Animal responses- muscles

Question 1

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

Answer 1

1. Muscle fibres are enclosed within a plasma membrane known as the sarcolemma
2. The muscle fibres contain a number of nuclei and are much longer than normal cells, as they are formed as a result of many individual embryonic muscle cells fusing together
3. This makes the muscle stronger, as the junction between adjacent cells would act as a point of weakness.
4. The shared cytoplasm within a muscle fibre is known as sarcoplasm
5. Parts of the sarcolemma folds inwards (known as transverse or t-tubules) to help spread electrical impulses throughout the whole sarcoplasm. THis ensures the whole of the fibre receives the impulse to contract at the same time
6. Muscle fibres have lots of mitochondria to provide ATP that is needed for muscle contraction
7. They also have modified version of the endoplasmic reticulum, known as the sarcoplasmic reticulum. This extends throughout the muscle fibre and contains calcium ions required for muscle contraction.

Question 2

Define 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 myofibril

Answer 3

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

Question 4

Define sarcolemma

Answer 4

The cell membrane of a muscle fibre cell

Question 5

Define sarcoplasm

Answer 5

The cytoplasm of muscle cells

Question 6

Define sarcoplasmic reticulum

Answer 6

The specialized endoplasmic reticulum of muscle fibres that functions especially as a storage and release area for calcium.

Question 7

Define transverse tubule (T-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 sarcomere

Answer 8

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

Question 9

List the 3 types of muscle and state where they occur and their basic function

Answer 9

1. Skeletal muscle- make up the bulk of body muscle tissue. These are responsible for movement
2. Cardiac muscle- cardiac muscle cells are found only in the heart. These cells are myogenic, meaning they contract without the need for a nervous stimulus
3. Involuntary muscle (smooth muscle)- Found in many parts of the body e.g. walls of stomach and bladder. They are also found in the walls of blood vessels and the digestive tract where through peristalsis they move food along the gut

Question 10

Describe skeletal muscles structure and function

Answer 10

1. Fibre appearance- striated
2. Control- conscious (voluntary)
3. Arrangement- regularly arranged so muscle contracts in one direction
4. Contraction speed- rapid
5. Length of contraction- short
6. Structure- muscles showing cross striations are known as striated or striped. Fibres are tubular and mulitnucleated

Question 11

Describe cardiac muscles structure and function

Answer 11

1. Fibre appearance- specialised-striated
2. Control- involuntary
3. Arrangement- cells branch and interconnect resulting in simultaneous contraction
4. Contraction speed- intermediate
5. Length of contraction- intermediate
6. Structure- cardiac muscle does show striations but they are much fainter than those in skeletal muscle. Fibres are branched and uninucleated

Question 12

Describe involuntary (smooth) muscles structure and function

Answer 12

1. Fibre appearance- non-striated
2. Control- involuntary
3. Arrangement- no regular arrangement- different cells can contract in different directions
4. Contraction speed- slow
5. Length of contraction- can remain contracted for a relatively long time
6. Structure- muscles showing no cross striations are called non-striated or unstriped muscles. Fibres are spindle shaped and uninuleated

Question 13

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

Answer 13

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

Question 14

What are myofibrils

Answer 14

1. Each muscle fibre contains many myofibrils
2. They are long cylindrical organelles made of proteins and specialised for contraction
3. On their own they provide almost no force but collectively they are very powerful.
4. Myofibrils are lined up in parallel to provide maximum force when they all contract together

Question 15

Explain why myofibrils have a striped appearance

Answer 15

1. Light bands- these areas appear light as they are the region where the actin and myosin filaments do not overlap. Also known as isotonic bands or I-bands
2. Dark bands- these areas appear dark because of the presence of thick myosin filaments. The edges are particularly dark as the myosin is overlapped with actin. Also known as anisotropic bands or A-bands.
3. Z-line- this is a line found at the centre of each light band. The distance between adjacent z-lines is called a sarcomere.
4. H-zone- this is a lighter coloured region in the centre of each dark band. Only myosin filaments are present at this point. When the muscle contracts the H-zone decreases.
   see p371

Question 16

What features should you be able to identify from a photomicrograph of skeletal muscle

Answer 16

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 structured arrangement of sarcomeres which as appear dark and light bands
3. Streaks of connective adipose tissue
4. Capillaries running in between the fibres

Question 17

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

Answer 17

1. Muscle contraction is triggered when an action potential arrives at a neuromuscular junction- point where a motor neurone and a skeletal muscle fibre meet
2. When an action potential reaches the neuromuscular junction, it stimulates calcium ion channels to open.
3. Calcium ions then diffuse from the synapse into the synaptic knob, where they cause synaptic vesicles to fuse with the presynaptic membrane.
4. Acetylcholine is released into the synaptic cleft by exocytosis and diffuses across the synapse.
5. It binds to receptors on the postsynaptic membrane (sarcolemma), opening sodium ion channels and resulting in depolarisation.
6. Acetylcholine is then broken down by acetylcholinesterase into choline and ethanoic acid.
7. This prevents the muscle being overstimulated.
8. Choline and ethanoic acid diffuse back into the neurone, where they are recombined into acetylcholine, using energy provided by mitochondria.

