3.6.3 Skeletal muscles

Question 1

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

Answer 1

Cardiac: heart
Smooth: walls of blood vessels and intestines
Skeletal: attached to incompressible skeleton by tendons

Question 2

What does antagonistic pair of muscles mean

Answer 2

Muscles 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 muscles

Answer 4

Myofibrils; site of contraction
Sarcoplasm; shared nuclei and cytoplasm with lots of mitochondria and endoplasmic reticulum
Sarcolemma; folds inwards towards sarcoplasm to form transverse (T) tubules

Question 5

What does each line/band/zone contain

Answer 5

Z line - boundary between sarcomeres
I band - only actin
A band - actin and myosin
H zone - only myosin

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 and fuse with presynaptic membrane
3. Exocytosis of 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+ 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 forms cross bridge with actin
2. Power stroke: myosin head changes shape and 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 reattaches 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 eachother
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 ( myosin filaments dont shorten)

Question 12

What happens during muscle relaxation

Answer 12

1. Ca2+ is actively transported back into the 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 eg 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; sites of sustained contraction eg calf muscle
Fast; sites of short term rapid powerful contraction eg biceps

Question 16

Explain the role of slow and fast twitch muscle fibres

Answer 16

Slow; long duration contraction; well adapted to aerobic respiration to prevent lactate buildup
Fast; 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 pp
Many mitochondria; aerobic respiration produces more ATP
Surrounded by many blood vessels; high supply of oxygen and 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 conc of enzymes involved in anaerobic respiration
Extensive sarcoplasmic reticulum; rapid uptake and 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