Respuesta :
For contraction to occur, a neuron must send a message to the muscle cell through Acetylcholine. By cascade effect, the muscle cell depolarizes, triggering interaction between filaments, which leads to fiber contraction.
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Let us first describe different steps needed to produce muscle contraction.
1) The central nervous system generates an action potential that travels to the muscle fiber activating the calcium channels.
2) Calcium triggers vesicles fusion to the presynaptic membrane releasing acetylcholine (Ach) into the synaptic space. This process is known as exocytosis.
3) Ach diffuses across the synaptic cleft forward to the muscle membrane.
4) Once there, Ach binds to its receptors on the postsynaptic membrane of the skeletal muscle fiber, causing ion channels to open.
5) Positively charged sodium ions cross the membrane to get into the muscle fiber (sarcoplasm) and potassium leaves the cell.
6) The difference in charges caused by these ions' transport charges the muscle fiber membrane positively. It depolarizes.
7) Action potential enters the t-tubules, depolarizing the inner portion of the muscle fiber.
8) Contraction initiates when the action potential depolarizes the inner portion of the muscle fiber.
9) Calcium channels activate in the T tubules membrane, releasing calcium into the sarcolemma.
10) At this point, the muscle is at rest, and the tropomyosin is inhibiting the attraction strengths between myosin and actin filaments.Tropomyosin is obstructing binding sites for myosin on the thin filament.
11) When calcium binds to troponin C, troponin T alters the tropomyosin position by moving it and unblocking the binding sites.
12) Myosin heads join the uncovered actin-binding points forming cross-bridges, and while doing so, ATP turns into ADP and inorganic phosphate, which is released.
13) Myofilaments slide impulsed by chemical energy collected in myosin heads, producing a power stroke. The power stroke initiates when the myosin cross-bridge binds to actin. As they slide, ADP molecules are released.
14) A new ATP links to myosin heads and breaks the bindings to the actin filament. Then ATP splits into ADP and phosphate, and the energy produced is accumulated in the myosin heads, which starts a new binding cycle to actin.
15) Finally, Z-bands are pulled toward each other, shortening the sarcomere and the I-band, producing muscle fiber contraction.
Now, let us complete the following sentenses.
Note: As I do not have available options, I will complete the sentences using my words. But if you have options, take a general idea and use it to complete your work with your options.
1) Start: neuronal action potential reaches the axon terminal !
2) _Acetylchonine (neurotransmitter)_ is released from synaptic vesicles by ___exocytosis event (fusion of the vesicle membrane with cellular membrane)___!
3) __Acetylchonine (neurotransmitter)__ diffuses across the _synaptic cleft or space_ !
4) acetylcholine binds to receptors on the motor end plate !
5) _ion channels open in the _postsynaptic muscle membrane_ !
6) _Sodium (Na+)_ diffuses into the __sarcoplasm_ and begins an _action potential_(electrical current) !
7) the __action potential__ propagates along the __t-tubules__ and __produce contraction_ !
8) _Calcium__ ions are released from the _from sarcoplasmic reticulum_ !
9) calcium ions bind to troponin !
10) troponin pulls _tropomyosin_ away from _actin-binding sites_ !
11) _ATP molecule_ is hydrolyzed, which _provides energy for filaments to slide_(power strocke) ; actin binds myosin heads_ !
12) _Actin_ dissociate from _myosine_ and it undergoes a new binding cycle to actin_ !
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