Support and Movement

Support and movement in humans depend on a coordinated interaction between the skeletal, joint and muscular systems. The AAMC MCAT content outline expects you to know the divisions of the human skeleton, classify joints, explain the sliding-filament theory of muscle contraction, distinguish the three muscle types, and recognize common disorders such as arthritis. This is a high-yield area for the Bio/Biochem section.

Key exam topics. Bone structure and the skeleton, joint types, and the three muscle types — but the highest-yield material is skeletal muscle at the molecular level: sarcomere structure, the sliding filament theory, the cross-bridge cycle, the role of Ca²⁺/troponin/tropomyosin, and the neuromuscular junction.

Human Skeleton

The adult human skeleton consists of 206 bones (about 270 at birth; many fuse with growth). It supports the body, protects vital organs, anchors muscles for movement, makes blood cells in red marrow (hematopoiesis) and stores calcium and phosphate.

Two main divisions

Axial skeleton — 80 bones: skull (22), hyoid (1), auditory ossicles (6), vertebral column (26), sternum and ribs (25). Forms the central axis.
Appendicular skeleton — 126 bones: pectoral girdle (4), upper limbs (60), pelvic girdle (2), lower limbs (60). Concerned with movement.

Vertebral column

26 bones in the adult: 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum (5 fused), 1 coccyx (4 fused). Curvatures (cervical, thoracic, lumbar, sacral) absorb shock and balance the body.

Bone tissue

Bone is a connective tissue with a matrix of collagen fibers (tensile strength) hardened by hydroxyapatite — calcium phosphate (compressive strength). Compact bone has Haversian systems (osteons); spongy bone has trabeculae and houses red marrow. Bone cells: osteoblasts (build), osteocytes (mature), osteoclasts (resorb).

Common trap. Cartilage is a separate connective tissue from bone — it has no blood vessels and heals slowly. The skeleton at birth is largely cartilaginous and is replaced by bone in the process of endochondral ossification.

Joints

A joint (articulation) is the site where two or more bones meet. Joints are classified by their structure (fibrous, cartilaginous, synovial) or by their range of movement (immovable, slightly movable, freely movable).

Fibrous (synarthrosis) — immovable: Bones held together by dense fibrous connective tissue. Examples: sutures of the skull.
Cartilaginous (amphiarthrosis) — slightly movable: Bones connected by cartilage. Examples: intervertebral discs, pubic symphysis.
Synovial (diarthrosis) — freely movable: Most joints in the body. Articular cartilage covers the bone ends; a fibrous capsule encloses a fluid-filled cavity (synovial fluid lubricates).

Types of synovial joint

Ball-and-socket — movement in all planes. Examples: shoulder, hip.
Hinge — movement in one plane (flexion-extension). Examples: elbow, knee, fingers.
Pivot — rotation of one bone around another. Atlas-axis (C1-C2) for head shaking.
Saddle — biaxial. Carpometacarpal joint of the thumb.
Condyloid (ellipsoidal) — biaxial. Wrist (radiocarpal) joint.
Gliding (plane) — small sliding movements. Between carpals or tarsals.

Types of Muscles

Three types in vertebrates — classified by structure (striated or not), control (voluntary or involuntary) and location.

Skeletal vs Cardiac vs Smooth muscle
Property	Skeletal	Cardiac	Smooth
Striations	Striated	Striated	Non-striated
Cell shape	Long cylindrical fibers	Branched cells	Spindle-shaped
Nuclei per cell	Many (peripheral)	Usually 1 (central)	1 (central)
Control	Voluntary (somatic NS)	Involuntary (auto-rhythmic)	Involuntary (autonomic / hormones)
Speed of contraction	Fast	Medium	Slow but sustained
Fatigue	Tires easily	Never fatigues	Highly resistant
Special feature	Attached to bones via tendons	Intercalated discs with gap junctions; SA-node pacemaker	Found in walls of viscera
Examples / location	Biceps, quadriceps	Heart wall (myocardium) only	Gut, blood vessels, uterus, bronchi, iris

Skeletal Muscles

A skeletal muscle is built up hierarchically: muscle → fascicle → muscle fiber (cell) → myofibril → sarcomere → thick (myosin) and thin (actin) filaments.

