Joints and muscle movement — A/L Biology

What must I remember about Joints and muscle movement?

• Joints (articulations) are junctions between bones classified structurally by binding material: Fibrous joints (immovable / synarthroses, e.g., skull sutures), Cartilaginous joints (slightly movable / amphiarthroses, e.g., pubic symphysis, intervertebral discs), and Synovial joints (freely movable / diarthroses).
• Synovial Joint components: Articular cartilage (hyaline cartilage covering bone ends), Joint Cavity containing viscous Synovial Fluid (lubrication, shock absorption, chondrocyte nutrition secreted by synovial membrane), Fibrous Articular Capsule, and reinforcing Ligaments.
• Synovial Joint subtypes: Ball-and-socket (multiaxial: shoulder, hip), Hinge (uniaxial flexion/extension: elbow, knee), Pivot (rotation: atlantoaxial joint, proximal radioulnar joint), Condyloid / Ellipsoid (biaxial: radiocarpal joint), Saddle (biaxial: carpometacarpal joint of thumb), and Gliding / Plane (non-axial sliding: intercarpal joints).
• Skeletal muscle ultrastructure: Epimysium encloses muscle -> Perimysium wraps fascicles -> Endomysium encloses muscle fibers (multinucleated cells packed with parallel myofibrils); Sarcomere is the functional contractile unit bounded by Z-discs, containing overlapping Thin Filaments (Actin, Tropomyosin, Troponin complex) and Thick Filaments (Myosin with ATP-binding heads).
• Sliding Filament Mechanism: Motor nerve action potential triggers acetylcholine release -> sarcolemma depolarizes through T-tubules -> Sarcoplasmic Reticulum releases Ca2+ -> Ca2+ binds Troponin C -> Tropomyosin shifts exposing actin active sites -> Myosin head hydrolyzes ATP, forms cross-bridge, executes Power Stroke pulling thin filaments toward M-line -> new ATP binding releases myosin head -> active Ca2+ reuptake terminates contraction.

Test yourself (1 minute)

1. Rigor Mortis mechanism

   Following death, ATP synthesis ceases completely; without fresh ATP to bind myosin heads, cross-bridges remain locked to actin filaments, producing muscular stiffness until proteolytic decomposition sets in ~12-24 hours later.

2. Antagonistic muscle pairs

   Skeletal muscles can only pull (contract actively), not push; locomotion requires paired antagonistic muscles acting around joints (e.g., Biceps brachii flexes elbow while Triceps brachii extends elbow).

3. Motor Unit recruitment

   A motor unit consists of a single somatic motor neuron and all muscle fibers it innervates; fine motor control uses small motor units (e.g., 2-5 fibers in eye muscles), while gross power uses large units (e.g., 1000+ fibers in gastrocnemius).

Practise Joints and muscle movement questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 5

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Where does Joints and muscle movement fit in the syllabus?

Year 2 · Term 1