3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors

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

How do the I-band and A-band appear under an optical microscope?

Answer 5

I-band: light
A-band: dark

Question 6

How is muscle contraction stimulated?

Answer 6

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

Question 7

Explain the role of Ca2+ ions in muscle contraction.

Answer 7

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 8

Outline the ‘sliding filament theory’

Answer 8

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 9

How does sliding filament action cause a myofibril to shorten?

Answer 9

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 10

State 4 pieces of evidence that support the ‘sliding filament theory’

Answer 10

• H-zone narrows
• I-band narrows
• Z-line gets closer (sarcomere shortens)
• A-zone remains same width (proves that myosin filaments do not shorten)

Question 11

What happens during muscle relaxation?

Answer 11

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

Question 12

Explain the role of phosphocreatine in muscle contraction.

Answer 12

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

Question 13

How could a student calculate the length of one sarcomere?

Answer 13

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 14

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

Answer 14

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

Question 15

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

Answer 15

Slow-twitch: long-duration contraction; well-adapted to aerobic respiration to prevent lactate build up.
Fast-twitch: powerful short-term contraction; well-adapted to anaerobic respiration.

Question 16

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

Answer 16

• 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 17

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

Answer 17

• Large store of phosphocreatine
• More myosin filaments
• Thicker myosin filaments
• High conc. of enzymes involved in aerobic respiration
• Extensive sarcoplasmic reticulum: rapid uptake & release of Ca2+

Question 18

What is a motor unit?

Answer 18

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