The sarcomere

The functional contractile unit, running from one Z line to the next. Components on a striated diagram:

Z line — boundary of the sarcomere; thin filaments anchor here.
I band — light band; only thin (actin) filaments.
A band — dark band; full length of thick (myosin) filaments — remains constant during contraction.
H zone — center of A band; only thick filaments. Shortens during contraction.
M line — midline of the sarcomere; anchors thick filaments.

Slow-twitch (Type I) vs fast-twitch (Type II) fibers
Property	Type I (slow oxidative)	Type II (fast glycolytic)
Contraction speed	Slow	Fast
Main ATP source	Aerobic (oxidative phosphorylation)	Anaerobic glycolysis
Mitochondria & myoglobin	High (“red” muscle)	Low (“white” muscle)
Fatigue resistance	High — endurance	Low — tires quickly
Best for	Posture, marathon	Sprints, power lifts

Muscle Contraction

Muscle contracts by the sliding filament theory (Huxley, 1954): thin actin filaments slide past thick myosin filaments — the filaments themselves do not shorten.

The neuromuscular junction (excitation-contraction coupling)

An action potential reaches the axon terminal of a motor neuron → voltage-gated Ca²⁺ channels open.
Ca²⁺ influx triggers exocytosis of acetylcholine (ACh) into the synaptic cleft.
ACh binds nicotinic ACh receptors (ligand-gated cation channels) on the motor end plate → Na⁺ entry depolarizes the sarcolemma.
The action potential spreads along the sarcolemma and down the T-tubules, where the voltage sensor (DHP receptor) triggers the ryanodine receptor to release Ca²⁺ from the sarcoplasmic reticulum.
Acetylcholinesterase in the cleft degrades ACh, terminating the signal so the muscle can relax. (Curare blocks the receptor; organophosphates block the esterase → sustained contraction.)

Steps of the cross-bridge cycle

A motor neuron releases acetylcholine at the neuromuscular junction; the sarcolemma depolarises.
Action potential travels down T-tubules → Ca²⁺ released from sarcoplasmic reticulum.
Ca²⁺ binds troponin C, shifting tropomyosin away from the myosin-binding sites on actin.
Energised myosin heads (already loaded with ADP + P_i) bind actin to form cross-bridges.
Power stroke: P_i is released; the head pivots and pulls the thin filament toward the M line.
A new ATP binds the myosin head, detaching it from actin.
ATP is hydrolysed, re-energising the head; cycle repeats as long as Ca²⁺ and ATP are available.

Result: I band and H zone shorten; A band length is unchanged; sarcomere shortens; muscle contracts.

Relaxation and energy supply

Relaxation is active: when nerve stimulation stops, the SERCA pump uses ATP to pump Ca²⁺ back into the sarcoplasmic reticulum. As cytosolic Ca²⁺ falls, it leaves troponin, tropomyosin slides back to re-cover the binding sites, and cross-bridges can no longer form. Note that ATP is required for both contraction (the power stroke and detaching myosin) and relaxation (SERCA).

Muscle draws ATP from three sources in order: (1) creatine phosphate instantly regenerates ATP for a few seconds; (2) anaerobic glycolysis gives fast ATP but produces lactate; (3) aerobic respiration (oxidative phosphorylation) supplies the bulk of ATP for sustained activity, aided by myoglobin as an O₂ store.

Rigor mortis

After death, ATP synthesis stops. Without ATP, myosin heads cannot detach from actin, so the muscle stiffens. Rigor mortis sets in within 2-4 hours and resolves over 24-72 hours as proteins begin to autolyse.

Memory aid for the sarcomere. "A is constant; I and H shrink." The thick-filament A band stays the same length; the I band (only thin) and H zone (only thick) shorten because thin filaments slide into the A band.

