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 the 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 endomucium: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

Slide 12

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 move 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
3. 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 direction = actin filaments are pulled towards each other

Distance between adjacent sacromere 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 sacromere?

Answer 14

1. View thin slice of muscle under optical microscope
2. Calibrate eyepiece graticle
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 pressure
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 sores do phosphocreatine
More myosin filaments
Thicker myosin filaments
High concentration of enzymes involved in anaerobic respiration
Extensive sacroplasmic 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