Sarcomere Structure

Question 1

What are myoblasts ?

Answer 1

Embryonic mesoderm cells

Question 2

What is a myotube ?

Answer 2

Immature multinucleate muscle fibre

Question 3

Briefly describe the development of muscle

Answer 3

Myoblasts undergo cell division to increase in number and enlarge.

Several myoblasts fuse together to form a myotube.

Myotube matures into skeletal muscle fibre.

Question 4

Outline the steps involved in muscle contraction

Answer 4

Neuronal stimulus

Transfer of signal to muscle cell

Relay of signal inside the cell

Conversion of electrical signal to mechanical signal

Muscle contraction

Question 5

What is the sarcomere ?

Answer 5

The sarcomere is the main contractile unit of muscle fiber in the skeletal muscle.

Can be known as the structure between 2 Z discs.

Question 6

Thick filaments

Answer 6

Myosin

Question 7

Thin filaments

Answer 7

Actin

Question 8

What is each sarcomere composed of ?

Answer 8

Myofilaments containing actin and myosin.

Bundles of myofilaments are called myofibrils.

Question 9

A bands

Answer 9

Dark bands that contain whole thick filaments (myosin)

Question 10

What does the outer edge of the A band contain ?

Answer 10

Thick and Thin filaments which overlap.

Thick - myosin
Thin - actin

Question 11

I bands

Answer 11

Light bands that contain only the thin filaments (actin) and are located between the two thick filaments.

Question 12

Z disc

Answer 12

The area that traverses the I bands and marks the point of the connection between the two neighboring actin filaments.

Question 13

M line

Answer 13

Marks the middle of the sarcomere and contains the protein called myomesin.

Question 14

H zone

Answer 14

The H zone contains only myosin.

Question 15

Name some supporting proteins

Answer 15

Alpha-Actinin
Titin
Nebulin
Dystrophin

Question 16

At muscle rest describe the bands of the sarcomere

Answer 16

The A and I bands are similar widths

Question 17

During muscle contraction describe the bands of the sarcomere

Answer 17

Myosin binds to actin, pulling inwards shortening the sarcomere.

The Z discs move closer together.

The I band reduces in size.
The A band remains the same width, but has greater actin and myosin overlap.

Question 18

What does each myosin-II molecule consist of ?

Answer 18

2 Heavy intertwined chains
2 Essential light chains (which stabilises the myosin head)
2 Regulatory light chains (which regulates ATPase activity of myosin)

Question 19

What does each actin consist of ?

Answer 19

2 intertwined alpha-helical chains of actin

Tropomyosin (prevents myosin binding to actin in the relaxed state)

Troponin (binds actin, tropomyosin and calcium)

Question 20

Function of tropomyosin

Answer 20

Prevents myosin binding to actin in the relaxed state

Question 21

Function of TnC

Answer 21

Binds Ca2+

Question 22

Function of TnT

Answer 22

Binds Tropomyosin

Question 23

Function of TnI

Answer 23

Binds actin : Inhibits contraction

Question 24

Explain what happens after neuronal stimulation and depolarisation of the muscle cell.

Answer 24

Ca2+ is released from the sarcoplasmic reticulum and binds TnC.

This binding causes a conformational change in TnI and TnT.

This rotates tropomyosin, to reveal myosin binding sites on actin.

In presence of ATP, myosin binds to actin.

Sarcomeres shorten and muscle contracts.

Question 25

Describe what happens when myosin binds to actin

Answer 25

The sarcomeres shorten and muscle contracts. Calcium couples the electrical stimulation into mechanical contraction.

Question 26

Function of calcium in muscle contraction

Answer 26

Calcium couples the electrical stimulation into mechanical contraction.

Question 27

Function of TnT in relation to tropomyosin

Answer 27

TnT rotates tropomyosin to reveal myosin bonding sites on actin.

Question 28

What is cross bridge formation ?

Answer 28

Energised myosin head attaches to an actin myofilament, forming a cross-bridge.

Question 29

What is the power (working) stroke ?

Answer 29

ADP + Pi are released, the myosin head pivots and bends, changing to its bent low-energy state. As a result of this it pulls the actin filament towards the M line.

Question 30

What happens in cross bridge detachment ?

Answer 30

After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches. i.e. the cross bridge "breaks"

Question 31

What happens in the cocking of the myosin head ?

Answer 31

As ATP is hydrolysed to ADP + Pi, the myosin head returns to its pre stroke high-energy, or cocked position.

Question 32

Explain what happens at the neuromuscular junction.

Answer 32

Action potential travels down motor neurone to the terminal motor end plate. Voltage gated calcium channels open and influx of Ca2+ enters. This initiates vesicles containing acetylcholine to fuse with membrane. Ach is released into the synaptic cleft. Ach attaches to Nicotonic Ach receptors. Acetylcholinesterase rapidly breaks down Ach in the synaptic cleft;

Question 33

Function of acetylcholinesterase

Answer 33

Rapidly breaks down Ach in the synaptic cleft

Question 34

What are T tubules ?

Answer 34

T Tubules are extensions of the sarcolemma - invaginate into the cell.

Question 35

What is the function of T tubules ?

Answer 35

They transmit the electrical impulse deep within the cell structure. They are closely associated with the sarcoplasmic reticulum to stimulate release of Ca2+, which enables the whole cell to contract almost simultaneously.

Question 36

What is the sarcoplasmic reticulum ?

Answer 36

An elaborate smooth endoplasmic reticulum. A vast network of interconnecting tubules surrounding myofibril.

Question 37

What is the function of the sarcoplasmic reticulum ?

Answer 37

Stores calcium ---> releases on stimulation ---> contraction.

Question 38

Explain what happens at the end of muscle contraction.

Answer 38

The action potential stops Ca2+ is pumped back into the sarcoplasmic reticulum by active transport. SERCA Calcium may also be transported extracellularly via symporters.

Question 39

Name some major Ca2+ binding proteins in skeletal muscle, within the sarcoplasmic reticulum.

Answer 39

Within the sarcoplasmic reticulum, calsequestrin and calreticulin are major Ca- binding proteins in skeletal muscle.

Question 40

What are the 2 phases in muscle contraction ?

Answer 40

1. Motor neuron stimulates muscle fibre. 2. Excitation-contraction coupling occurs.

Question 41

Summarise what happens in Phase 1 of muscle contraction.

Answer 41

Action potential arrives at axon terminal at neuromuscular junction. ACh released; binds to receptors on sarcolemma. Ion permeability of sarcolemma changes. Local change in membrane voltage (depolarisation) occurs. Local depolarisation ignites action potential in sarcolemma.

Question 42

Summarise what happens in Phase 2 of muscle contraction.

Answer 42

Action potential travels across the entire sarcolemma. Action potential travels along T tubules Sarcoplasmic reticulum releases Ca2+; Ca2+ binds to troponin; myosin binding sites on actin are exposed. Myosin heads bind to actin; contraction begins.