6.3 Skeletal Muscles Flashcards
3 types of muscle in the body and where they are located
- Cardiac: found in heart
- Smooth: walls of blood vessels and intestines
- Skeletal: attached to incompressible skeletons by tendons
What phrase does ‘antagonistic pair of muscles’ mean?
- Muscles can only pull, so they work in pairs to move bones around joints
- Pairs pull in opposite directions: agonist contracts while antagonist is relaxed
Describe the gross structure of skeletal muscle
- Muscle cells are fused together to form bundles of parallel muscle 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 and endoplasmic reticulum
- Sarcolemma: folds inwards towards sarcoplasm to form transverse tubules
Draw a diagram to show the ultrastructure of a myofibril
- Z line: boundary between sarcomeres
- I band: only actin
- A band: overlap of actin and myosin
- H zone: only myosin
How does each band appear under an optical microscope?
- I band: light
- A band: dark
How is muscle contraction stimulated?
1) Neuromuscular junction: action potential = voltage gated Ca2+ channels open
2) Vesicles move towards and fuse with presynaptic membrane
3) Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft
4) ACh binds to receptors on Na+ channel proteins on skeletal muscle cell membrane
5) Influx of Na+ = depolarisation
Explain the role of Ca2+ ions in muscle contraction
1) Action potential moves through T-tubules in the sarcoplasm = Ca2+ channels in sarcoplasmic reticulum open
2) Ca2+ binds to troponin, triggering conformational change in tropomyosin
3) Exposes binding sites on actin filaments so actinomyosin bridges can form
Outline the sliding filament theory
1) Myosin head with ADP attached forms cross bridge with actin
2) Power stroke: myosin head changes shape and loses ADP, pulling actin over myosin
3) ATP attaches to myosin head, causing it to detach from actin
4) ATPase hydrolyses ATP->ADP+Pi, so myosin head can return to original position
5) 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 sarcomere Z lines shorten
- Sliding filament action occurs up to 100 times per second in multiple sarcomeres
State 4 pieces of evidence that support the sliding filament theory
- H zone narrows
- I band narrows
- Z lines get closer (sarcomere shortens)
- A band remains same width (proves that myosin filaments do not shorten)
What happens during muscle contraction?
1) Ca2+ is actively transported back into endoplasmic reticulum
2) Tropomyosin once again blocks actin binding site
Explain the role of phosphocreatine in muscle contraction
Phosphorylates 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 sarcomere?
1) View thin slice of muscle under optical microscope
2) Calibrate eyepiece graticule
3) 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 contractions e.g. calf muscle
- Fast twitch: sites of short term, rapid, powerful contractions e.g. biceps
