13.9-13.10 Muscles

Question 1

What are the three types of muscle in the body?

Answer 1

1) Skeletal muscle
2) Cardiac muscle
3) Involuntary muscle (also known as smooth muscle)

Question 2

Where is skeletal muscle found?

Answer 2

• Skeletal muscles make up the bulk of body muscle tissue.

- These are the cells responsible for movement e.g biceps and triceps

Question 3

Where is cardiac muscle found?

Answer 3

• Cardiac muscle cells are only found in the heart.
• These cells are myogenic, meaning they contract without the need for a nervous stimulus, causing the heart to beat in a regular rhythm.

Question 4

Where is involuntary muscle found?

Answer 4

• Involuntary muscle cells are found in many parts of the body e.g the walls of hollow organs such as the stomach and bladder.
• They are also found in the walls of blood vessels and the digestive tract

Question 5

What are skeletal muscles made up of?

Answer 5

Skeletal muscles are made up of bundles of muscle fibres.

Question 6

What is the plasma membrane around the muscle fibres called?

Answer 6

Sarcolemma

Question 7

What is the shared cytoplasm within a muscle fibre called?

Answer 7

Sarcoplasm

Question 8

Why are muscle fibres formed from individual muscle cells fusing together?

Answer 8

This makes the muscle stronger, as the junction between adjacent cells would act as a point of weakness,

Question 9

What is it called when parts of the sarcolemma fold inwards?

Answer 9

Transverse/T-tubules

Question 10

Why do parts of the sarcolemma fold inwards?

Answer 10

• Parts of the sarcolemma fold inwards to help spread electrical impulses throughout the sarcoplasm.
• This ensures that the whole of the fibre receives the impulse to contract at the same time.

Question 11

What else do muscles fibres contain and why?

Answer 11

1) Lots of mitochondria to provide the ATP that is needed for muscle contraction.
2) Sarcoplasmic reticulum which extends throughout the muscle fibre and contains calcium ions required for muscle contraction.

Question 12

What are myofibrils?

Answer 12

Myofibrils are long cylindrical organelles found in muscle which are made of protein and specialised for contraction.

Question 13

Which two types of protein filaments are myofibrils made up of?

Answer 13

1) Actin: The thinner filament and consists of two strands twisted around each other.
2) Myosin: The thicker filament and consists of long rod-shaped fibres that project to one side.

Question 14

What is a sarcomere?

Answer 14

• A sarcomere is the functional unit of a myofibril.

- When a muscle contracts, the sarcomere contracts.

Question 15

What are light bands?

Answer 15

Light bands are areas that appear light as they are the region where the actin and myosin filaments do not overlap. Known as I(isotopic)-bands

Question 16

What are dark bands?

Answer 16

• Dark bands are areas that appear dark because of the presence of thick myosin filaments.
• The edges are particularly dark as the myosin is overlapped with actin.
• a bands

Question 17

What is the Z-line?

Answer 17

• The Z line is a line found at the centre of each light band.
• The distance between adjacent Z-lines is called a sarcomere.

Question 18

What is the H-zone?

Answer 18

• The H zone is a lighter coloured region found in the centre of each dark band.
• Only myosin filaments are present at this point.

Question 19

What is the sliding filament model?

Answer 19

• During contraction the myosin filaments pull the actin filaments towards the centre of the sarcomere.
• This results in the light band (I band) and H-zone becoming narrower, and the Z lines moving closer together causing the sarcomere to shorten
• The dark band (A band) remains the same width, as the myosin filaments themselves have not shortened, but now overlap the actin filaments by a greater amount.
• The simultaneous contraction of lots of sarcomeres means that myofibrils and muscle fibres contract.
• This results in enough force to pull on a bone and cause movement.
• When sarcomeres return to their original length the muscle relaxes.

Question 20

What is the structure of myosin?

Answer 20

• Myosin filaments have globular heads that are hinged which allows them to move back and forwards.
• On the head is a binding site for each of actin and ATP.
• The tails of several hundred myosin molecules are aligned together to form the myosin filament.

Question 21

What are actin-myosin sites?

Answer 21

• Actin filaments have binding sites for myosin heads and these are called actin-myosin sites.

Question 22

What are the actin-myosin sites blocked by?

Answer 22

Actin-myosin binds sites are blocked by the presence of TROPOMYOSIN which is held in place by TROPONIN.

Question 23

When are the actin-myosin sites blocked by tropomyosin?

Answer 23

• When a muscle is in a resting state (relaxed) the actin-myosin sites are blocked by tropomyosin.
• The myosin heads can therefore not bind to the actin, and the filaments cannot slide past each other.

Question 24

When a muscle is stimulated to contract what happens?

Answer 24

• myosin heads form bonds with actin filaments known as actin-myosin cross-bridges.
• The myosin heads then flex (change direction) in unison, pulling the actin filament along the myosin filament.
• The myosin detaches from the actin and its head returns to its original angle, using ATP.
• The myosin then reattaches further along the actin filament and the process occurs again.

Question 25

When is muscle contraction triggered?

Answer 25

Muscle contraction is triggered when an action potential arrives at a neuromuscular junction.

Question 26

What is the neuromuscular junction?

Answer 26

The point where a motor neurone and a skeletal muscle fibre meet.

Question 27

What happens when an action potential reaches the neuromuscular junction?

Answer 27

• When an action potential reaches the neuromuscular junction it stimulates calcium ion channels to open.
• Calcium ions diffuse from the synapse into the synaptic knob where they cause synaptic vesicles (containing acetylcholine) to fuse with the presynaptic membrane.
• Acetylcholine is released into the synaptic cleft by exocytosis and diffuses across the synapse.
• It binds to receptors on the postsynaptic membrane (sarcolemma), opening sodium ion channels and causes depolarisation.
• Acetylcholine is then broken down by acetylcholinesterase into choline and ethanoic acid.

Question 28

How does muscle contraction occur?

Answer 28

1) The depolarisation of the sarcolemma travels deep into the muscle fibre by spreading through the T tubules which are in contact with the sarcoplasmic reticulum.
2) When the action potential reaches the sarcoplasmic reticulum it stimulates calcium ion channels to open.
3) The calcium ions diffuse down their concentration gradient flooding the sarcoplasm with calcium ions.
4) The calcium ions bind to troponin causing it to change shape.
5) This pulls on the tropomyosin moving it away from the actin-myosin binding sites on the actin filament. Now that the binding sites have been exposed, the myosin head binds to the actin filament forming an actin-myosin cross bridge.
6) Once attached to the actin filament the myosin head flexes, pulling the actin filament along.
7) The molecule of ADP bound to the myosin head is released. An ATP molecule can now bind to the myosin head causing the head to detach from the actin filament.
8) The calcium ions present in the sarcoplasm activate the ATPase activity of myosin. This hydrolyses the ATP to ADP and phosphate, releasing energy which the myosin head uses to return to its original position.

Question 29

How is energy supplied during muscle contraction & why is it needed?

Answer 29

• Hydrolysis of ATP into ADP and phosphate.
• Energy required for the movement of myosin heads .
• Energy required for sarcoplasmic reticulum to actively reabsorb calcium ions from the sarcoplasm.

Question 30

What are the 3 main ways ATP is generated?

Answer 30

1) Aerobic respiration
2) Anaerobic respiration
3) Creatine Phosphate

Question 31

How is ATP generated via creatine phosphate?

Answer 31

• Creatine phosphate acts as a reserve supply of phosphate, which is available immediately to combine with ATP, reforming ATP.
• This generates ATP rapidly, but the store of phosphate is used up quickly.