6.3 - Skeletal muscles Flashcards
Name the 3 types of muscle in the body and where they are located.
- Cardiac: exclusively found in heart
- Smooth: walls of blood vessels and intestines
- Skeletal: attached to incompressible skeleton by tendons
What does the phrase ‘antagonistic pair of muscles pair’ mean?
Muscles can only pull, so they work in pairs to move bones around joints.
Pairs pull in oppposite directions: agonist contracts while antagonist is relaxed.
Describe the gross structure of skeletal muscle.
- Muscle cells are fused together to form bundles of parallel muscles fibres (myofibrils).
- Arrangement ensures there is no point of weakness between cells.
- Each bundle is surrounded by endomycium: loose connective tissue with many capillaries.
Describe the microscopic structure of skeletal muscle.
Myofibrils: site of contraction
Sarcoplasm: shared nuclei and cytoplasm with lots of mitochondria & endoplasmic reticulum
Sacrolemma: folds inwards towards sacroplasm to form transverse (T) tubules.
How does each band appear under an optical microscope?
I-band: light
A-band: dark
How is muscle contraction stimulated?
- Neuromuscular junction: action potential = voltage-gated Ca2+ channels open.
- Vesicles move towards & fuse with presynaptic membrane.
- Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft.
- ACh binds to receptors on Na+ channel proteins on skeletal muscle cell membrane.
- Influx of Na+ = depolarisation.
Explain the role of Ca2+ ions in muscle contraction.
- Action potential moves through T-tubules in the sacroplasm = Ca2+ channels in sacroplasmic reticulum open.
- Ca2+ binds to troponin, triggering conformational change in tropomyosin.
- Exposes binding sites an actin filaments so actinomyosin bridges can form.
Outline the ‘sliding filament theory’.
- Mysosin head with ADP attached forms cross bridge with actin.
- Power stroke: myosin head changes shape & loses ADP, pulling actin over myosin.
- ATP attaches to myosin head, causing it to detach from actin.
- ATPase hydrolyses ATP -> ADP + Pi so myosin head can return to original position.
- Myosin head re-attaches to actin further along filament.
How does sliding filament action cause a myofibril to shorten?
- Myosin heads flex in opposite directions = actin filaments are pulled towards each other.
- Distance between adjacent sacromere Z lines shortens.
- Sliding filament action occurs up to 100 times per second in multiple sacromeres.
Stage 4 evidence that support the sliding filament theory.
- H-zone narrows
- I-band narrows
- Z-lines get closer (sacromere shortens)
- A-zoze remains same width (proves that myosin filaments do not shorten)
What happens during muscle relaxation?
- Ca2+ is actively transported back into endoplasmic reticulm.
- Tropomyosin once again blocks actin binding site.
Explain the role of phosphocreatine in muscle contraction.
Phophorylates ADP directly to ATP when oxygen for aerobic respiration is limited e.g. during vigorous exercise.
How could a student calculate the length of one sacromere?
- View thin slice of muscle under optical microscope.
- Calibrate eyepiece graticule.
- Measure distance from middle of one light band to middle of another.
Where are slow and fast-twitch muscle fibres found in the body?
Slow-twitch: sites of sustained contraction e.g. calf muscle.
Fast-twitch: sites of short-term, rapid, powerful contraction e.g. biceps.
Explain the role of slow and fast-twitch muscle fibres.
- Slow-twitch: long-duration contraction; well-adapted to aerobic respiration to prevent lactate buildup.
- Fast-twitch: powerful short-term contraction; well-adapted to anaerobic respiration.
