🦠 Locomotion & Movement Study Notes

4.1 Movement & Locomotion

Movement is a defining feature of all living things — even rooted plants and microbes move parts of themselves. When movement results in an animal changing its place (walking, running, climbing, flying, swimming), it is called locomotion.

The structures used for movement and locomotion are often the same: a fish's fins help it swim, our limbs help us walk. Locomotion is needed to find food, shelter, a mate, suitable breeding grounds, or to escape danger.

4.2 Types of Movement

Human cells show three main kinds of movement:

4.3 Muscle — Properties, Types & Structure

Muscle is a specialised tissue of mesodermal origin that makes up about 40–50% of adult body weight. It shows four properties: excitability, contractility, extensibility and elasticity.

Organisation of skeletal muscle:

Each myofibril shows alternating dark A bands (anisotropic, thick myosin) and light I bands (isotropic, thin actin). A Z line bisects each I band; the region between two Z lines is the sarcomere — the functional unit of contraction. The central thin-filament-free part of the A band is the H zone, crossed by the M line.

4.4 Contractile Proteins

Thin filament (Actin): two helical F-actin strands (each a polymer of globular G-actin), with two tropomyosin filaments alongside and troponin at intervals. At rest, a troponin subunit masks the actin binding sites for myosin.

Thick filament (Myosin): polymerised meromyosin. Each meromyosin has a globular head + short arm (heavy meromyosin, HMM) that projects out as the cross-bridge, plus a tail (light meromyosin, LMM). The head carries ATPase and binding sites for ATP and actin.

4.5 Mechanism of Muscle Contraction

Contraction is explained by the sliding filament theory: thin filaments slide over thick filaments, so the sarcomere shortens while the A band stays the same length.

• A new ATP binding to the myosin head breaks the cross bridge; the head re-cocks for another cycle.
• I bands and the H zone shorten; the A band is constant.
• Relaxation: Ca²⁺ is pumped back into the sarcoplasmic reticulum, troponin re-masks the sites and the muscle lengthens.
• Repeated stimulation → fatigue from lactic acid (anaerobic glycolysis).

4.6 Skeletal System

The skeleton is a framework of bones and cartilages — 206 bones in an adult, split into the axial (80) and appendicular (126) skeleton.

• Skull: only the lower jaw (mandible) is movable; the skull rests on the first vertebra, the atlas.
• Vertebral column (26): cervical 7, thoracic 12, lumbar 5, sacrum 1 (fused), coccyx 1 (fused). It protects the spinal cord and anchors the ribs.
• Ribs: 1–7 true (joined to sternum by cartilage), 8–10 false (joined to the 7th rib's cartilage), 11–12 floating (no ventral attachment). Ribs + thoracic vertebrae + sternum = the rib cage.

4.7 Joints

Joints are points of contact between bones (or bone and cartilage). They are essential for movement, since bones act as levers moved by muscles.

4.8 Disorders of the Muscular & Skeletal System

• Myasthenia gravis: autoimmune disorder of the neuromuscular junction → fatigue, weakening and paralysis of skeletal muscle.
• Muscular dystrophy: genetic; progressive degeneration of skeletal muscle.
• Tetany: rapid, wild muscle spasms from a fall in calcium ions in body fluid.
• Arthritis: inflammation of joints.
• Osteoporosis: age-related fall in bone mass and higher fracture risk; often from decreased oestrogen.
• Gout: joint inflammation from accumulation of uric acid crystals.