💛 3.6 - 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only) Flashcards
What type of muscle do you use to move?
Skeletal muscle.
What are skeletal muscles attached to bones by?
Tendons.
What attach bones to other bones to hold them together?
Ligaments.
Pairs of skeletal muscle contract and relax to move bones at a what?
Joint.
What are muscles that work together to move a bone called?
Antagonistic pairs.
The contracting muscle is called the agonist and the relaxing muscle is called the antagonist.
Why do muscles work in pairs?
Because they can only pull when they contact, they can’t push.
What is an example of muscles acting in antagonistic pairs?
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.
What is skeletal muscle made up of?
Long muscle fibres which are made up of large bundles of long cells.
What do muscles act as? and what are they stimulated to contract by?
Effectors and are stimulated to contract by neurones.
What is the cell membrane of muscle fibre cells called?
Sarcolemma.
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?
Spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.
What is the sarcoplasmic reticulum? What does it store and release and why?
A network of internal membranes which run through the sarcoplasm.
It stores and releases calcium ions that are needed for muscle contraction.
Why do muscle fibres have lots of mitochondria?
To provide the ATP that is needed for muscle contraction.
What does it means when it says that muscle fibres are multinucleate?
That they contain many nuclei.
What are myofibrils? What are they made up of? What are they specialised for?
They are long, cylindrical organelles which are made up of proteins and are highly specialised for contraction.
What filaments do myofibrils contain?
Thick myosin filaments and thin actin filaments.
What are thick myofilaments made of?
The protein myosin.
What are thin myofilaments made of?
The protein actin.
What will you see if you look at a myofibril under an electron microscope?
A pattern of alternating dark and light bands.
What do dark bands contain in myofibrils? What band are they called?
The thick myosin filaments and some overlapping thin actin filaments - called A-bands.
What do light bands contain in myofibrils? What band are they called?
The thin actin filaments only - called I-bands.
A myofibril is made up of many short units called what?
Sarcomeres.
What line are the ends of each sarcomere marked with?
Z.
In the middle of each sarcomere is an M-line. Where is the M-line?
In the middle of the myosin filaments.
What zone is around the M-line? What does this zone only contain?
H.
This zone only contains myosin filaments.
What theory is muscle contraction explain by?
The sliding filament theory.
What is the sliding filament theory?
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.
What happens to the bands and zones and the sarcomeres in the sliding filament theory?
A-bands stay the same length.
I-bands get shorter.
H-zones get shorter.
The sarcomeres get shorter.
What are two features myosin filaments have?
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.
What are actin-myosin binding sites in resting (unstimulated) muscles blocked by? Why?
Tropomyosin.
So myofilaments cannot slide past each other because the myosin heads cannot bind to the actin-myosin binding site on the actin filaments.
What influx of ions is muscle contraction triggered by?
An influx of calcium ions.
What is the process of muscle contraction when it is triggered by an influx of calcium ions?
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.
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).
Active transport.
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?
They move back, so they block the actin-myosin binding sites again.
When the muscle stop being stimulated, muscles are not contracting because?
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.
What are two things that provide the energy which is needed for muscle contraction?
ATP and phosphocreatine.
As ATP has to be continuously generated so exercise (muscle contraction) can continue, what are the three main ways the happens?
1) Aerobic respiration.
2) Anaerobic respiration.
3) ATP-Phosphocreatine (PCr) System.
How is ATP generated for muscle contraction in aerobic respiration?
- 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.
How is ATP generated for muscle contraction in anaerobic respiration?
- 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.
How is ATP generated for muscle contraction in ATP-Phosphocreatine (PCr) System?
- 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).
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?
Kidney damage.
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?
Slow twitch and fast twitch.
What are the properties of slow twitch muscle fibres?
- 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.
What are the properties of fast twitch muscle fibres?
- 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).
