Skeletal muscles Flashcards
Name the 3 types of muscles
Cardiac: Exclusively found in the heart.
Smooth: walls of blood vessels and intestines
Skeletal: Attached to incompressible skeleton by tendons
What does the 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 muscles
Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils)
Arrangment ensures there is no point of weakness between cells.
Each bundle is surrounded by endomycium: loose connective tissue will many capillaries.
Describe the microscopic structure of skeletal muscle.
Myofibrils: site of contraction
Sacroplasm: shared nuclei and cytoplasm with lots of mitochondria & endoplasmic reticulum
Sarcolemma: folds inwards towards sacroplasm to form transverse (T) tubles.
Draw a diagram to show the ultrastructure of a myofibril.
Z-line: boundary between sacromeres
I-bands: only actin
A-band: overlap of actin & myosin
H-zone: only myosin
Refer to diagram on PMT
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 (ACh), which diffuses across synaptic cleft.
- ACh binds to receptors 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 sacroplasm = Ca2+ channels in sacroplasmic reticulum open.
- Ca2+ binds to troponin, triggering conformational change in tropomyosin
- Exposes binding sites on actin filaents so actinomyosin 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) myosin head can return to original postion.
- Myosin head re-attches to actin further along filament
How does sliding filament action cause a myofibril to shorten?
Myosin heads flex opposite directions = actin filaments are pulled towards each other.
Distance between adjacent sacromere Z lines shortens.
Sliding filaments action occurs up to 100 times per second in multiple sacromeres.
State 4 pieces of evidence that support the sliding filament theory
- H-zones
- I-band narrows
- Z-lines get closer (Sacromere shortens)
- A-zone remains same width (proves that myosin filaments do not shorten)
What happens during muscle contraction?
- Ca2+ is actively transported into endoplasmic reticulum
2. Tropomysoin 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 sacromere?
- 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.
