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

Advantage - the second muscle required to reverse movement caused by the first
(muscles can only pull) and contraction of both muscles helps maintain posture

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 and relaxation

Answer 6

1. Depolarisation spreads down sarcolemma via T tubules causing Ca2+
   release from sarcoplasmic reticulum, which diffuse to myofibrils
2. Calcium ions bind to tropomyosin, causing it to move → exposing
   binding sites on actin
3. Allowing myosin head, with ADP attached, to bind to binding sites on
   actin → forming an actinomyosin crossbridge
4. Myosin heads change
   angle, pulling actin along
   myosin, (ADP released), using energy from ATP hydrolysis
5. New ATP binds to myosin head causing it to detach from binding site
6. Hydrolysis of ATP by ATP(hydrol)ase (activated by Ca2+) releases
   energy for myosin heads to return to original position
7. Myosin reattaches to a different binding site further along actin
   Process is repeated as long as calcium ion conc. is high

During muscle relaxation:
1. Ca2+ 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

Answer 7

● A source of inorganic phosphate (Pi) → 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 8

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

Answer 8

Slow twitch
general properties
● Specialised for slow, sustained
contractions (eg. posture, long
distance running)
● Obtain ATP mostly from aerobic
respiration → release energy slowly
● Fatigues slowly
location
● High proportion in muscles used for
posture eg. back, calves
● Legs of long distance runners
structure
● High conc. of myoglobin → stores
oxygen for aerobic respiration
● Many mitochondria → high rate of
aerobic respiration
● Many capillaries → supply high conc. of
oxygen / glucose for aerobic
respiration and to prevent build-up of
lactic acid causing muscle fatigue

fast twitch
general properties
● Specialised for brief, intensive
contractions (eg. sprinting)
● Obtain ATP mostly from anaerobic
respiration → release energy quickly
● Fatigues quickly due to high lactate conc
location
● Specialised for brief, intensive
contractions (eg. sprinting)
● Obtain ATP mostly from anaerobic
respiration → release energy quickly
● Fatigues quickly due to high lactate conc
structure
● Low levels of myoglobin
● Lots of glycogen → hydrolysed to provide
glucose for glycolysis / anaerobic
respiration which is inefficient so large
quantities of glucose required
● High conc. of enzymes involved in
anaerobic respiration (in cytoplasm)
● Store phosphocreatine