Skeletal Muscles Flashcards
Name the three types of muscle in the body and where they are located.
- Cardiac: heart
- Smooth: walls of blood vessels and intestines
- Skeletal: attached to incompressible skeleton by tendons
What does the phrase ‘antagonistic 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 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 ER
Sarcolemma: folds inwards towards sarcoplasm 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 which diffuses across synaptic cleft
- Acetylcholine binds to recptors 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 sarcoplasm = Ca2+ channels in sarcoplasmic reticulum open
- Ca2+ binds to troponin triggering conformational change in tropomyosin
- Exposes binding sites on actin filaments so actomyosin bridges can form
Outline the ‘sliding filament theory’.
- Myosin 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 oosition
- Myosin head re-attaches to actin further along filament
Outline the ‘sliding filament theory’.
- Myosin 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 oosition
- 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 shortens
Sliding filament action occurs up to 100 times per second in multiple sarcomeres
State 4 pieces of evidence that support sliding filament theory.
- H-zone narrows
- I-band narrows
- Z-lines get closer (sarcomere shortens
- A-zone remains same width
What happens during muscle relaxation?
- Ca2+ is actively transported back into ER
- Tropomyosin once again blocks actin binding sites on
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 a sarcomere?
- 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 erm, rapid powerful contraction e.g. biceps
