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Place the following events of excitation in the correct order. 1) ion channels on the sarcolemma open and sodium ions enter the muscle fiber 2) the entry of sodium ions into the muscle fiber depolarizes the sarcolemma locally 3) acetylcholine is released from vesicles in the motor neuron into the synaptic cleft 4) an action potential arrives at the axon terminal of a motor neuron 5) acetylcholine binds to ligand-gated sodium ion channels in the motor end plate

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eyitbay

Answer:

4, 3 5, 1, 2.

Explanation:

The events of muscle contraction begins from the presynaptic neuron, which cause the influx of Ca+, and the release of acetycholine neurotransmitter  into the synaptic cleft.

The Ach, binds with the receptors in the sarcolemma, leading to opening of Ligand gated Na+ channels. Therefore Na+diffuses in, causing depolarization, and propagation of action potential along the sarcolemma  spreading to the  T-tubules.

This is followed by opening of Calcium channels leading  exist of Ca+ from the sarcoplasmic reticulum.

The Calcium binds with the troponin, and  Tropomysoin exposed the myosin binding sites  on the actin filament.Thus cross-bridge is formed by the myosin head  with thin filaments which   shortened the sarcomere, leading to contraction of muscle fibres.

marianaegarciaperedo

An action potential in the presynaptic cell activates calcium channels. Ca+ triggers the release of Acetylcholine by exocytosis. Ach binds its receptors in the postsynaptic membrane, opening Na+ channels. The Order is 4, 3, 5, 1, 2.

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  • The central nervous system generates an action potential that travels to the target activating the calcium channels ⇒ event (4)

  • Calcium triggers vesicles fusion to the presynaptic membrane. By exocytosis, vesicles release the neurotransmitter -Acetylcholine- into the synaptic space ⇒ event (3)

  • Once there, Ach binds to its receptors on the postsynaptic membrane of the skeletal muscle fiber, causing ion channels to open ⇒ event (5)

  • Positively charged sodium ions cross the membrane to get into the muscle fiber (sarcoplasm) and potassium leaves the cell ⇒ event (1)

  • The difference in charges, caused by these ions' transport, positively charges the fiber membrane. The postsynaptic cell depolarizes ⇒ Event (2)

  • The action potential enters the t-tubules depolarizing the inner portion of the muscle fiber.

  • Contraction initiates when the action potential depolarizes the inner portion of the muscle fiber.

  • Calcium channels activate in the T tubules membrane, releasing calcium into the sarcolemma.

  • 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.

  • When calcium enters the sarcolemma, it binds to troponin C, and troponin T alters the tropomyosin position by moving it and unblocking the binding sites.

  • 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.

  • 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. 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.

  • Finally, Z-bands are pulled toward each other, and the sarcomere and the I-band get shorter, producing muscle fiber contraction.

According to these steps, the correct order of events is 4, 3, 5, 1, 2.

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