Arthritis

Arthritis is inflammation of one or more joints, causing pain, stiffness and reduced mobility. Two important types to know:

Osteoarthritis (OA): Most common form. Wear-and-tear degeneration of articular cartilage with age, repetitive use or obesity. Affects weight-bearing joints (knees, hips, spine). Pain on movement; relieved by rest. Treated with weight loss, NSAIDs, physiotherapy, joint replacement.
Rheumatoid arthritis (RA): An autoimmune disease — antibodies attack the synovial membrane, leading to inflammation, pannus formation and joint destruction. Symmetrical, affects small joints first (fingers, wrists). Morning stiffness for > 1 hour. Treated with DMARDs (methotrexate), NSAIDs, biologics.
Gouty arthritis: Caused by deposition of monosodium urate crystals in joints (often the big toe) due to hyperuricemia. Treated by xanthine oxidase inhibitors (allopurinol).

Worked MCQs

Five MCQs covering the high-yield testing patterns for support and movement.

Q1. The total number of bones in the adult human skeleton is:

An adult has 206 bones — 80 in the axial skeleton (skull, vertebrae, ribs, sternum, hyoid, ossicles) and 126 in the appendicular skeleton (limbs and girdles). At birth there are about 270 bones; many fuse during growth.

Q2. The shoulder joint is an example of which type of synovial joint?

Hinge
Ball-and-socket
Pivot
Saddle

The shoulder (and the hip) is a ball-and-socket joint, allowing movement in three planes — flexion-extension, abduction-adduction, and rotation. Hinge joints (elbow, knee) move in a single plane.

Q3. Which band of the sarcomere remains the same length during muscle contraction?

I band
A band
H zone
Z line distance

The A band is the full length of the thick (myosin) filament. The thick filaments themselves do not shorten in contraction; only the relative position of thin filaments changes — so the A band length is constant. The I band and H zone, by contrast, shrink as actin slides into the A band.

Q4. Cardiac muscle differs from skeletal muscle in that it is:

Non-striated
Voluntary
Connected by intercalated discs and is involuntary
Multinucleated and rapidly fatiguing

Cardiac muscle is striated like skeletal muscle but its cells are uninucleated, branched, and joined by intercalated discs that contain gap junctions and desmosomes. Contraction is involuntary and inherently rhythmic, driven by SA-node pacemaker cells.

Q5. In the cross-bridge cycle of skeletal muscle, calcium ions trigger contraction by binding to:

Myosin heads
Actin directly
Troponin C
Tropomyosin

Ca²⁺ binds troponin C, causing a conformational change in the troponin-tropomyosin complex that uncovers the myosin-binding sites on actin. Cross-bridges then form and the power stroke pulls the thin filament inwards.

Quick Recap

206 bones in adult: 80 axial + 126 appendicular.
Joint types: fibrous (immovable), cartilaginous (slightly movable), synovial (freely movable).
Synovial subtypes: ball-and-socket, hinge, pivot, saddle, condyloid, gliding.
Muscle types: skeletal (striated, voluntary), cardiac (striated, involuntary, intercalated discs), smooth (non-striated, involuntary).
Fiber types: Type I slow oxidative (fatigue-resistant, red) vs Type II fast glycolytic (powerful, fatigable, white).
Sarcomere: Z to Z; A band (thick) constant; I band and H zone shrink in contraction.
NMJ: ACh → nicotinic receptors → sarcolemma/T-tubule depolarization → Ca²⁺ released from sarcoplasmic reticulum; acetylcholinesterase ends the signal.
Sliding filament theory: Ca²⁺ → troponin → tropomyosin shift → myosin-actin cross-bridges → power stroke (uses ATP).
Relaxation is active too: SERCA pumps Ca²⁺ back into the SR using ATP.
Rigor mortis = no ATP → myosin cannot detach → cross-bridges stuck.
Arthritis: osteoarthritis (wear-and-tear), rheumatoid arthritis (autoimmune), gouty arthritis (urate crystals).

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.