscular junction and muscular dystrophy affects the muscle fibres themselves.

5.3 Exercise practice

The chapter has one exercise section of ten questions and all are represented across the pages: the sarcomere and the contractile proteins on page 1, the sliding filament mechanism on page 2, the skeleton and the rib cage on page 3, the girdles and joints on page 4, and the remaining comparisons and blanks here.

Exercise 1. A sarcomere is the stretch from one Z line to the next. On the sketch mark a Z line at each end, the I band on both sides of each Z line, the A band carrying the H zone in the middle, and the M line through the centre of the H zone. Label the thin actin filaments and the thick myosin filaments so that their overlap inside the A band is visible.

Exercise 2. The sliding filament theory states that muscle contraction happens because the thin actin filaments slide over the thick myosin filaments, pulled by the repeated forming and breaking of cross bridges. The filaments themselves do not shorten, but the sarcomere becomes shorter because the filaments overlap more.

Exercise 3. The important steps are: the signal arrives from the motor neuron and calcium is released from the sarcoplasmic reticulum; calcium binds troponin, which moves tropomyosin and exposes the actin sites; myosin heads bind actin and form cross bridges; the heads tilt and pull the actin; ATP breaks each bridge and re-cocks the head; and the cycle repeats until the calcium is pumped back and the muscle relaxes.

Exercise 4. (a) True, actin is present in the thin filament. (b) False, because the H zone contains only the thick filaments and no thin filaments. (c) True, the human skeleton has 206 bones. (d) False, because there are 12 pairs of ribs. (e) True, the sternum lies on the ventral side.

Exercise 5. Actin is the thin filament protein and myosin is the thick filament protein, and myosin has the ATPase head that binds actin. Red muscle fibres have plenty of myoglobin and mitochondria and resist fatigue, while white fibres have little myoglobin and tire quickly. The pectoral girdle has the clavicle and scapula and joins the upper limb to the sternum, while the pelvic girdle has two coxal bones and joins the lower limb to the sacrum; the pectoral girdle is lighter and more mobile and the pelvic girdle is heavier and stronger.

Exercise 6. Smooth muscle goes with involuntary, tropomyosin with the thin filament, red muscle with myoglobin, and the skull with sutures.

Exercise 7. The cells of the body show amoeboid movement in macrophages and leucocytes, ciliary movement in the ciliated epithelium, flagellar movement in the sperm, and muscular movement in the limbs and the body.

Exercise 8. Skeletal muscle is striated, voluntary and attached to bones, and it tires quickly; cardiac muscle is striated, involuntary and forms the wall of the heart, and it contracts rhythmically without rest.

Exercise 9. The joint between the atlas and axis is a pivot joint; between the carpal and metacarpal of the thumb it is a saddle joint; between the phalanges it is a hinge joint; between the femur and acetabulum it is a ball and socket joint; between the cranial bones it is a fibrous joint or suture; and between the pubic bones it is a cartilaginous joint.

Exercise 10. (a) Seven cervical vertebrae are found in almost every mammal. (b) A human limb carries fourteen phalanges. (c) The thin filament has two F actins plus tropomyosin and troponin. (d) The calcium of a muscle fibre sits in the sarcoplasmic reticulum. (e) The two lowest pairs, the eleventh and the twelfth, are the floating ribs. (f) Eight bones join to build the human cranium.