Types of movement
Movement is any change in position of the body or a part of it, like blinking. Locomotion is movement that takes the whole body from one place to another, like walking or swimming. All locomotion is movement, but not all movement is locomotion.
Three kinds of movement in the human body
- Amoeboid: cells push out pseudopodia (false feet) by streaming of cytoplasm, helped by microfilaments. Examples: macrophages and leucocytes in blood.
- Ciliary: many tiny hair-like cilia beat together. In our windpipe they sweep out dust and germs; in the oviduct they move the egg.
- Muscular: muscles contract to move limbs, jaws, tongue and the whole body. Needs muscles, bones and nerves working together.
Flagellar movement (one long whip-like flagellum) helps sperm swim and is also seen in Euglena. Paramecium uses cilia to move and to push food into its gullet.
Types of muscle
Muscle is a tissue that can contract, get excited by signals, stretch and spring back. It makes up about 40 to 50% of an adult's body weight.
| Type | Look | Control | Where |
|---|---|---|---|
| Skeletal | Striped (striated) | Voluntary | Attached to bones |
| Visceral (smooth) | No stripes | Involuntary | Walls of gut, blood vessels, bladder |
| Cardiac | Striped, branched | Involuntary | Heart only |
Structure of a skeletal muscle and the sarcomere
- A muscle is made of bundles called fascicles, held together by a tough layer, the fascia.
- Each fascicle has many muscle fibres. A fibre is one long cell with many nuclei (a syncytium). Its membrane is the sarcolemma, its cytoplasm the sarcoplasm, and its ER the sarcoplasmic reticulum, which stores Ca2+.
- Each fibre is packed with thread-like myofibrils with light and dark bands, so the muscle looks striped.
The sarcomere
The part of a myofibril between two Z lines is a sarcomere, the working unit of contraction.
- I band (isotropic, light): only thin actin filaments. The Z line runs through its middle.
- A band (anisotropic, dark): the full length of thick myosin filaments, overlapping with actin at both ends.
- H zone: the middle part of the A band where actin does not reach.
- M line: a thin line in the centre of the A band that holds myosin in place.
The contractile proteins
- Actin (thin filament): two F-actin strands twisted together (each made of G-actin units), with tropomyosin running along them and troponin at intervals. At rest, troponin covers the myosin-binding sites on actin.
- Myosin (thick filament): many myosin units (meromyosins). Each has a head and short arm (heavy meromyosin) that sticks out as a cross arm, and a tail (light meromyosin). The head has an ATPase site and an actin-binding site.
Mechanism of muscle contraction: sliding filament theory
- A motor neuron sends a signal. At the neuromuscular junction it releases acetylcholine, which starts an action potential in the sarcolemma. One motor neuron with the fibres it controls is a motor unit.
- The signal spreads and the sarcoplasmic reticulum releases Ca2+.
- Ca2+ binds to troponin, which moves tropomyosin and uncovers the binding sites on actin.
- The myosin head, energised by breaking ATP → ADP + Pi, binds to actin and forms a cross bridge.
- The head tilts and pulls actin toward the centre (the power stroke); ADP and Pi are released.
- A new ATP binds, the cross bridge breaks, the head is re-energised and the cycle repeats.
Result: actin slides over myosin, Z lines come closer, the sarcomere shortens. The I band and H zone shorten, while the A band stays the same. When Ca2+ is pumped back, the sites are covered again and the muscle relaxes.
Red and white fibres; fatigue
- Red fibres: much myoglobin (red, stores O2), many mitochondria: aerobic, slow to tire.
- White fibres: little myoglobin, few mitochondria but lots of sarcoplasmic reticulum: fast, anaerobic.
- Repeated contraction without enough O2 makes lactic acid build up, causing fatigue.
The human skeletal system
The adult skeleton has 206 bones and some cartilages. It is divided into two parts.
Axial skeleton (80 bones)
- Skull (22): 8 cranial bones protect the brain; 14 facial bones. Plus the U-shaped hyoid (1) and 3 ear ossicles in each middle ear (malleus, incus, stapes) = 6. The skull joins the spine through two occipital condyles (dicondylic skull).
- Vertebral column (26): cervical 7, thoracic 12, lumbar 5, sacral 1 (5 fused), coccygeal 1 (fused). It protects the spinal cord. Mammals, including giraffes, have 7 cervical vertebrae.
