Animal Responses - Muscle.

Question 1

Draw, label and annotate a diagram of a muscle fibre to show the components of the cell and their functions.

Answer 1

• Muscle fibres are enclosed within a plasma membrane known as sarcolemma.
• Muscle fibres contain a number of nuclei and are much longer than normal cells, they are formed as a result of many embryonic muscle cells fusing together.
• This makes muscle stronger as the junction between adjacent cells would act as a point of weakness.
• The shared cytoplasm within a muscle fibre is known as sarcoplasm.
• Parts of the sarcolemma fold inwards to help spread electrical impulses throughout the sarcoplasm. This ensure that the whole fibre receives the impulse to contract at the same time.
• Muscle fibres have lots of mitochondria to provide the ATP that is needed for muscle contraction.
• They also have a modified version of the endoplasmic reticulum, known as the sarcoplasmic reticulum. This extends throughout the muscle fibre and contain calcium ions required for contractions.

Question 2

Define the term “muscle fibre”.

Answer 2

A muscle cell, especially one of the cylindrical, multinucleate cells that make up skeletal muscles and are composed of numerous myofibrils that contract when stimulated.

Question 3

Define the term “myofibril”.

Answer 3

Long cylindrical organelles found in the muscle which are made of proteins and specialised for contraction.

Question 4

Define the term “sarcolemma”.

Answer 4

The cell membrane of a muscle fibre cell.

Question 5

Define the term “sarcoplasm”.

Answer 5

The cytoplasm of muscle cells.

Question 6

Define the term “sarcoplasmic reticulum”.

Answer 6

The a specialised endoplasmic reticulum of muscle cells that functions especially as a storage and release area for calcium ions.

Question 7

Define the term “transverse tubule”.

Answer 7

Any of the small tubules which run transversely through a muscle fibre and through which electrical impulses are transmitted.

Question 8

Define the term “sarcomere”.

Answer 8

The sarcomere is the functional unit of myofibril. When a muscle contracts the sarcomere shortens.

Question 9

What is a myofibril?

Answer 9

Myofibrils are long cylindrical organelles made of protein specialised for contraction. Collectively, they provide a lot of force. They are lined up in two parallel lines to maximise force when they contract together.

Question 10

List the 3 types of muscle and state where they occur.

Answer 10

1) Skeletal - make up the bulk of the body muscle tissue. These are cells responsible for movement (e.g. for the movement of biceps and triceps).
2) Cardiac - found only in the heart. These cells are myogenic, meaning they contract without need the for a nervous stimulation, causing the heart to beat in a regular rhythm.
3) Smooth muscle - involuntary. Found in the walls of the stomach and bladder, the walls of blood vessels and the digestive tract where it performs peristalsis.

Question 11

Describe the structure, location and function of skeletal muscle.

Answer 11

1) appearance = striated.
2) Control = voluntary/ conscious.
3) Arrangement = regularly arranged so that muscle contracts in one direction.
4) Speed of contraction = rapid.
5) Length of contraction = short.
6) Structure = Muscle shows cross striations so known as striated. Fibres are tubular and multinucleated.

Question 12

Describe the structure, location and function of cardiac muscle.

Answer 12

1) Appearance = specialised striated.
2) Control = involuntary.
3) Arrangement = cells branch and interconnect resulting in simultaneous contraction.
4) Speed of contraction = intermediate.
5) Length of contraction = intermediate.
6) Structure = Fainter striations. Fibres are branched and uninucleated.

Question 13

Describe the structure, location and function of smooth muscle.

Answer 13

1) Appearance = non-striated.
2) Control = involuntary.
3) No regular arrangement = different cells can contract in different directions.
4) Speed of contraction = slow.
5) Length of contraction = can remain contracted for a long time.
6) Structure = No cross striations so non-striated. Fibres are spindle shaped and uninucleated.

Question 14

What are the two types of protein filaments that make up myofibrils?

Answer 14

1) Actin - the thinner filament. Consists of two strands twisted around each other.
2) Myosin - the thicker filament. Consists of long rod-shaped fibres with bulbous heads that project to one side.

Question 15

Explain why myofibrils have a striped appearance.

Answer 15

see pp. 371 for diagram.
They have alternating dark and light bands which result in their striated appearance:
- Light bands: these areas appear light as they are the region where the actin and myosin filaments do not overlap.
- Dark bands: appear dark because of presence of thick myosin filaments. The edges are particularly dark as the edges are overlapped with myosin.
- Z-line: this is the line found at the centre of each light band. The distance between the adjacent Z-lines is called a sarcomere.
- H-zone: this is a lighter coloured region found in the centre of each dark band. Only myosin filaments are present at this point. When muscle contracts, the H-zone decreases.

Question 16

Label a photomicrograph of skeletal muscle.

Answer 16

Should be able to identify the following things in a section of skeletal muscle:

1) Individual muscle fibres - long and thin multinucleated fibres that are crossed with a regular pattern of fine red and white lines.
2) The highly structure arrangement of sarcomeres which appear as dark and light bands.
3) Streaks of connective and adipose tissue.
4) Capillaries running in between fibres.

