Chapter Summary and Practice

Movement and locomotion are connected but distinct

Movement includes changes in cellular shape, organ position and limb posture; locomotion is a movement that carries an organism from one place to another. Paramecium uses cilia to swim and to move food, while Hydra uses body and tentacle movements for several tasks. Cells of the human body show amoeboid movement through pseudopodia, ciliary movement on selected epithelial surfaces and muscular movement through specialised contractile cells. Human locomotion requires coordinated muscular, skeletal and neural systems, but a muscle can also act to hold a posture without changing location.

Skeletal muscles are striated and under voluntary somatic motor control; visceral smooth muscles are nonstriated and generally involuntary; cardiac muscle is striated, branching and involuntary. Their four major properties are excitability, contractility, extensibility and elasticity. The appearance and control characteristics must be considered separately: cardiac muscle is not voluntary merely because it is striated, and smooth muscle does not lack neural or hormonal influences just because its action is involuntary.

Skeletal muscle has a nested structural hierarchy

A muscle organ contains fascicles of muscle fibres held together by connective tissue. A fibre is an elongated multinucleate cell bounded by sarcolemma; sarcoplasmic reticulum stores Ca²⁺ needed for contraction. Within the fibre lie parallel myofibrils composed of repeating sarcomeres. Each sarcomere is bounded by two Z lines. Thin actin filaments extend from Z lines toward the centre and thick myosin filaments lie centrally, aligned near the M line. Their regular spatial arrangement creates the striated microscopic pattern.

The A band is the full region occupied by thick myosin filaments, including parts overlapped by thin actin. The I band contains only thin filaments and is bisected by a Z line. The H zone is the central region of the A band with thick myosin but no thin-filament overlap in a resting sarcomere. During active sliding the Z lines approach, the I band and H zone narrow, but the A band retains its length. Neither actin nor myosin monomers become shorter: filament overlap increases.

Contractile proteins regulate exposure and produce work

The thin filament contains two helical F-actin strands built from G-actin monomers, together with tropomyosin and troponin. At rest, regulatory proteins hinder myosin binding. Calcium binding to troponin moves tropomyosin, uncovering sites on actin. Thick filaments contain assembled myosin molecules with projecting heads that possess ATPase and actin-binding functions. Myosin heads form temporary cross-bridges and use ATP reaction energy to move actin. The functional role of a myosin head differs from its tail, which participates in thick-filament assembly.

A motor-neuron impulse releases acetylcholine at a neuromuscular junction, triggering a muscle action potential. This leads to Ca²⁺ release from sarcoplasmic reticulum. Calcium binds troponin, exposing actin binding sites. A primed myosin head attaches, performs a power stroke and releases products; new ATP permits detachment, and ATP hydrolysis re-primes the head. Repetition slides thin filaments inward. When Ca²⁺ is pumped back into storage, thin-filament regulation returns towards rest and cross-bridge cycling stops. ATP is essential both for force production and relaxation.

Red and pale muscle fibres differ in metabolism

Myoglobin is an oxygen-binding red pigment in skeletal muscle. Red oxidative fibres tend to have more myoglobin, more mitochondria and relatively strong fatigue resistance during sustained work. Paler fibres tend towards rapid glycolytic ATP production and may fatigue faster under prolonged intense activity. These descriptions are teaching categories, not an absolute division: real skeletal muscles have mixed and intermediate fibre types. Repeated activity may be associated with lactate formation, but fatigue is influenced by several biochemical and neural mechanisms rather than by lactate alone.

Skeletal system counts follow a clear anatomical scheme

A typical adult has 206 bones, divided into 80 axial and 126 appendicular bones. Axial elements include 22 skull proper bones (8 cranial and 14 facial), one hyoid, six middle-ear ossicles, 26 adult vertebral units, 24 ribs and one sternum. The vertebral column is arranged as seven cervical, twelve thoracic, five lumbar, a fused sacrum and a fused coccyx. The skull meets atlas through two occipital condyles. Among the twelve rib pairs, first seven are true, next three vertebrochondral false and last two floating.

Each upper limb contains thirty bones: humerus, radius, ulna, eight carpals, five metacarpals and fourteen phalanges. Each lower limb also has thirty: femur, tibia, fibula, patella, seven tarsals, five metatarsals and fourteen phalanges. Both pairs of limbs contribute 120. The pectoral girdle contributes two scapulae and two clavicles (four) and the pelvic girdle contributes two coxal bones, reaching 126 appendicular bones. The shoulder's glenoid cavity receives the humerus and the hip's acetabulum receives the femur.

Joint type predicts possible movement

Fibrous joints join bones through dense connective tissue and have little mobility, as in cranial sutures. Cartilaginous joints join bones through cartilage, as between adjacent vertebral bodies and at the pubic symphysis, allowing limited movement. Synovial joints have cartilage-covered surfaces, a lubricated cavity, capsule and supporting structures, allowing substantial movement. Ball-and-socket examples include shoulder and hip; a hinge joint occurs between phalanges and at the modified hinge of the knee; atlas–axis provides a pivot, intercarpal joints glide and the thumb's first carpometacarpal joint is saddle-shaped.

