6.3 skeletal muscles are stimulated to contract by nerves and act as effectors

Question 1

Describe how muscles work

Answer 1

● Work in antagonistic pairs → pull in opposite directions eg. biceps / triceps
○ One muscle contracts (agonist), pulling on bone / producing force
○ One muscle relaxes (antagonist)
● Skeleton is incompressible so muscle can transmit force to bone

Question 2

Describe the gross and microscopic structure of skeletal muscle

Answer 2

● Made of many bundles of muscle fibres (cells) packaged together
● Attached to bones by tendons
● 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 ultrastructure of a myofibril

Answer 3

● Made of two types of long protein filaments, arranged in parallel
○ Myosin - thick filament
○ Actin - thin filament
● Arranged in functional units called sarcomeres
○ Ends – Z-line / disc
○ Middle – M-line
○ H zone – contains only myosin

Question 4

Explain the banding pattern to be seen in myofibrils

Answer 4

● I-bands - light bands containing only thin actin filaments
● 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

Answer 5

● Myosin heads slide actin along myosin causing the sarcomere to contract
● Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract
● When sarcomeres contract (shorten)…
○ H zones get shorter
○ I band get shorter
○ A band stays the same
○ Z lines get closer

Question 6

Describe the roles of actin, myosin, calcium ions, tropomyosin and ATP in
myofibril contraction

Answer 6

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Question 7

During muscle relaxation:

Answer 7

1. Ca
   2+ 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 8

Describe the role of phosphocreatine in muscle contraction

Answer 8

● A source of inorganic phosphate → rapidly phosphorylates ADP to regenerate ATP
○ ADP + phosphocreatine → ATP + creatine
● Runs out after a few seconds → used in short bursts of vigorous exercise
● Anaerobic and alactic

Question 9

Compare the structure, location and general properties of slow and fast
skeletal muscle fibres

Answer 9

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