A muscle fibre receives a normal nerve signal, and its sarcolemma makes a normal action potential…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A muscle fibre receives a normal nerve signal, and its sarcolemma makes a normal action potential. But a drug stops the sarcoplasmic reticulum from releasing calcium ions.
- Does the muscle fibre contract? Why?
- Which two proteins stay unchanged in this case?
- Would adding ATP to the fibre make it contract? Give the reason.
Need a reminder? The key ideaShow
The signal goes from the central nervous system along a motor neuron to the neuromuscular junction. Acetylcholine makes an action potential in the sarcolemma, which spreads through the fibre and releases Ca from the sarcoplasmic reticulum. The calcium binds to troponin and unmasks the sites on the actin. A motor unit is a motor neuron with all the fibres that it supplies.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Normal signal and action potential. No release of calcium ions.
Plan. Follow the chain of events until it breaks.
Step 1. (a) The signal and the action potential are normal, but without calcium ions in the sarcoplasm, troponin does not move. The active sites on the actin stay masked. The myosin heads cannot bind, and the fibre does not contract.
Step 2. (b) Troponin stays in its resting position, and the actin keeps its sites covered.
Step 3. (c) No. ATP gives energy to the heads and breaks the bridges, but the heads still need uncovered sites to bind.
Answer: (a) No, the sites on the actin stay masked. (b) Troponin and actin. (c) No, the sites are still covered.
Check. Calcium is the switch. ATP is the fuel. A switch that is off cannot be replaced by more fuel.
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