Six disorders disturb different structural levels

Myasthenia gravis is an autoimmune neuromuscular transmission disorder producing fatigable weakness. Muscular dystrophy includes mainly inherited conditions leading to progressive muscle fibre degeneration. Tetany describes involuntary spasms commonly related to low ionised extracellular Ca²⁺, not the same as tetanus infection. Arthritis broadly refers to joint inflammation, whereas gout is a specific crystal-associated inflammatory arthritis caused by urate deposits. Osteoporosis reduces bone strength and raises fracture risk, commonly influenced by age and reduced oestrogen. Each disorder affects a different part of the movement system or mechanism.

Use source-style reasoning, not isolated recall

A good explanation of a sarcomere identifies two Z lines, the M line, A band, I band and H zone and connects filament movement to band change. A contraction question should proceed from acetylcholine to Ca²⁺ release, troponin activation, ATP-powered cross-bridge cycling and Ca²⁺ reuptake. A bone-count question should state which fused and separate elements are included. A joint identification question needs the pair of bones or surfaces, not just a body part. A disorder question should identify the biological site before discussing symptoms.

The following practice set covers the source's drawing, true/false, matching, comparison, joint-identification and fill-in exercises. It also adds a small number of integrated questions that test the cause-and-effect relations between muscle structure, energy supply and joint movement. Where an illustration is requested, draw a labelled schematic rather than copying an unlabelled figure. In every explanation distinguish what a diagram directly shows from the underlying physiological mechanism it illustrates.

Quick Quiz

Model Answers

1. Draw two Z lines with thin actin entering from each side; thick myosin at centre aligned to M line. Label A band (full myosin length), I band (thin-only), H zone (central myosin-only) and sarcomere between Z lines.

2. The theory states that contraction results from thin actin filaments sliding over thick myosin filaments, increasing overlap and shortening the Z-to-Z sarcomere distance without shortening the individual filaments.

3. Motor neuron signal → acetylcholine release → muscle action potential → SR Ca²⁺ release → troponin activation and binding-site exposure → myosin cross-bridge ATP cycle → actin sliding and Z-line approach. Calcium reuptake and cessation of cycling enable relaxation.

4. True; False—H zone is myosin-only in resting sarcomere; True for standard adult count; False—there are 12 pairs; True—the sternum is on anterior/ventral midline.

5. Actin provides thin filaments and myosin-binding sites, regulated by troponin/tropomyosin. Myosin forms thick filaments whose ATPase motor heads bind actin and generate power strokes.

6. Red fibres tend to have more myoglobin and mitochondria and high oxidative endurance; pale fibres tend to have less myoglobin, greater rapid glycolytic contribution and earlier fatigue during sustained high-power work.

7. Pectoral: clavicle/scapula halves, glenoid cavity articulating with humerus, substantial shoulder mobility. Pelvic: two coxal bones fused from ilium/ischium/pubis with acetabulum for femur and pubic symphysis, optimised for load bearing.

8. Smooth muscle → involuntary; tropomyosin → thin filament; red muscle → myoglobin; skull → sutures.

9. Amoeboid movement (e.g., leukocytes), ciliary movement (e.g., epithelial surfaces of airways and reproductive tubes) and muscular movement (e.g., limbs and other structures moved by muscle).

10. Skeletal fibres are long multinucleate voluntary striated cells controlled by somatic motor neurons. Cardiac cells are branching striated involuntary cells with specialised connections and intrinsic rhythmic activity.

11. Atlas–axis pivot; thumb first carpometacarpal saddle; interphalangeal hinge; femur–acetabulum ball-and-socket; cranial bones fibrous sutures; pubic bones cartilaginous symphysis.

12. 7; 14; tropomyosin and troponin; sarcoplasmic reticulum; 11th and 12th; 8.

13. The A band spans the full length of unchanged thick myosin filaments. Actin slides farther inward, shortening I bands and H zone while Z-line distance falls.

14. Axial 80 = skull 22 + ossicles 6 + hyoid 1 + vertebral column 26 + ribs 24 + sternum 1. Appendicular 126 = four limbs × 30 + pectoral girdle 4 + pelvic girdle 2. Total = 206.

15. ATP hydrolysis primes myosin and powers cycling; new ATP facilitates cross-bridge detachment. ATP also drives sarcoplasmic-reticulum calcium pumps during relaxation.

16. Myasthenia gravis disrupts neuromuscular transmission through autoimmunity; dystrophies cause progressive muscle degeneration often due to genetic defects; tetany involves involuntary spasms associated with low extracellular Ca²⁺.

17. Arthritis is joint inflammation broadly; osteoporosis is decreased bone strength and increased fracture risk; gout is a urate-crystal-triggered inflammatory arthritis.

18. The rib cage protects thoracic organs yet its joints and muscles permit breathing-related motion; the vertebral column protects the spinal cord but permits limited bending. Structural design balances stability and flexibility.