Question

Put the items in the correct order. Items to order:______. 1. Calcium is released from the sarcoplasmic reticulum.2. Acetylcholine is released.3. Action potential gets to axon terminal of a motor neuron.4. Troponin-tropomyosin blockade is removed.5. Action potential travels across sarcolemma.6. Nicotinic receptors are activated.7. Action potential travels down T tubule.8. Myosin binds to actin and slides the thin filament

Answer 1

Answer:

The correct sequence of the given statements are:

• 3. Action potential gets to axon terminal of a motor neuron.
• 2. Acetylcholine is released.
• 6. Nicotinic receptors are activated.
• 5. Action potential travels across sarcolemma.
• 7. Action potential moves down T tubule.
• 1. Calcium moves out of the SR (sarcoplasmic reticulum) of the muscle cell.
• 4. Troponin-tropomyosin blockade is removed.
• 8. Myosin binds to actin and slides the thin filament

Explanation:

• During the transmission of an impulse in the neuro-muscular junction the following events occur sequentially which causes the muscle cells to contract.

1. A neural impulse that carries the signal to cause muscle contraction reaches the axon terminal at the neuro-muscular junction in the form of an action potential.
2. This action potential causes the entry of calcium ions into the axon terminal through the voltage gated calcium ion channels.
3. The increased calcium ion concentration in the axon terminal causes the fusion of the acetylcholine containing vesicles with the pre-synaptic membrane. This is followed by the release of acetylcholine into the synaptic cleft at the neuro-muscular junction.
4. The acetylcholine binds to its cognate nicotinic receptors present on the sarcolemma of the muscle cell located in the post-synaptic region.
5. This triggers an action potential in the post-synaptic sarcolemma.
6. The action potential also spreads in the T-tubules of the muscle cells. (T-tubules can be defined as the structure formed by the protrusion of the plasma membrane towards the interior of the muscle cell).
7. This causes the migration of calcium ions from the sarcoplasmic reticulum into the muscle cell's cytoplasm.
8. In absence of any contraction inducing impulse, the Troponin filaments present in the cytoplasm of the muscle cell remain associated with the Tropomyosin proteins. These tropomyosin proteins prevent the association of the troponin filaments with the myosin filaments, which is necessary for the contraction to occur.
9. In presence of a conduction inducing impulse, the calcium ions released from the sarcoplasmic reticulum associates with the troponin proteins and this causes the dissociation of tropomyosin proteins from  the troponin filaments.
10. The troponin filaments are now free to associate with the myosin filaments.
11. Now, the myosin filaments can associate with the actin filaments and cause muscle contraction.