Topic 6.3 - Skeletal Muscle

Question 1

Describe how muscles work (2)

Answer 1

1) Work in antagonistic pairs -> pull in opposite directions e.g. biceps / triceps
- One muscle contracts (agonist), pulling on bone / producing force
- One muscle relaxes (antagonist)

2) Skeleton is incompressible so muscle can transmit force to bone

Question 2

Describe the gross and microscopic structure of skeletal muscle (3+6)

Answer 2

1) Made of many bundles of muscle fibres (cells) packaged together

2) Attached to bones by tendons

3) Muscle fibres contain:
- Sarcolemma (cell membrane) which folds inwards (invagination) to form transverse (T) tubules
- Sarcoplasm (Cytoplasm)
- Multiple nuclei
- Many myofibrils
- Sarcoplasmic reticulum (endoplasmic reticulum)
- Many mitochondria

Question 3

Describe the ultra structure of a myofibril (1+2, 1+3)

Answer 3

1) Made of 2 types of long protein filaments, arranged in parallel
- Myosin -> thick filament
- Actin -> thin filament

2) Arranged in functional units called sarcomeres
- Ends -> Z line / disc
- Middle -> M line
- H zone - contains only myosin

Question 4

Explain the banding patter to be seen in myofibrils (2+2)

Answer 4

1) I-bands -> light bands containing only thin actin filaments

2) A-bands -> dark bands containing thick myosin filaments (and some actin filaments)
- H zone contains only myosin
- Darkest region contains overlapping actin and myosin

Question 5

Give an overview of muscle contraction (3+4)

Answer 5

1) Myosin heads slide actin along myosin causing the sarcomere to contract

2) Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract

3) When sarcomeres contract (shorten)…
- H zones get shorter
- I bands get shorter
- A band stays the same
- Z lines get closer

Question 6

What happens during muscle relaxation? (2)

Answer 6

1) Ca2+ is actively transported back into the endoplasmic reticulum using energy from ATP

2) Tropomyosin moves back to block myosin binding site on actin again -> no actinomyosin cross bridges

Question 7

Describe the role of phosphocreatine in muscle contraction (3)

Answer 7

1) A source of inorganic phosphate (Pi) -> rapidly phosphorylates ADP to regenerate ATP
- ADP + phosphocreatine -> ATP + creatine

2) Runs out after a few seconds -> used in short bursts of vigorous exercise

3) Anaerobic and alactic

Question 8

Compare the general properties of slow twitch and fast twitch muscle fibres (3)

Answer 8

Slow twitch:

1) Specialised for slow, sustained contractions (e.g. posture, long distance running)

2) Produce more ATP slowly from aerobic respiration

3) Fatigues slowly

Fast twitch:

1) Specialised for brief, intensive contractions (e.g. sprinting)

2) Produce less ATP rapidly from anaerobic respiration

3) Fatigues quickly due to high lactate concentration

Question 9

Compare the location of slow twitch and fast twitch muscle fibres (2)

Answer 9

Slow twitch:

1) High proportion in muscles used for posture e.g. back, calves

2) Legs of long distance runners

Fast twitch:

1) High proportion in muscles used for fast movement e.g. biceps, eyelids

2) Legs of sprinters

Question 10

Compare the structure of slow twitch and fast twitch muscle fibres (3/4)

Answer 10

Slow twitch:

1) High concentration of myoglobin -> stores oxygen for aerobic respiration

2) Many mitochondria -> high rate of aerobic respiration

3) Many capillaries -> supply high concentration of oxygen / glucose for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue

Fast twitch:

1) Low levels of myoglobin

2) Lots of glycogen -> hydrolysed to provide glucose for glycolysis / anaerobic respiration which is inefficient so large quantities of glucose required

3) High concentration of enzymes involved in anaerobic respiration (in cytoplasm)

4) Stores phosphocreatine