6.3 Skeletal Muscles

Question 1

Name the 3 types of muscle in the body and where they are located.

Answer 1

● Cardiac: exclusively found in heart. ● Smooth: walls of blood vessels and
intestines.
● Skeletal: attached to incompressible
skeleton by tendons.

Question 2

What does the phrase ‘antagonistic pair of muscles’ mean?

Answer 2

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.

Question 3

Describe the gross structure of skeletal muscle.

Answer 3

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.

Question 4

Describe the microscopic structure of skeletal muscle.

Answer 4

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.

Question 5

Draw a diagram to show the ultrastructure of a myofibril.

Answer 5

Z-line: boundary between sarcomeres
I-band: only actin
A-band: overlap of actin & myosin H-zone: only myosin

check picture

Question 6

How does each band appear under an optical microscope?

Answer 6

I-band: light A-band: dark

Question 7

How is muscle contraction stimulated?

Answer 7

1. Neuromuscular junction: action potential = voltage-gated Ca2+ channels open.
2. Vesicles move towards & fuse with presynaptic membrane.
3. Exocytosis of acetylcholine (ACh), which diffuses across
   synaptic cleft.
4. ACh binds to receptors on Na+ channel proteins on skeletal
   muscle cell membrane.
5. Influx of Na+ = depolarisation.

Question 8

Explain the role of Ca2+ ions in muscle contraction.

Answer 8

1. Action potential moves through T-tubules in the sarcoplasm = Ca2+ channels in sarcoplasmic reticulum open.
2. Ca2+ binds to troponin, triggering conformational change in tropomyosin.
3. Exposes binding sites on actin filaments so actinomyosin bridges can form.

Question 9

Outline the ‘sliding filament theory’.

Answer 9

1. Myosin head with ADP attached forms cross bridge with actin.
2. Power stroke: myosin head changes shape & loses ADP, pulling
   actin over myosin.
3. ATP attaches to myosin head, causing it to detach from actin.
4. ATPase hydrolyses ATP→ADP(+Pi) so myosin head can return to
   original position.
5. Myosin head re-attaches to actin further along filament.

Question 10

How does sliding filament action cause a myofibril to shorten?

Answer 10

Myosin heads flex in opposite directions = 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.

Question 11

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

Answer 11

● H-zone narrows
● I-band narrows
● Z-lines get closer (sarcomere shortens)
● A-zone remains same width (proves that
myosin filaments do not shorten)

Question 12

What happens during muscle relaxation?

Answer 12

1. Ca2+ is actively transported back into endoplasmic reticulum.
2. Tropomyosin once again blocks actin binding site.

Question 13

Explain the role of phosphocreatine in muscle contraction.

Answer 13

Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited e.g. during vigorous exercise.

Question 14

How could a student calculate the length of one sarcomere?

Answer 14

1. View thin slice of muscle under optical microscope.
2. Calibrate eyepiece graticule.
3. Measure distance from middle of one light
   band to middle of another.

Question 15

Where are slow and fast-twitch muscle fibres found in the body?

Answer 15

Slow-twitch: sites of sustained contraction e.g. calf muscle.
Fast-twitch: sites of short-term, rapid, powerful contraction e.g. biceps.

Question 16

Explain the role of slow and fast-twitch muscle fibres.

Answer 16

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.

Question 17

Explain the structure and properties of slow-twitch muscle fibres.

Answer 17

● 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.

Question 18

Explain the structure and properties of fast-twitch muscle fibres.

Answer 18

● 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+.

Question 19

What is a motor unit?

Answer 19

One motor neuron supplies several muscle fibres, which act simultaneously as one functional unit.