6.3 Skeletal Muscles Flashcards by Lucy Williams

Name 3 types of muscle in the body.

Cardiac
Smooth
Skeletal

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Where is cardiac muscle located?

Only in the heart

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Where is smooth muscle located?

Walls of blood vessels and intestines

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Where is skeletal muscle located?

Attached to skeleton by tendons

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What is meant by an “antagonistic pair of muscles”?

Muscles can only pull so work in pairs to move bones around joints
Pairs pull in opposite directions

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What are myofibrils?

Muscle cells fused together to form bundles of parallel muscle fibres

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Why are muscle cells in myofibrils?

Arrangement ensures there is no point of weakness between cells

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What is endomycium?

Loose connective tissue with many capillaries surrounding myofibrils

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What is the function of myofibrils?

Site of contraction

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What is sarcoplasm?

Shared nuclei and cytoplasm with lots of mitochondria & endoplasmic reticulum

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What is the sarcolemma?

Folds inwards towards sarcoplasm to form transvere tubules

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What four sections is a myofibril said to have?

Z line
I band
A band
H zone

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What is the Z line of a myofibril?

Boundary between sarcomeres

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What is the I band of a myofibril?

Actin only

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What is the A band of a myofibril?

Overlap of actin & myosin

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What is the H zone of a myofibril?

Myosin only

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How does the I band of a myofibril appear under a microscope?

Light

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How does the A band of a myofibril appear under a microscope?

Dark

Describe how muscle contraction is stimulated.

Action potential arrives at neuromuscular junction and opens voltage-gated Ca²⁺ channels
Vesicles move towards and fuse with presynaptic membrane
Exocytosis of acetylcholine, diffuses across synaptic cleft
ACh binds to receptors on Na⁺ channel proteins on skeletal muscle cell membrane
Depolarisation due to influx of Na⁺

Explain the role of Ca²⁺ ions in muscle contraction.

Action potential moves through T tubules in sarcoplasm and opens Ca²⁺ channels in sarcoplasmic reticulum
Ca²⁺ binds to troponin triggering shape change in tropomyosin
Exposes binding sites on actin filaments 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, puling actin over myosin
ATP attaches to myosin head, causing it to detach from actin
ATPase hydrolyses ATP to ADP so myosin head can return to original position
Myosin head reattaches to actin further along filament

How does sliding filament action cause a myofibril to shorten?

Myosin heads flex in opposite directions so actin filaments are pulled towards each other

State 4 pieces of evidence that support the sliding filament theory.

H zone narrows
I band narrows
Z lines get closer
A zone remains same width

What does the Z lines of sarcomeres getting closer prove?

Sarcomere shortens

What does the A zone of sarcomeres remaining the same prove?

Myosin filaments do not shorten

What happens during muscle relaxation?

Ca²⁺ is actively transported back into endoplasmic reticulum Tropomyosin blocks actin binding site

Explain the role of phosphocreatine in muscle contraction.

Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited

How could the length of one sarcomere be calculated?

View thin slice of muscle under optical microscope Calibrate eyepiece graticule Measure distance from middle of one light hand to middle of the next

Where are slow-twitch muscle fibres found in body?

Sites of sustained contraction e.g. calf

Where are fast-twitch muscle fibres found in body?

Sites of short-term, rapid, powerful contraction e.g. biceps

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

State the properties of slow-twitch muscle fibres.

Glycogen store Contain myoglobin Many mitochondria Surrounded by many blood vessels

Why do slow-twitch muscle fibres have a glycogen store?

Can be hydrolysed to release glucose for respiration

Why do slow-twitch muscle fibres contain myoglobin?

Higher affinity for oxygen that haemoglobin at lower partial pressures

Why do slow-twitch muscle fibres have many mitochondria?

Aerobic respiration produces more ATP

Why are slow-twitch muscle fibres surrounded by many blood vessels?

High supply of oxygen & glucose

State the 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

Why do fast-twitch muscle fibres have a large store of phosphocreatine?

Formation of ATP in anaerobic conditions

Why do fast-twitch muscle fibres have more myosin filaments?

Myosin provies energy

Why do fast-twitch muscle fibres have thicker myosin filaments?

More heads to form cross bridges

Why do fast-twitch muscle fibres have a high concentration of aerobic respiration enzymes?

For generation of ATP

Why do fast-twitch muscle fibres have an extensive sarcoplasmic reticulum?

For rapid uptake & release of Ca²⁺