- Sternum (1) and ribs (12 pairs = 24). The first 7 pairs join the sternum directly (true ribs). The 8th, 9th and 10th join through cartilage of the 7th rib (false or vertebrochondral ribs). The 11th and 12th are free in front (floating ribs). The ribs, spine and sternum make the rib cage.
Appendicular skeleton (126 bones)
- Each arm (30): humerus, radius, ulna, carpals (8), metacarpals (5), phalanges (14).
- Each leg (30): femur (longest bone), tibia, fibula, tarsals (7), metatarsals (5), phalanges (14), and the knee cap, patella.
- Pectoral girdle (each side: clavicle + scapula) joins the arms. The scapula has a cup, the glenoid cavity, for the head of the humerus.
- Pelvic girdle (two coxal bones, each from ilium, ischium and pubis) joins the legs. The cup for the femur is the acetabulum. The two halves meet in front at the pubic symphysis.
Joints
A joint is where two or more bones (or bone and cartilage) meet. Muscles pull on bones, and the joint acts as the fulcrum.
- Fibrous joints: no movement; bones joined by dense fibres. Example: sutures between skull bones.
- Cartilaginous joints: little movement; bones joined by cartilage. Example: joints between adjacent vertebrae.
- Synovial joints: free movement; a fluid-filled synovial cavity between the bones.
- Ball and socket: humerus with scapula (shoulder); femur with pelvis (hip). Moves in all directions.
- Hinge: knee, elbow. Bends one way.
- Pivot: between atlas and axis (turning the head).
- Gliding: between carpals.
- Saddle: between carpal and metacarpal of the thumb.
Disorders of the muscular and skeletal system
- Myasthenia gravis: an autoimmune disease that harms the neuromuscular junction, causing tiredness, weakness and paralysis of skeletal muscle.
- Muscular dystrophy: gradual breakdown of skeletal muscle, mostly genetic.
- Tetany: fast, painful spasms (cramps) in muscle due to low Ca2+ in body fluid.
- Arthritis: swelling (inflammation) of joints.
- Osteoporosis: age-related loss of bone mass, so bones break easily. A fall in oestrogen is a common cause in women.
- Gout: swelling of joints due to build-up of uric acid crystals.
Key formulas and definitions
- Sarcomere = distance between two Z lines
- On contraction: A band constant; I band ↓, H zone ↓, sarcomere ↓
- Total shortening = number of sarcomeres × shortening per sarcomere
- Bones: 206 = axial 80 + appendicular 126
- Vertebrae: C7 + T12 + L5 + S1 + Co1 = 26
- Each limb: 30 bones
Worked examples
1. A sarcomere shortens from 2.5 µm to 2.1 µm. A myofibril has 10,000 sarcomeres in series. How much does it shorten?
Per sarcomere = 0.4 µm. Total = 10,000 × 0.4 = 4000 µm = 4 mm.
2. A band = 1.6 µm, sarcomere = 2.4 µm. Find the total I band length in this sarcomere.
I band (both halves together) = sarcomere − A band = 2.4 − 1.6 = 0.8 µm (0.4 µm on each side).
3. Each actin filament is 1.0 µm long. The sarcomere is 2.4 µm. Find the H zone.
H zone = sarcomere − 2 × actin = 2.4 − 2.0 = 0.4 µm.
4. Count the bones in the axial skeleton.
Skull 22 + hyoid 1 + ear ossicles 6 + vertebrae 26 + sternum 1 + ribs 24 = 80.
5. Count the bones in one arm.
Humerus 1 + radius 1 + ulna 1 + carpals 8 + metacarpals 5 + phalanges 14 = 30.
6. Count the appendicular bones and check the total of 206.
Arms 2 × 30 = 60, legs 2 × 30 = 60, pectoral girdles 2 × 2 = 4, pelvic girdle 2. Total = 126. 80 + 126 = 206.
7. A muscle fibre shortens by 20% of its resting length of 3 cm. What is its new length?
Shortening = 0.2 × 3 = 0.6 cm. New length = 3 − 0.6 = 2.4 cm.
8. A runner's leg muscle gets tired and sore after a sprint. Explain.
White fibres work anaerobically in a sprint. Without enough O2, glycogen breaks down to lactic acid, which builds up and causes fatigue.
Common mistakes
- Saying the A band shortens during contraction. The A band stays the same; the I band and H zone shorten.
- Thinking actin and myosin themselves get shorter. They slide past each other; their lengths do not change.
- Calling the knee a ball-and-socket joint. The knee is a hinge; the shoulder and hip are ball and socket.
- Mixing up tetany (low Ca2+ spasms) with tetanus (a bacterial infection).