week 2- lesson 1 Muscle Contraction

Question 1

describe the 7 steps leading to muscle contraction

Answer 1

1) AP travels down axon of a motor neurone to axon terminal
2) calcium ion channels open, ca2+ released and they diffuse in terminal
3) synaptic vesicles release acetylcholine by exocytosis
4)this diffuses across synaptic cleft, binds to its receptors. (contain ligand-gates cation channels)
5)channels open
6) sodium ions enter muscle fibre
potassium exits
membrane potentials become less negative
7) threshold value reached, AP propagates along the sarcolemma

Question 2

what would a cease of acetylcholine prevent?

Answer 2

neural transmission to a muscle fibre

Question 3

give 2 examples of how acetylcholine is ceased

Answer 3

1) it diffuses away from the synapse
2) its broken down by acetylcholinesterase to acetic acid and choline. Choline is then transported to axon terminal for re-synthesis of acetylcholine

Question 4

what is the process called between an AP generated to a contraction

Answer 4

EXCITATION-CONTRACTION COUPLING

Question 5

give functions of T tubule

Answer 5

Wrap around the myofibril underneath themembrane+Rich in ion channels+Release ions to SR

Question 6

give functions of sarcoplasmic reticulum

Answer 6

Channel along the fulllength of the fibril filledwith calcium (Ca++)+When signal receivedfrom T-tubule releasescalcium (Ca++)+This signals contractioninitiation of the sarcomere

Question 7

give functions of sarcomere

Answer 7

-functional contractile unit of the striated muscle fibre
-The region of the myofibril between two successive Z-discs
-Composed of myofilaments made up of contractile proteins (alternating thick and thin filaments givingstriated appearance)

Question 8

describe the contractile proteins (2)

Answer 8

Thick filament: A bundle of myosin molecules. Each muscle has a projecting regionwhich has ATPase activity (Myosin ATPase) ATP is bound to the myosin head
Thin filament: Two strands of f-actin with regularly spaced Ca++ regulatory proteins,troponin and tropomyosin

Question 9

name the 3 types of fibres

Answer 9

type 1, type 2a, type 2b

Question 10

which colour is type 2b, explain why

Answer 10

white, (1, 2a are red) because of a lower blood supply and less mitochondria

Question 11

describe the cross-bridge cycle

Answer 11

1) activated myosin head binds to actin forming a C-B, Pi released + bond becomes stronger between actin and myosin.
2) ADP is released, activating the myosin head to PIVOT, sliding the thin myofilament toward the centre of the sarcomere.
3) ATP binds to myosin head. C-B link weakens. Myosin head detaches.
4) Reactivation of myosin head. ATP hydrolysed to ADP and Pi. The energy released reactivates myosin head, returning it to the ‘cocked’ position.

Question 12

what does a motor nerve look like

Answer 12

(myelinated) branches at itsterminal and each branch (axon) ends on aseparate muscle fibre.

Question 13

what is a motor end plate?

Answer 13

the ending at the skeletal muscle fibre.

Question 14

what activates the muscle fibre?

Answer 14

action potential

Question 15

how does an action potential activate the muscle fibre?

Answer 15

by releasing acetylcholine

Question 16

what is the function of the neuromuscular junction

Answer 16

synaptic Vesicles+ Contain Neurotransmitter: A substance that isreleased from the presynaptic membrane thatdiffuses across the synaptic cleft and stimulates theproduction of an action potential in the post-synapticmembrane+ Acetylcholine

Question 17

what occurs at the NMJ?

Answer 17

1) an AP transmitted along nerve to nerve ending
2) at the arrival of the AP, Ca2+ channels open, allowing influx of Ca2+
3) the rise of calcium ions in the nerve terminal causes vesicles to fuse with the presynaptic membrane- these vesicles contain acetylcholine
4) vesicle fusion releases the acetylcholine into the synaptic cleft
5)acetylcholine binds to a receptor on the muscle membrane
6) ACh binding to its receptor opens it to allow Na+ to flow into the cell. this depolarises the muscle cell and if great enough generates an AP in the muscle cell to initiate muscular contraction

Question 18

what events happen in the muscle cell to cause Ca2+ to be released

Answer 18

-AP
-conducted down T-Tubules
-Arrive at the triad with SR
-stimulates release of Ca2+

Question 19

What proteins make up the thick and thin filaments?

Answer 19

Thick – myosin Thin - actin

Question 20

What is the function of the tropomyosin complex?

Answer 20

To prevent actin and myosin from creating a crossbridge and generating unwanted muscle contractions.

Question 21

What happens to the tropomyosin complex when calcium ions bind onto troponin?

Answer 21

Calcium binds to a protein (troponin) that is attached to tropomyosin that wraps its way around the myosin and blocks actin and myosin from interacting. Binding of calcium to troponin causes it to move the tropomyosin which allows myosin to attach to its binding site on actin to generate a crossbridge.

Question 22

Where does calcium come from and how is it released into the muscle fibre?

Answer 22

SR

Question 23

Describe how a neural impulse from a motor neuron causes the release of calcium described in the previous section. Your answer
should describe how the electrical signal is convert to a chemical signal in the NMJ.

Answer 23

A motor neurone from the spinal chord sends electrical signals down its axon and to its axon terminal. At the neuromuscular junction a neurotransmitter called acetylcholine is released into the synaptic cleft. Acetylcholine diffuses across the cleft and binds to receptors on the sarcolemma . This stimulates changes in the membrane that cause it to depolarise which spreads deep into the cell via the T-Tubule. The T-tubule is linked to the sarcoplasmic reticulum which is especially adapted for the storage of calcium. The depolarisation of the membrane stimulates the sarcoplasmic reticulum to release calcium into the cell allowing myosin to bind to actin and contraction to occur.

Question 24

Describe a motor unit and explain how they are used to allow a muscle to carry out a variety of tasks of different intensities. For example stroking a dog and carrying shopping.
A motor unit is the motor neurone and all of the individual muscle fibres(cells) it innervates (connects to).

Answer 24

Motor units vary in size, with smaller units connected to fewer, smaller fibres which are involved in finer movements.
Larger motor units are generally connected to more and large muscle fibres.
Small motor units are easily stimulated by low frequency impulses, where as larger motor units require much bigger stimulus.
During muscle recruitment, the frequency of impulses increases when more tension is required to move a heaver load and small, medium and larger muscle units will be recruited which combined generate enough tension to move the load.

Question 25

What is the peri and endomysium?

Answer 25

Its is connective tissue that surrounds the muscle fibres and muscle bundles. It functions to transmit the force(tension) generated by contacting muscles fibres to the tendon.

Question 26

What name is given to a muscle cell?

Answer 26

muscle fibre

Question 27

What is the sarcolemma?

Answer 27

The membrane that surrounds the muscle cell.

Question 28

What are the names of the contractile proteins of the myofibril?

Answer 28

Actin and myosin

Question 29

Describe attachment

Answer 29

Myosin head is bound to actin forming a cross bridge.
Phosphate and ADP are bound to myosin head

Question 30

Describe power stroke

Answer 30

Myosin head pulls actin towards the m lines
Phosphate and adp are released from myosin head

Question 31

Describe release

Answer 31

Myosin head gets uncooked from actin
ATP binds to myosin

Question 32

Describe cocking

Answer 32

Myosin head is undergoing conformational change to high energy state
ATP hydrolysed to adp and pi, ready to restart cycle again

Question 33

Which region shortens during muscle contraction

Answer 33

I band