Skeletal Muscles Flashcards
Topic 6.3
Name 3 types of muscle in body & where they are located
- Cardiac: exclusively found in heart.
- Smooth: walls of blood vessels and intestines
- Skeletal: attached to in-compressible skeleton to tendons
What does ‘antagonsitic 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 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 capilaries
Describe microscopic structure of skeletal muscle
Myofibrils: site of contraction
Sarcoplasm: shared nuclei and cytoplasm with lots of mitochondria & endoplasmic reticulum
Sarcolemma: folds inwards towards sarcoplasm to form transverse (T) tubules
How is muscle contraction stimulated?
- Neuromuscular junction: action potential = voltage-gated Ca+ channels open
- Vesicles move towards & fuse with presynaptic membrane.
- Exocytosis of acetlycholine (ACh), which diffuses across synaptic cleft.
- ACh binds to receptors on Na+ channel proteins on skeletal muscle cell membrane.
- Influx of Na+ = depolarisation
Explain role of Ca+ ions in muscle contraction
- Action potential moves through T-tubules in sarcoplasm= Ca+ channels in sarcoplasmic reticulum open.
- Ca+ binds to troponin, triggering conformational change in tropomyosin.
- Exposes binding site on actin filaments so actinomyosin bridges can form
Outline ‘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 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 direction = 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.
What happens during muscle relaxation?
- Ca+ is actively transported back into endoplasmic reticulum.
- Tropomyosin once again block actin binding site.
Role of phospho-creatine in muscle contraction
Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited (e.g. during vigorous exercise)
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)
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
Explain structure and properties of slow-twitch muscle fibres
- Glycogen store: many terminal ends ca be hydrolysed to release glucose for respiration
- Contain myoglobin: higher affinity for oxygen than haemoglobin at lower partial pressures.
- Many mitochondria: aerobic respiration produces more ATP
- Surrounded by many blood vessels: high supply of oxygen & glucose
Explain structure and properties of fast-twitch muscle fibres
- Large store of phosphocreatine
- More myosin filaments
- Thicker myosin filaments
- High concentration of enzymes involved in anaerobic respiration
- Extensive sarcoplasmic reticulum: rapid uptake & release of Ca2+
What is a motor unit?
One motor unit supplies several muscle fibres, which act simultaneously as one functional unit
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 (proves that myosin filaments do not shorten)
