6.3 - Skeletal Muscles Flashcards
Name the 3 types of muscles
Cardiac
Smooth
Skeletal
Where are cardiac muscles found
In the heart
Where are smooth muscles found
In the walls of blood vessels and intestines
Where are skeletal muscles found
Attached to skeleton by tendons.
What is meant by “antagonistic pair of muscles”?
Muscles can only pull, so they work in pairs to move bone around joints.
Pairs pull in opposite directions: agonist contracts, antagonist relaxes
Describe the gross structure of skeletal muscle.
Muscle cells fused together form bundles of parallel muscle fibres, myofibrils.
Arrangement ensures there’s 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 & cytoplasm with lots of mitochondria and endoplasmic reticulum
Sarcolemma: folds inwards towards sarcoplasm to form transverse (T) tubules.
What are sarcomeres?
Short units that make up a myofibril
End of each sarcomere is marked with a Z-line
Middle of each sarcomere is an M-line, which is the middle of the myosin filaments
Around the M-line is the H-zone which contains only myosin filaments
Describe the 2 filaments that make up myofibrils
Thick myofilaments are made of the protein myosin.
Thin myofilaments are made up of the protein actin.
What are I-bands?
Light bands in myofibrils that contain thin actin filaments only
What are A-bands?
Dark bands in myofibrils that contain thick myosin filaments and some overlapping thin actin filaments.
Outline the process of muscle contraction.
- Action potential from a motor neurone stimulates muscle cell.
- Depolarises sarcolemma.
- Depolarisation spreads down T-tubules into sarcoplasmic reticulum
- SR releases stored calcium ions into sarcoplasm.
- Calcium ions bind to protein attached to tropomyosin, causing it to change shape.
- Tropomyosin moves out of actin-myosin binding site on actin filament.
- Myosin head binds to actin-myosin binding site.
- Calcium ions activate enzyme ATP hydrolase to break down ATP to provide energy
- Energy causes myosin heads to bend, pulling actin filament along.
- Another ATP molecule provides energy to break actin-myosin cross bridge.
- Myosin attached to another actin filament further along.
This shortens the sarcomere, causing the muscle to contract.
How is muscle contraction stimulated?
- At neuromuscular junction, action potential causes voltage-gated calcium ion channels to open.
- Vesicles move towards & fuse with presynaptic membrane.
- Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft.
- ACh binds to receptors on sodium ion channel proteins on skeletal muscle cell membrane.
- Influx of sodium ions = depolarisation.
Explain the role of calcium ions in muscle contraction.
- Action potential moves T-tubules in the sarcoplasm = calcium ion channels in sarcoplasmic reticulum.
- Calcium ions bind to troponin, triggering conformational change in tropomyosin.
- Exposes binding sites on actin filaments, so actin-myosin cross bridges can form.
- Also activates the enzyme ATP hydrolase, which hydrolyses ATP to provide energy needed to cause myosin head to bend which pulls actin filament along.
Outline the ‘sliding-filament theory’
- Myosin head with ADP attached forms cross bridge with actin.
- Myosin head changes shape & loses ADP, pulling actin over myosin.
- ATP attaches to myosin head, causing it to detach from actin.
- ATP hydrolyse 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 sarcomere Z line shortens.
Sliding filament action occurs up to a 100 times per second in multiple sarcomeres.
State 4 pieces of evidence that support the sliding filament theory.
H-zone & 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?
- Calcium ions are actively transported back into SR (sarcoplasmic reticulum)
- Tropomyosin blocks actin binding site.
Explain the role of phosphocreatine in muscle contraction.
Phosphorylates ADP to ATP when oxygen for aerobic respiration is limited.
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-twitch muscle fibres found?
Sites of sustained contraction, such as muscles used for maintaining posture (e.g. in the back)
Where are fast-twitch muscle fibres found?
Site of short-term, rapid, powerful contractions, e.g in the legs & arms
Explain the role of slow-twitch muscle fibres.
Long-duration contraction
Well-adapted to aerobic respiration
To prevent lactate buildup
Explain the role of fast-twitch muscle fibres.
Powerful short-term contraction
Well-adapted to anaerobic respiration
Explain the structure and properties of slow-twitch muscle fibres.
- glycogen store = many ends can be hydrolysed to release glucose for respiration.
- contain myoglobin = protein that stores oxygen.
- many mitochondria = aerobic respiration produces more ATP.
- surrounded by many blood vessels = high supply of oxygen & glucose.
Explain the structure and properties of fast-twitch muscle fibres.
- large store of phosphocreatine
- more & thicker myosin filaments
- high concentration of enzymes involved in anaerobic respiration.
- large SR = rapid release of calcium ions
- not much myoglobin = can’t store much oxygen
What is a motor unit?
One motor neurone supplies several muscle fibres, which act as one functional unit.
