week 2- lesson 1 Muscle Contraction Flashcards
describe the 7 steps leading to muscle contraction
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
what would a cease of acetylcholine prevent?
neural transmission to a muscle fibre
give 2 examples of how acetylcholine is ceased
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
what is the process called between an AP generated to a contraction
EXCITATION-CONTRACTION COUPLING
give functions of T tubule
Wrap around the myofibril underneath themembrane+Rich in ion channels+Release ions to SR
give functions of sarcoplasmic reticulum
Channel along the fulllength of the fibril filledwith calcium (Ca++)+When signal receivedfrom T-tubule releasescalcium (Ca++)+This signals contractioninitiation of the sarcomere
give functions of sarcomere
-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)
describe the contractile proteins (2)
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
name the 3 types of fibres
type 1, type 2a, type 2b
which colour is type 2b, explain why
white, (1, 2a are red) because of a lower blood supply and less mitochondria
describe the cross-bridge cycle
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.
what does a motor nerve look like
(myelinated) branches at itsterminal and each branch (axon) ends on aseparate muscle fibre.
what is a motor end plate?
the ending at the skeletal muscle fibre.
what activates the muscle fibre?
action potential
how does an action potential activate the muscle fibre?
by releasing acetylcholine
what is the function of the neuromuscular junction
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
what occurs at the NMJ?
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
what events happen in the muscle cell to cause Ca2+ to be released
-AP
-conducted down T-Tubules
-Arrive at the triad with SR
-stimulates release of Ca2+
What proteins make up the thick and thin filaments?
Thick – myosin Thin - actin
What is the function of the tropomyosin complex?
To prevent actin and myosin from creating a crossbridge and generating unwanted muscle contractions.
What happens to the tropomyosin complex when calcium ions bind onto troponin?
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.
Where does calcium come from and how is it released into the muscle fibre?
SR
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.
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.
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).
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.
What is the peri and endomysium?
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.
What name is given to a muscle cell?
muscle fibre
What is the sarcolemma?
The membrane that surrounds the muscle cell.
What are the names of the contractile proteins of the myofibril?
Actin and myosin
Describe attachment
Myosin head is bound to actin forming a cross bridge.
Phosphate and ADP are bound to myosin head
Describe power stroke
Myosin head pulls actin towards the m lines
Phosphate and adp are released from myosin head
Describe release
Myosin head gets uncooked from actin
ATP binds to myosin
Describe cocking
Myosin head is undergoing conformational change to high energy state
ATP hydrolysed to adp and pi, ready to restart cycle again
Which region shortens during muscle contraction
I band
