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The Musculoskeletal System

Chapter 11: The Musculoskeletal System

3 min read Updated Mar 26, 2026
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🎯 Diagnostic: Test Your Starting Level 24 questions (2 per section). No prior reading required, see what you already know.

Aim to answer every question before checking. Missed questions point you to the sections you need most.

1. (11.1) Compact bone is organized into structural units called:
B. Each osteon is a concentric ring of lamellae around a central Haversian canal that carries blood vessels. Lacunae house osteocytes; canaliculi connect them.
2. (11.1) The bone matrix consists primarily of:
C. Organic collagen gives tensile strength; inorganic hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) gives compressive strength.
3. (11.2) Osteoblasts function to:
D. Osteoblasts lay down collagen-rich osteoid and alkaline phosphatase activity mineralizes it. When trapped in matrix, they become osteocytes.
4. (11.2) Osteoclasts are:
A. Osteoclasts secrete acid and proteases into a sealed lacuna on the bone surface, dissolving mineral and digesting matrix during remodeling.
5. (11.3) Long-bone lengthening during growth occurs at the:
B. Cartilage in the growth plate proliferates on the epiphyseal side and ossifies on the diaphyseal side. Fusion of the plate ends growth in height.
6. (11.3) Which hormone raises blood calcium by driving bone resorption?
C. PTH activates osteoblasts to produce RANKL, which stimulates osteoclast formation. Calcitonin does the opposite (opposes resorption).
7. (11.4) Hyaline cartilage is characterized by:
D. Hyaline cartilage is the most abundant cartilage type. The trachea, costal cartilages, and joint surfaces use it for smooth gliding.
8. (11.4) Fibrocartilage is found in:
A. Fibrocartilage has thick type I collagen bundles for toughness under load. Elastic cartilage is in the ear and epiglottis; hyaline is in joints and trachea.
9. (11.5) Synovial joints feature:
B. Synovial joints are freely movable. Synovial fluid (made by the synovial membrane) lubricates and nourishes the avascular articular cartilage.
10. (11.5) Ligaments connect:
C. Ligaments (bone to bone) stabilize joints; tendons (muscle to bone) transmit force. Both are dense regular collagen.
11. (11.6) A sarcomere is defined as the region:
D. Sarcomeres are lined up end-to-end along the myofibril. Their alternating thick and thin filaments give striated muscle its banded look.
12. (11.6) Thick filaments in skeletal muscle are composed of:
A. Myosin II hexamers with two heads form the thick filament. Thin filaments are F-actin with troponin-tropomyosin regulatory proteins.
13. (11.7) The sliding filament model states that contraction occurs because:
C. Myosin heads attach to actin, rotate, release using ATP, and re-attach further along, dragging the thin filament toward the M-line.
14. (11.7) Which sarcomere zones shorten during contraction?
B. The A band equals the length of thick filaments and does not change. H (thick only) and I (thin only) zones shrink as overlap increases.
15. (11.8) Ca²⁺ triggers skeletal muscle contraction by:
A. With tropomyosin moved aside, the myosin head can now bind actin and power-stroke. In smooth muscle, Ca²⁺ acts through calmodulin/MLCK instead.
16. (11.8) Ca²⁺ used in excitation-contraction coupling is stored in:
D. T-tubule depolarization opens ryanodine receptors on the SR, flooding the cytosol with Ca²⁺. SERCA then pumps it back to relax the muscle.
17. (11.9) Type I (slow-twitch) muscle fibers:
C. "Slow red oxidative" fibers dominate postural muscles and distance runners. Type II fibers are faster and favor glycolytic metabolism.
18. (11.9) Type IIx (fast-twitch glycolytic) fibers:
B. Think sprinter: few mitochondria, pale color, fast firing, but tired in seconds.
19. (11.10) Cardiac muscle differs from skeletal muscle because:
A. Intercalated discs contain gap junctions (for electrical coupling) and desmosomes (for mechanical linkage), enabling the myocardium to contract as a syncytium.
20. (11.10) Smooth muscle is best described as:
D. Smooth muscle has no sarcomeres. Actin attaches to dense bodies, and MLCK-phosphorylated myosin slowly pulls, allowing sustained tone with little ATP.
21. (11.11) Acetylcholine released at the neuromuscular junction binds:
C. Nicotinic ACh receptors are ligand-gated cation channels. Curare blocks them (paralysis); myasthenia gravis features autoantibodies against them.
22. (11.11) Acetylcholinesterase in the synaptic cleft:
B. AChE hydrolyzes ACh to choline and acetate. Organophosphates (nerve agents, pesticides) inhibit AChE and cause continuous muscle activation.
23. (11.12) The epidermis is composed primarily of:
D. Epidermal layers (basale → spinosum → granulosum → lucidum → corneum) reflect keratinocyte maturation, creating a tough keratinized barrier.
24. (11.12) Melanocytes in the epidermis function to:
A. Melanocytes in the stratum basale transfer melanosomes to keratinocytes. Melanin shields DNA from UV damage and gives skin its pigmentation.

Try this right now: make a fist and squeeze as hard as you can. In the two seconds it took you to do that, your brain fired an electrical signal down a motor neuron, released a chemical messenger across a tiny gap, triggered calcium to flood out of storage compartments inside your muscle cells, and set off a cascade where millions of molecular motors ratcheted protein filaments past each other like tiny rowing teams - all to produce the force you just felt in your hand.

Now open your hand. That required its own set of signals, energy, and molecular machinery. Every movement you make - from blinking to sprinting - depends on the seamless coordination of bones providing leverage, joints allowing motion, and muscles generating force. Break any link in that chain, and movement stops.

This chapter covers the two systems that make movement possible. The skeletal system provides the rigid framework: bones that store minerals, produce blood cells, and act as levers. The muscular system provides the engine: contractile tissue that converts chemical energy (ATP) into mechanical force. We will also cover the skin, the body’s largest organ, which protects everything underneath.

The Construction Site and the Engine

Think of your body as a building under permanent construction. Your bones are the steel beams - they provide structure, but they are not static. A crew of construction workers (osteoblasts) is constantly laying down new bone, while a demolition crew (osteoclasts) tears old bone apart. The balance between building and demolition determines whether your skeleton gets stronger or weaker over time.

Your muscles are the engines bolted to that steel frame. They do not push - they can only pull. Every movement requires one muscle pulling in one direction and an opposing muscle pulling the other way. Inside each muscle cell, the actual force comes from protein filaments sliding past each other, powered by ATP. It is an elegant, repeating cycle that looks a lot like rowing a boat.

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