💛 3.6 - 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only)

Question 1

What type of muscle do you use to move?

Answer 1

Skeletal muscle.

Question 2

What are skeletal muscles attached to bones by?

Answer 2

Tendons.

Question 3

What attach bones to other bones to hold them together?

Answer 3

Ligaments.

Question 4

Pairs of skeletal muscle contract and relax to move bones at a what?

Answer 4

Joint.

Question 5

What are muscles that work together to move a bone called?

Answer 5

Antagonistic pairs.
The contracting muscle is called the agonist and the relaxing muscle is called the antagonist.

Question 6

Why do muscles work in pairs?

Answer 6

Because they can only pull when they contact, they can’t push.

Question 7

What is an example of muscles acting in antagonistic pairs?

Answer 7

The bones of your lower arm are attached to a biceps muscle and a triceps muscle by tendons. The biceps and triceps work together to move your arm - as one contracts, the other relaxes.

When your B contracts your T relaxes. This pulls the bone so your arm bends (flexes) at the elbow.
When your T contracts your B relaxes. This pulls the bone so your arm straightens (extends) at the elbow.

Question 8

What is skeletal muscle made up of?

Answer 8

Long muscle fibres which are made up of large bundles of long cells.

Question 9

What do muscles act as? and what are they stimulated to contract by?

Answer 9

Effectors and are stimulated to contract by neurones.

Question 10

What is the cell membrane of muscle fibre cells called?

Answer 10

Sarcolemma.

Question 11

Bits of the sarcolemma fold inwards across the muscle fibre and stick into the sarcoplasm (a muscle cells cytoplasm). These folds are called transverse (T) tubules and what do they help to do?

Answer 11

Spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.

Question 12

What is the sarcoplasmic reticulum? What does it store and release and why?

Answer 12

A network of internal membranes which run through the sarcoplasm.
It stores and releases calcium ions that are needed for muscle contraction.

Question 13

Why do muscle fibres have lots of mitochondria?

Answer 13

To provide the ATP that is needed for muscle contraction.

Question 14

What does it means when it says that muscle fibres are multinucleate?

Answer 14

That they contain many nuclei.

Question 15

What are myofibrils? What are they made up of? What are they specialised for?

Answer 15

They are long, cylindrical organelles which are made up of proteins and are highly specialised for contraction.

Question 16

What filaments do myofibrils contain?

Answer 16

Thick myosin filaments and thin actin filaments.

Question 17

What are thick myofilaments made of?

Answer 17

The protein myosin.

Question 18

What are thin myofilaments made of?

Answer 18

The protein actin.

Question 19

What will you see if you look at a myofibril under an electron microscope?

Answer 19

A pattern of alternating dark and light bands.

Question 20

What do dark bands contain in myofibrils? What band are they called?

Answer 20

The thick myosin filaments and some overlapping thin actin filaments - called A-bands.

Question 21

What do light bands contain in myofibrils? What band are they called?

Answer 21

The thin actin filaments only - called I-bands.

Question 22

A myofibril is made up of many short units called what?

Answer 22

Sarcomeres.

Question 23

What line are the ends of each sarcomere marked with?

Answer 23

Z.

Question 24

In the middle of each sarcomere is an M-line. Where is the M-line?

Answer 24

In the middle of the myosin filaments.

Question 25

What zone is around the M-line? What does this zone only contain?

Answer 25

H. This zone only contains myosin filaments.

Question 26

What theory is muscle contraction explain by?

Answer 26

The sliding filament theory.

Question 27

What is the sliding filament theory?

Answer 27

1) Myosin and actin filaments slide over one another to make the sarcomeres contract - the myofilaments themselves don't contract. 2) The simultaneous contraction of lots of sarcomeres means the myofibrils and muscle fibres contract. 3) Sarcomeres return to their original length as the muscle relaxes.

Question 28

What happens to the bands and zones and the sarcomeres in the sliding filament theory?

Answer 28

A-bands stay the same length. I-bands get shorter. H-zones get shorter. The sarcomeres get shorter.

Question 29

What are two features myosin filaments have?

Answer 29

Globular heads and binding sites. Globular heads that are hinged, so they can move back and forth. Each myosin head has a binding site for actin and a binding site for ATP. Actin filaments have binding sites for myosin heads, called actin-myosin binding sites. Another protein called tropomyosin is found between actin filaments. It helps myofilaments move past each other.

Question 30

What are actin-myosin binding sites in resting (unstimulated) muscles blocked by? Why?