Question 17

Describe the events that occur at a neuromuscular junction in order to cause (and then stop) muscle contraction.

Answer 17

1) Action potential reaches neuromuscular junction and stimulates calcium ion channels to open.
2) Calcium ions then diffuse from the synapse into the synaptic knob, where they cause synaptic vesicles carrying acetyl choline to fuse with the presynaptic membrane.
3) Acetyl choline is then released into the synaptic cleft by exocytosis and diffuses across the synaptic cleft.
4) It binds to receptors on the post synaptic membrane (the sarcolemma), opening sodium ion channels.
5) Sodium ions which had been free in the synaptic cleft diffuse into the sarcolemma down their electrochemical gradient via the opened sodium ion channels.
6) This depolarises the sarcolemma, resulting in a new action potential.

Question 18

Explain the sliding filament hypothesis of muscle contraction.

Answer 18

(Pre-cursor is events at neuromuscular junction)

1) Initiation
- Action potential arrives at the sarcolemma, causing it to depolarise. The depolarisation spreads deep into the muscle fibre via T-tubules.
- T-tubles are in contact with sarcoplasmic reticulum, which has stored calcium ions. When the action potential reaches the SR calcium ion channels are stimulated to open and calcium ions diffuse down their electrochemical gradient into the sarcoplasm.

2) Binding
- The released calcium ions bind to the troponin protein and change its shape.
- This causes the tropomyosin molecule to pull away from the actin-myosin binding site on the actin filament that it had been blocking.
- This means myosin heads are able to attach to actin-myosin binding site. This forms a cross-bridge.

3) Bending
- Once attach to binding site, the myosin head bends, pulling actin filament along with it (called a power stroke).
- As myosin head pulls actin along, it releases its ADP
and phosphate.

4) Straighten
- ATP molecule fixes to myosin head, causing it to detach from the actin filament.
- ATP is hydrolysed into ADP and Phosphate by ATPase (ATPase activated by calcium ions in sarcoplasm).
- The hydrolysis of ATP releases energy which is used to return the myosin head to its original position.

5) Repeat
- Myosin head re-attaches to another binding site further along the actin filament and the cycle is repeated.
- Many actin-myosin bridges form and break rapidly, causing actin to be pulled along.
- Causes sarcomere to shorten and so muscle contracts.

Question 19

Describe the structure of myosin.

Answer 19

• They have hinged globular heads (mysoin heads) which move back and forth in a muscle contraction.
• On the head, is a binding for site actin and a binding site for ATP.
• The tails of several hundred myosin molecules are aligned together to form a myosin filament.

Question 20

Describe the structure of actin.

Answer 20

• Actin filaments have binding sites for myosin heads. These are called actin-myosin binding sites.
• When the muscle is in resting state, these binding sites are blocked by tropomyosin which is held in place by a protein called troponin.
• In resting state, myosin heads cannot bind to the actin because the binding site is blocked by tropomyosin.

Question 21

How does muscle contraction change the arrangement of the actin and myosin filaments, as they appear on an electromicrograph.

Answer 21

During contraction the myosin filaments pull the actin filaments inwards towards the centre of the sarcomere. These are the results:

• The light band/ A band becomes shorter.
• The z lines move closer together, shortening the sarcomere.
• The h-zone becomes shorter.
• The overall width of the dark band/ A band stays the same buts its darker regions get longer as they now overlap more with the actin filaments.

Question 22

State the roles of ATP in muscle contraction.

Answer 22

• Active reabsorption of calcium ions into the sarcoplasmic reticulum from the sarcoplasm. Allows cycle of muscle contraction to continue.
• Return myosin head to its original position by the energy provided from the hydrolysis of ATP into ATP and P.
• Release of myosin head from actin filament. Binding of ATP causes myosin head to detach, breaking cross-bridge.

Question 23

Explain how a sarcomere can lengthen after contraction.

Question 24

Describe the similarities and differences between synapses and neuromuscular junctions.

Answer 24

1) Cellular connection: Synapse = neurone. NMJ = muscle fibre.
2) Effect on post-synaptic cell: Synapse = AP created which travels along neurone to next synapse. NJM = sarcolemma depolarised.
3) Shape of post-synaptic membrane: Synapse = smooth. NJM = folded.
4) Neurotransmitter: Synapse = acetyl choline, but also noradrenaline and adrenaline. NJM = just acetyl choline.
5) Role of vesicles, movement of neurotransmitter, action of neurotransmitter on post-synaptic cell and removal of neurotransmitter is the same in both.

Question 25

State the three ways in which ATP is regenerated in muscle fibres.

Answer 25

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

Question 26

Describe how creatine phosphate regenerates ATP.

Answer 26

• Creatine phosphate is stored in muscle.
• To form ATP, ADP has to be phosphorylated ( a phosphate group is added).
• Creatine Phosphate acts as a reserve supply of phosphate, which is available immediately to combine with ADP, reforming ATP.
• This generates ATP rapidly, but the store of phosphate is used up quickly. As a result, this system is used for short bursts of vigorous exercise (e.g. a tennis serve).
• When the muscle is relaxed, the creatine phosphate store is replenished using ATP.