Question 18

Why are there many neuromuscular junctions along a length of a muscle and how does the response change for different required forces

Answer 18

1. There are many neuromuscular junctions along the length of a muscle to ensure that all the muscle fibres contract simultaneously.
2. If there was only one the muscle fibres would not contract together and therefore the contraction would not be as powerful and it would also be much slower, as a wave on contraction would have to travel across the muscle to stimulate the individual fibres to contract
3. All the muscle fibres supplied by a single motor neurone are known as a motor unit- the fibres act as a single nit
4. If a strong force is need, a large number of motor units are stimulated, whereas only a small number are stimulated if a small force is required.

Question 19

Explain the releasing of calcium ions stage in the sliding filament hypothesis of muscle contraction.

Answer 19

1. First the events at the neuromuscular junction occur causing depolarisation
2. The depolarisation of the sarcolemma travels deep into the muscle fibre by spreading through the T-tubules. These are in contact with the sarcoplasmic reticulum
3. The sarcoplasmic reticulum contains stored calcium ions which it actively absorbs from the sarcoplasm.
4. When the action potential reaches the sarcoplasmic reticulum it stimulates calcium channels to open.
5. The calcium ions diffuse down their concentration gradient flooding the sarcoplasm with calcium ions.

Question 20

Explain the sliding filament hypothesis of muscle contraction.

Answer 20

1. Initiation
   - Tropomyosin molecule prevents myosin head from attaching to the binding site on the actin molecule
   - Ca2+ ions bind to troponin causing it to change shape.
   - This pulls on the tropomyosin moving it away from the actin-myosin binding sites on the actin filament.
2. Attach
   - Myosin heads now attaches to teh bidning stie of the actin filament forming a cross-bridge
3. Bend
   - The myosin head bends/flexes, pulling the acting filament along the myosin- power stroke
   - The molecule of ADP bound to the myosin head is released.
4. Release
   - An ATP molecule can now bind to the myosin head.
   - This causes the head to released from the myosin binding site on actin, breaking the cross-bridge
5. Straighten
   - The calcium ions present in the sarcoplasm also activate the ATPase activity hydrolysing ATP releasing energy
   - This energy is used to return the myosin head to its original position.
6. The myosin head can now return to its original position and reattaches to another actin-myosin binding site further along the actin filament and the cycle is repeated.
7. During the period of stimulation many actin-myosin bridges form and break rapidly, pulling the actin filament along. This shortens the sarcomere and causes muscle to contract.

Question 21

How does muscle contraction change the arrangement of the actin and myosin filaments as they appear on an electron micrograph

Answer 21

1. The z lines moving closer together, shortening the sarcomere
2. The H-zone (A band that isn’t overlapped with I band) becomes narrower
3. I band/light band gets shorter
4. A band/dark band stays the same
5. The dark part of the dark band remains the same width, as the myosin filaments themselves have not shortened, but now overlap the actin filaments by a greater amount

Question 22

Describe the structure of myosin

Answer 22

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

Question 23

Describe the structure of actin

Answer 23

1. Actin filaments have binding sites for myosin heads- actin-myosin binding sites.
2. However, these binding sites are often blocked by the presence of another protein called tropomyosin which is held in place by the protein troponin
3. When a muscle is in a resting state the actin-myosin sites are blocked by tropomyosin. The myosin heads therefore cannot bind to the actin and the filaments cannot slide past each other.

Question 24

State the roles of ATP in muscle contraction.

Answer 24

1. Muscle contraction requires large quantities of energy,
2. This is provided by the hydrolysis of ATP into ADP and phosphate.
3. The energy is required for the movement of the myosin heads and to enable the sarcoplasmic reticulum to actively reabsorb calcium ions from the sarcoplasm.

Question 25

Describe the similarities and differences between synapses and neuromuscular junctions.

Answer 25

1. Cellular connection made in a synapse is with a neurone but with neuromuscular junction it is a muscle cell
2. Effect on post-synaptic cell- Synapse is action potential, Neuromuscular junction leads to depolarisation of sarcolemma and contraction of cell
3. Shape of post-synaptic membrane- synapse has a normal smooth membrane, neuromuscular junction is wiggly membrane
4. Neurotransmitter- synapse has acetylcholine but can use other stuff, Neuromuscular junction has just acetylcholine.
5. Everything else is the same. e.g role of vesicles is both to store acetylcholine. And they remove the neurotransmitter in the same way.

Question 26

State the 3 ways that ATP is regenerated in muscle fibres

Answer 26

1. Aerobic respiration
2. Anaerobic respiration
3. Creatine phosphate

Question 27

Describe how creatine phosphate regenerates ATP

Answer 27

1. Creatine is stored in muscle
2. To form ATP, ADP has to phosphorylated- a phosphate group has to be added.
3. Creatine phosphate acts as a reserve supply of phosphate, which is available immediately to combine with ADP rapidly, but the store of phosphate is used up quickly .
4. As a result this is used for short bursts of vigorous exercise such as a tennis serve
5. When the muscle is relaxed, the creatine phosphate store is replenished using phosphate form ATP .