6.3: Skeletal Muscles Flashcards
Name the 3 types of muscles in the body and where they are found
Cardiac- exclusively found in the heart
Smooth- walls of blood vessels and intestines
Skeletal- attached to incompressible skeletons by tendons
What does phrase ‘antagonist 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 & endoplasmic reticulum
Sarcolemma: folds inwards towards sarcoplasm to form transverse (T) tubules
Describe bands and zones of structure of myofibril
Z-line: a boundary between sarcomeres
I-band: only actin
A-band: overlap of actin & myosin
H- zone: only myosin
How does each band appear under an optical microscope?
I-band: dark
A-band: dark
How is muscle contraction stimulated?
- Neuromuscular junction: action potential= voltage-gated Ca2+ channels open
- Vesicles move towards and 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 role of Ca2+ ions in muscle contraction
- Action potential moves through T-tubules in sarcoplasm= Ca2+ channels in sarcoplasmic reticulum open
- Ca2+ bind to troponin, triggering conformational change in tropomyosin
- Exposes binding sites on actin filaments so actinmyosin bridges can form
Outline ‘sliding filament’ theory
- Myosin head with ADP attached forms cross bridge with actin
- Power stroke: myosin head changed shape & loses ADP, pulling actin over myosin
- ATP attached to myosin head, causing it to detach from actin
- ATPase hydrolyses ATP -> ADP + Pi so myosin head can return to original position
- Myosin head reattached to actin further along filament
How does sliding filament action cause a myofibril to shorten?
Myosin heads flex to 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 supports the 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)
What happens during muscle relaxation?
- Ca2+ is actively transported back into endoplasmic reticulum
- Tropomyosin once agains blocks actin binding site
Explain the role of phosphocreatine in muscle contraction
Phosphorylates ADP directly to ATP when oxygen for aerobic exercise is limited e.g. during vigorous exercise
How could a student calculate the length of one 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-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
Explain structure and properties of slow-twitch muscle fibres
- Glycogen store: many terminal ends can 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 stores 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 neuron supplies several muscle fibres, which act simultaneously as one functional unit
