3.6.3 Skeletal Muscles are Stimulated to Contract by Nerves & Act as Effectors

Question 1

Where is cardiac muscle located?

Answer 1

Exclusively found in heart.

Question 2

Where is smooth muscle located?

Answer 2

Walls of blood vessels and intestines.

Question 3

Where is skeletal muscle located?

Answer 3

Attached to incompressible skeleton by tendons.

Question 4

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

Answer 4

• 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 5

What are muscle cells fused together to form?

Answer 5

Bundles of parallel muscle fibres (myofibrils)

Question 6

What does the arrangement of muscle cells ensure?

Answer 6

There is no point of weakness between cells

Question 7

What surrounds each bundle of muscle fibres?

Answer 7

Endomycium

Question 8

What type of tissue is endomycium?

Answer 8

Loose connective tissue with many capillaries

Question 9

What is the site of contraction in skeletal muscle?

Answer 9

Myofibrils

Question 10

What is the function of sarcoplasm in skeletal muscle?

Answer 10

It contains shared nuclei and cytoplasm with lots of mitochondria and endoplasmic reticulum.

Question 11

What structure in skeletal muscle folds inwards to form transverse (T) tubules?

Answer 11

Sarcolemma

Question 12

Ultrastructure of a myofibril (4)

Answer 12

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

Question 13

Appearance of I-band under microscope (colour)

Answer 13

Light.

Question 14

Appearance of A-band under microscope (colour)

Answer 14

Dark.

Question 15

What happens at the neuromuscular junction when an action potential occurs?

Answer 15

Voltage-gated Ca2+ channels open.

Question 16

What occurs after voltage-gated Ca2+ channels open at the neuromuscular junction?

Answer 16

Vesicles move towards and fuse with the presynaptic membrane.

Question 17

What neurotransmitter is released during muscle contraction stimulation?

Answer 17

Acetylcholine (ACh)

Question 18

What happens to acetylcholine after it is released into the synaptic cleft?

Answer 18

It diffuses across the synaptic cleft.

Question 19

What does acetylcholine bind to on the skeletal muscle cell membrane?

Answer 19

Receptors on Na+ channel proteins.

Question 20

What is the result of Na+ influx in skeletal muscle cells?

Answer 20

Depolarisation.

Question 21

What happens when an action potential moves through T-tubules in muscle contraction?

Answer 21

Ca2+ channels in the sarcoplasmic reticulum open.

Question 22

What does Ca2+ bind to during muscle contraction?

Answer 22

Troponin

Question 23

What is triggered when Ca2+ binds to troponin?

Answer 23

A conformational change in tropomyosin.

Question 24

What does the conformational change in tropomyosin expose?

Answer 24

Binding sites on actin filaments.

Question 25

What can form when binding sites on actin filaments are exposed?

Answer 25

Actinomyosin bridges.

Question 26

What is the first step of the sliding filament theory?

Answer 26

Myosin head with ADP attached forms cross bridge with actin.

Question 27

What happens during the power stroke in the sliding filament theory?

Answer 27

Myosin head changes shape & loses ADP, pulling actin over myosin.

Question 28

What role does ATP play in the sliding filament theory?

Answer 28

ATP attaches to myosin head, causing it to detach from actin.

Question 29

What is the function of ATPase in the sliding filament theory?

Answer 29

ATPase hydrolyses ATP to ADP and inorganic phosphate (Pi) so myosin head can return to original position.

Question 30

What occurs after the myosin head returns to its original position in the sliding filament theory?

Answer 30

Myosin head re-attaches to actin further along the filament.

Question 31

What happens to myosin heads during myofibril shortening?

Answer 31

Myosin heads flex in opposite directions.

Question 32

What is the effect of myofibril shortening on actin filaments?

Answer 32

Actin filaments are pulled towards each other.

Question 33

What happens to the distance between adjacent sarcomere Z lines during myofibril shortening?

Answer 33

The distance shortens.

Question 34

How many times per second can sliding filament action occur in multiple sarcomeres?

Answer 34

Up to 100 times per second.

Question 35

What happens to the H-zone during muscle contraction?

Answer 35

The H-zone narrows.

Question 36

What happens to the I-band during muscle contraction?

Answer 36

The I-band narrows.

Question 37

What occurs to the Z-lines during muscle contraction?

Answer 37

The Z-lines get closer together, indicating that the sarcomere shortens.

Question 38

What happens to the A-zone during muscle contraction?

Answer 38

The A-zone remains the same width.

Question 39

What does the unchanged width of the A-zone indicate?

Answer 39

It proves that myosin filaments do not shorten.

Question 40

What happens during muscle relaxation? (2)

Answer 40

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

Question 41

Role of phosphocreatine in muscle contraction

Answer 41

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

Question 42

What is the first step in calculating the length of one sarcomere?

Answer 42

View thin slice of muscle under optical microscope.

Question 43

What is the second step in calculating the length of one sarcomere?

Answer 43

Calibrate eyepiece graticule.

Question 44

What is the third step in calculating the length of one sarcomere?

Answer 44

Measure distance from middle of one light band to middle of another.

Question 45

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

Answer 45

Sites of sustained contraction e.g. calf muscle.

Question 46

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

Answer 46

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

Question 47

Role of slow-twitch muscle fibres

Answer 47

Long-duration contraction; well-adapted to aerobic respiration to prevent lactate buildup.

Question 48

Role of fast-twitch muscle fibres

Answer 48

Powerful short-term contraction; well-adapted to anaerobic respiration.

Question 49

What is a key characteristic of slow-twitch muscle fibres regarding glycogen storage?

Answer 49

They have many terminal ends that can be hydrolysed to release glucose for respiration.

Question 50

What protein do slow-twitch muscle fibres contain that has a higher affinity for oxygen than haemoglobin?

Answer 50

Myoglobin

Question 51

What is the role of mitochondria in slow-twitch muscle fibres?

Answer 51

They produce more ATP through aerobic respiration.

Question 52

Why do slow-twitch muscle fibres have many blood vessels surrounding them?

Answer 52

To provide a high supply of oxygen and glucose.

Question 53

What is a key characteristic of fast-twitch muscle fibres regarding energy storage?

Answer 53

They have a large store of phosphocreatine.

Question 54

What type of filaments are more abundant in fast-twitch muscle fibres?

Answer 54

They have more myosin filaments.

Question 55

How do the myosin filaments in fast-twitch muscle fibres compare in thickness?

Answer 55

They have thicker myosin filaments.

Question 56

What is the concentration of enzymes involved in anaerobic respiration in fast-twitch muscle fibres?

Answer 56

They have a high concentration of these enzymes.

Question 57

What feature of the sarcoplasmic reticulum is extensive in fast-twitch muscle fibres?

Answer 57

It allows for rapid uptake and release of Ca2+.

Question 58

What is a motor unit?

Answer 58

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