1. Muscle structure and contraction

2. Muscle Contraction
Striated muscle is made from many fibres laid down in parallel.
The muscle fibres are made from several cells that share a common cell membrane:
the cytoplasm (sarcoplasm) contains many nuclei
technically this is a syncitium (cells fused)
The multinucleate muscle fibre is surrounded by a membrane called the sarcolemma
it has infoldings called T-tubules
cytoplasm has a large amount of smooth ER
stores Ca2+ ions that are needed for contraction
the SER is called sarcoplasmic reticulum 2

3. Striated muscle is made from many fibres laid down in parallel3

4. The muscle fibres are made up of two types of protein:
thick filaments of myosin with a ‘head’ projecting from it
thinner actin filaments made up of repeating protein units
tropomyosin and troponin are associated with the actin and are important for contraction 4

5. Bundles of myosin molecules make up the thick filaments.
Myosin molecules consist of a long filament with a ‘head’ projecting from it.
Myosin molecules are bundled together to produce thick bundles with equal numbers of myosin heads in each half.
Bundles of myosin molecules make up the thick filaments
Myosin head
M-line
Long myosin molecule 5

6. Actin consists of many globular protein units linked together into long chains.
Two of these actin chains are twisted together with a tropomyosin molecule lying in the groove between them.
At intervals, troponin molecules bind to the actin, covering the binding sites of the myosin heads.
Tropomyosin
Troponin
Globular protein that makes up the actin chain
Actin chain 6

7. Each muscle fibre is made from many myofibrils that are composed of thick myosin molecules and thin actin molecules.
The regular arrangement of these molecules produces a banding pattern typical of voluntary muscle.
This can be seen with the light microscope and electron microscope. 7

9. Z-line
sarcomere
H-band
I-band
A-band 9

10. The banded regions are given names or letters. A-bands:
are the broad dark bands made from myosin
they have an indistinct M-line (not labelled on the diagram) running down the centre of the band
I-bands:
are paler and not as broad
have a dark Z-line running down their middle
made from actin
Sarcomere:
the section between two Z-lines
in voluntary muscle these sarcomeres are lined up
give them a striped appearance 10

11. The banding pattern is clearly visible in electron micrographs of striated muscle. (Louisa Howard, public domain image)
A-band
Sarcomere
Z-line
I-band
H-band
M-line 11

13. During muscle contraction (details later), the actin filaments are pulled across the myosin filaments, towards the middle.
Consequently:
the I-bands and H-bands get smaller
the A-band remains the same
In the relaxed state, the sarcomere is 2.5 µm compared with 2.0 µm when contracted. 13

14. Z-line
sarcomere
Muscle relaxed:
Muscle contracted: 14

15. Relaxed state Contracted state Length of sarcomere H-band width
Complete the table to compare muscle when relaxed and contracted.
Relaxed state
Contracted state
Length of sarcomere
H-band width
I-band width
A-band width 15

16. Relaxed state Contracted state Length of sarcomere wider 2.5 µm
Complete the table to compare muscle when relaxed and contracted.
Relaxed state
Contracted state
Length of sarcomere
wider
2.5 µm
narrower
2.0 µm
H-band width
I-band width
A-band width
unchanged 16

17. The myosin head acts as an ATPase and hydrolyses the ATP
When the muscle is stimulated, Ca2+ ions are released from the sarcoplasmic reticulum
Ca2+ ions bind to troponin
results in the myosin binding sites on the actin being uncovered
The binding of the myosin head to the actin filament causes the head to bend
pulls the actin over the myosin in the ‘power stroke’
bending of the myosin head releases ADP and Pi
allows fresh ATP to bind
The myosin head acts as an ATPase and hydrolyses the ATP
breaks the bonds between the myosin head and actin
allows the myosin head to return to its original shape
then able to bind to other exposed binding sites and the cycle of contraction continues 17

18. Usually the myosin binding sites on the actin filaments are covered by the tropomyosin and troponin, so preventing binding of the myosin heads.
ADP + Pi
Actin filaments with binding sites covered
Myosin head not bound
When stimulated, Ca2+ ions are released from SER. This binds to the troponin, exposing the binding site which allows the myosin head to crosslink with the actin filament.
Actin filaments with exposed binding sites
Myosin head binds
ADP + Pi
The myosin head tilts (ca. 45o), moving the actin filament along by about 10 nm. This is the so-called power stroke. The change in shape causes the ADP and Pi to be released from the myosin head.
ADP + Pi
Actin filaments are pulled across the myosin 18

19. Fresh ATP binds to the myosin head and is hydrolysed to ADP + Pi
Fresh ATP binds to the myosin head and is hydrolysed to ADP + Pi. The energy released by the hydrolysis allows the actin and myosin to separate.
ATP
ATP binds to myosin and brings about separation of the myosin head
The detached myosin head flips back to the original position and is ready for another cycle to begin.
Actin filaments with exposed binding sites, so cycle can begin again
ADP + Pi 19

20. Q. What is the role of the ATP?
Q. The troponin and tropomyosin normally block the myosin binding sites on actin. What is the role of the Ca2+ ions released from the sarcoplasmic reticulum?
The Ca2+ ions bind to the troponin which then displaces the tropomyosin and so exposes the binding site on the actin. 
Q. What is the role of the ATP?
When it binds to the myosin head, it is hydrolysed.
Release of energy allows separation of the myosin from the actin and the resetting of the head in its pre-power stroke position. 20

21. Role and Supply of ATP to Muscle
ATP is required to:
break the cross bridge or links between the myosin heads and actin
reset the myosin head in its pre-power stroke form so that it can bring about further muscle contraction
There is actually very little ATP present in the muscle fibres
only enough for a few seconds contraction
it must be resupplied if contraction is to continue
This is achieved by:
respiration – aerobic and anaerobic
resupply from creatine phosphate reserves 21

22. Aerobic Respiration
The sarcoplasm contains many mitochondria that complete the link reaction, Krebs cycle and oxidative phosphorylation.
Respiration rates are high in active muscle and the supply of O2 must be maintained if aerobic respiration is to continue.
Q. How do the following help to maintain O2 supply?
Presence of myoglobin in muscle 
High levels of CO2 in active muscle 22

23. myoglobin is respiratory pigment with a high affinity for O2
saturates at low pp O2
only gives it up at very low O2
i.e. when respiratory demands are greatest
High levels of CO2 in active muscle:
elevated CO2 causes the Bohr effect
reduces haemoglobin’s affinity for O2
shifts O2 dissociation curve to the right
makes more O2 available at times of high demand 23

24. Anaerobic Respiration
Under anaerobic conditions, pyruvate can be diverted into the lactate pathway which ensures the resupply of NAD so that glycolysis and limited ATP production can continue.
Unfortunately this cannot continue indefinitely and only relatively small amounts of ATP are produced.
Q. Why is it not possible for anaerobic respiration to continue indefinitely?
Q. How much ATP is produced per molecule of glucose under anaerobic compared to aerobic conditions?
Suggest a reason for this. 24

25. Anaerobic respiration
cannot continue indefinitely because lactate is toxic
Limited ATP production
aerobic respiration produces 32–38 ATP per glucose
fully oxidised
anaerobic respiration only produces 2 ATP
glucose is not fully oxidised
much energy still remains in the lactate 25

26. Regeneration of ATP in Muscle
Creatine phosphate is a high energy compound.
It can be used to regenerate ATP that has been used by the muscle.
There is enough present in muscle for 2–4 seconds worth of contraction.
ADP + CP  ATP + C
The creatine phosphate can be made when aerobic respiration is taking place. 26