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

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

1
Q

What are the three different muscle types?

A
  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
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2
Q

Describe the breakdown of muscles from big to small:

A
  • Muscle bundles
  • Muscle fibres
  • Myofibrils
  • Myosin (thick, dark lines)
  • Actin (light)
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3
Q

What is a sacromere?

A

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

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4
Q

What is the z-line?

A

Divide myofibril up into smaller unites

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5
Q

What is the I-band?

A

Only Actin (light)

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6
Q

What is the M line?

A

Divides the sarcomere unit, where myosin attaches

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7
Q

What is the A-band?

A

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

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8
Q

What is the H zone?

A

Only myosin (no overlap)

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9
Q

What are the proteins in an actin filament?

A
  • Tropomyosin - forms a fibrous strand around the actin filament
  • Troponin
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10
Q

What does a myosin filament look like?

A

No other protein associated

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11
Q

What happens during contraction?

A
  • 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
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12
Q

How is muscle contraction stimulated?

A
  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
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13
Q

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

A

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

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14
Q

What is the first stage of the sliding filament hypothesis?

A
  • 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
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15
Q

What is the second stage of the sliding filament hypothesis?

A

This movement causes exposure of binding sites on the actin

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16
Q

What is the third stage of the sliding filament hypothesis?

A

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

17
Q

What is the fourth stage of the sliding filament hypothesis?

A

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

18
Q

What is the fifth stage of the sliding filament hypothesis?

A

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

19
Q

What is the sixth stage of the sliding filament hypothesis?

A

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

20
Q

What is the seventh stage of the sliding filament hypothesis?

A
  • 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
21
Q

What happens when there is no Ca2+ in place?

A
  • Tropomyosin returns
  • Troponin is in place
  • Actin myosin binding site hidden
22
Q

What happens following contraction?

A

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

23
Q

Why is a supply of ATP needed for muscle contraction?

A
  • 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
24
Q

What happens during aerobic respiration?

A
  • Maximum generation of ATP
  • Glycolysis, Kreb’s and ETC
  • Requires oxygen and glucose
  • Used for long, low intensity muscle contraction
25
What happens during anaerobic respiration?
- Reduced ATP generation - Glycolysis of stored glycogen - Lactate produced causes muscle fatigue - Used for short periods (up to 90s) of rapid/hard muscle contraction
26
What is phosphocreatine?
A molecule that can rapidly generate ATP from ADP in anaerobic conditions and therefore provide energy for muscle contraction, but it is short lived
27
Where is phosphocreatine stored?
In muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so re-form ATP.
28
What are slow-twitch fibres?
- 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
29
What are the adaptations of slow-twitch fibres?
- 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
30
What are fast-twitch fibres?
- 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
31
What are adaptations of fast-twitch fibres?
- 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
32
What is the structure of actin?
- Composed of thin filaments - Long protein molecules that form chains twisted around each other - Has myosin binding sites
33
What is the structure of myosin?
- 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
34
What is the structure of tropomyosin?
- Tropomyosin is a fibrous protein wound around actin - Has a calcium ion binding site - Acts as a 'switch' to control contraction

Biology (47 decks)

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