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

Question 1

What are the three different muscle types?

Answer 1

1. Skeletal/striated = voluntary, makes up the bulk of body muscle in vertebrates
2. Smooth eg/ blood vessels and the gut
3. Cardiac - heart, myogenic

Question 2

Describe the breakdown of muscles from big to small:

Answer 2

• Muscle bundles
• Muscle fibres
• Myofibrils
• Myosin (thick, dark lines)
• Actin (light)

Question 3

What is a sacromere?

Answer 3

Structural and functional unit for muscle contraction
- Attachment point for another protein

Question 4

What is the z-line?

Answer 4

Divide myofibril up into smaller unites

Question 5

What is the I-band?

Answer 5

Only Actin (light)

Question 6

What is the M line?

Answer 6

Divides the sarcomere unit, where myosin attaches

Question 7

What is the A-band?

Answer 7

Area of overlap between myosin and actin + area of just myosin

Question 8

What is the H zone?

Answer 8

Only myosin (no overlap)

Question 9

What are the proteins in an actin filament?

Answer 9

• Tropomyosin - forms a fibrous strand around the actin filament
• Troponin

Question 10

What does a myosin filament look like?

Answer 10

No other protein associated

Question 11

What happens during contraction?

Answer 11

• Actin and myosin filaments slide past each other
• The A bands stay the same length
• The I bands and H zones shorten
• Z lines are moved closer together
• Sarcomere decreases in length

Question 12

How is muscle contraction stimulated?

Answer 12

1. Action potential arrives at the neuromuscular junction (depolarised)
2. This causes an influx of Ca2+ and release of acetylcholine
3. Acetylcholine bind to receptor sites, causing influx in Na+ and action potential in the sarcolemma
4. Impulse is carried throughout the muscle fibre through T-tubules
5. Sarcoplasmic reticulum releases Ca2+ into the fibre

Question 13

What does acetylcholine break down by and why does it do that?

Answer 13

Acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not over-stimulated.
- The resulting choline and acetyl diffuse back into the neurone - recombine to form acetylcholine using energy provided by the mitochondria

Question 14

What is the first stage of the sliding filament hypothesis?

Answer 14

• An action potential arrives at the neuromuscular junction
• Calcium ions diffused into myofibrils from the sarcoplasmic reticulum (SR)
• Calcium ions bind to troponin molecules, stimulating them to change shape
• Pushes Tropomyosin out the way

Question 15

What is the second stage of the sliding filament hypothesis?

Answer 15

This movement causes exposure of binding sites on the actin

Question 16

What is the third stage of the sliding filament hypothesis?

Answer 16

Myosin head binds to binding site on actin forming actin-myosin cross bridge

Question 17

What is the fourth stage of the sliding filament hypothesis?

Answer 17

On binding with actin, myosin head rotates 45˚/bends, pulling the actin filaments towards the centre of the sarcomere, whilst hydrolysing ATP (“power stroke”)

Question 18

What is the fifth stage of the sliding filament hypothesis?

Answer 18

Attachment of a new ATP molecule to each myosin head causes myosin head to detach from actin sites

Question 19

What is the sixth stage of the sliding filament hypothesis?

Answer 19

Energy released from ATP hydrolysis straightens the myosin head and returns it to normal position

Question 20

What is the seventh stage of the sliding filament hypothesis?

Answer 20

• If Ca2+ are still present, the head binds to the next actin binding site again, a short distance along the strand from the first binding site
• ATP binds to the myosin heads once more in order for them to detach again
• As long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscles has a supply of ATP, this process repeats until the muscle is fully contracted

Question 21

What happens when there is no Ca2+ in place?

Answer 21

• Tropomyosin returns
• Troponin is in place
• Actin myosin binding site hidden

Question 22

What happens following contraction?

Answer 22

Relaxation
- The muscle enters a refractory period
- Acetylcholinesterase breaks down acetylcholine
- Ca2+ is actively transported back inside the sarcoplasmic reticulum
- The reabsorption of the calcium ions allows tropomyosin to block the actin filament again
- Myosin heads are now unable to bind to actin filament

Question 23

Why is a supply of ATP needed for muscle contraction?

Answer 23

• Energy is needed for the return movement of myosin heads that causes the actin filaments to slide
• The return of calcium ions back into the sarcoplasmic reticulum occurs via active transport

Question 24

What happens during aerobic respiration?

Answer 24

• Maximum generation of ATP
• Glycolysis, Kreb’s and ETC
• Requires oxygen and glucose
• Used for long, low intensity muscle contraction

Question 25

What happens during anaerobic respiration?

Answer 25

• Reduced ATP generation
• Glycolysis of stored glycogen
• Lactate produced causes muscle fatigue
• Used for short periods (up to 90s) of rapid/hard muscle contraction

Question 26

What is phosphocreatine?

Answer 26

A molecule that can rapidly generate ATP from ADP in anaerobic conditions and therefore provide energy for muscle contraction, but it is short lived

Question 27

Where is phosphocreatine stored?

Answer 27

In muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so re-form ATP.

Question 28

What are slow-twitch fibres?

Answer 28

• These contract more slowly than fast-twitch fibres and provide a less powerful contraction but over a longer period
• More common in calf muscles, which contract constantly to maintain upright position
• Adapted to aerobic respiration in order to avoid build up of lactic acid

Question 29

What are the adaptations of slow-twitch fibres?

Answer 29

• Large store of myoglobin, a bright red molecule that stores oxygen giving slow-twitch fibres a darker appearance
• A rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
• Many mitochondria to produce ATP

Question 30

What are fast-twitch fibres?

Answer 30

• These contract rapidly and produce powerful contractions but only for a short period of time (weight-lifting)
• More common in biceps and upper arm muscles, or muscles needing to do short bursts of intense activity

Question 31

What are adaptations of fast-twitch fibres?

Answer 31

• Thicker and more numerous myosin filaments
• High concentration of glycogen
• High concentration of enzymes involved with anaerobic respiration for fast ATP production
• A store of phosphocreatine

Question 32

What is the structure of actin?

Answer 32

• Composed of thin filaments
• Long protein molecules that form chains twisted around each other
• Has myosin binding sites

Question 33

What is the structure of myosin?

Answer 33

• Composed of thick filaments
• Fibrous protein with globular head shaped region at one end
• The head region has an ATPase activity and an actin binding site

Question 34

What is the structure of tropomyosin?

Answer 34

• Tropomyosin is a fibrous protein wound around actin
• Has a calcium ion binding site
• Acts as a ‘switch’ to control contraction