Answer 30

Tropomyosin. So myofilaments cannot slide past each other because the myosin heads cannot bind to the actin-myosin binding site on the actin filaments.

Question 31

What influx of ions is muscle contraction triggered by?

Answer 31

An influx of calcium ions.

Question 32

What is the process of muscle contraction when it is triggered by an influx of calcium ions?

Answer 32

1) When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum. 2) This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm. 3) Calcium ions bind to a protein attached to tropomyosin, causing the protein to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament. 4) This exposes the binding site, which allows the myosin head to bind. 5) The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge. 6) Calcium ions also activate the enzyme ATP hydrolyse which hydrolysis ATP to provide the energy needed for muscle contraction. 7) The energy released from ATP causes the myosin head to bend, which pulls the actin filament along in a kind of rowing action. 8) Another ATP molecule provides the energy to break the actin-myosin cross bridge, so the myosin head detaches from the actin filament after it's moved. 9) The myosin head then reattaches to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated (attach, move, detach, reattach to new binding site). 10) Many cross bridges form and break very rapidly, pulling the actin filament along - which shortens the sarcomere, causing the muscle to contract. 11) The cycle will continue as long as calcium ions are present.

Question 33

When the muscle stops being stimulated, calcium ions leave their binding sites and are moved, by what process back into the sarcoplasmic reticulum? (this needs ATP too).

Answer 33

Active transport.

Question 34

When the muscle stops being stimulated causing calcium ions to leave their binding sites and are moved by active transport back into the sarcoplasmic reticulum, what happens to the tropomyosin molecules?

Answer 34

They move back, so they block the actin-myosin binding sites again.

Question 35

When the muscle stop being stimulated, muscles are not contracting because?

Answer 35

No myosin heads are attached to actin filaments so there are no actin-myosin cross bridges. Therefore the actin filaments slide back to their relaxed position, which lengthens the sarcomere.

Question 36

What are two things that provide the energy which is needed for muscle contraction?

Answer 36

ATP and phosphocreatine.

Question 37

As ATP has to be continuously generated so exercise (muscle contraction) can continue, what are the three main ways the happens?

Answer 37

1) Aerobic respiration. 2) Anaerobic respiration. 3) ATP-Phosphocreatine (PCr) System.

Question 38

How is ATP generated for muscle contraction in aerobic respiration?

Answer 38

- Most ATP is generated via oxidative phosphorylation in the cells mitochondria. - Aerobic respiration only works when there is oxygen so it is good for long periods of low-intensity exercise.

Question 39

How is ATP generated for muscle contraction in anaerobic respiration?

Answer 39

- ATP is made rapidly by glycolysis. - The end product of glycolysis is pyruvate, which is converted to lactate by lactate fermentation. - Lactate can quickly build up in the muscles and cause muscle fatigue. - Anaerobic respiration is good for short period of hard exercise.

Question 40

How is ATP generated for muscle contraction in ATP-Phosphocreatine (PCr) System?

Answer 40

- ATP is made by phosphorylating ADP - adding phosphate groups taken from PCr. - PCr is stored inside cells and the ATP-PCr system generates ATP very quickly. - PCr runs out after a few seconds so it is used during short bursts of vigorous exercise. - The ATP-PCr system is anaerobic (does not need oxygen) and it is alactic (does not form any lactate).

Question 41

In ATP-Phosphocreatine (PCr) System, some of the creatine (Cr) gets broken down into creatinine, which is removed from the body via the kidneys. Creatine levels can be higher in people who exercise regularly and those with a high muscle mass. High creatinine levels may also indicate what?

Answer 41

Kidney damage.

Question 42

Skeletal muscles are made of two types of twitch muscle fibres. Different muscles have different proportions of these two twitch fibres. What are the two types?

Answer 42

Slow twitch and fast twitch.

Question 43

What are the properties of slow twitch muscle fibres?

Answer 43

- Contract slowly. - Muscles you use for posture. - Good for endurance activities. - Can work for a long time without getting tired. - Energy is released slowly through aerobic respiration - lots of mitochondria and blood vessels supply the muscles with oxygen. - Reddish in colour because they are rich in myoglobin which is a red-coloured protein that stores oxygen.

Question 44

What are the properties of fast twitch muscle fibres?

Answer 44

- Contract very quickly. - Muscles you use for fast movement. - Good for short bursts of speed and power. - Get tired very quickly. - Energy is released quickly through anaerobic respiration using glycogen (stored glucose) - there are few mitochondria or blood vessels. - Whitish in colour because they do not have much myoglobin (so cannot store much oxygen).