Excitation-contraction coupling

From the motor neuron's action potential to filament sliding, then relaxation.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Neuromuscular junction: what is it?
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The synapse between motor neuron and muscle fibre.
Normally, every motor neuron action potential triggers one in the fibre.
Excitation-contraction coupling: which step comes after "Motor neuron action potential"?
- Acetylcholine release
- Action potential along the T tubules
- Calcium released by the reticulum
- Calcium bound to troponin
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Acetylcholine release
From electrical signal to movement, then back to rest.
Acetylcholine: what is it?
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The neurotransmitter released at the neuromuscular junction.
It depolarises the fibre's membrane.
A poison blocks calcium uptake by the reticulum. What happens?
- The muscle no longer contracts
- Acetylcholine is no longer released
- The muscle can no longer relax
- Filaments shorten
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The muscle can no longer relax
Calcium stays on troponin: bridges keep cycling.
T tubules: what is it?
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Membrane infoldings carrying the action potential deep into the fibre.
So every myofibril contracts together.
Why does muscle relaxation also use energy?
- The fibre must resynthesise the acetylcholine released
- Troponin splits ATP to let go of calcium
- Myosin heads actively push actin back
- Reticulum pumps take calcium back using ATP
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Reticulum pumps take calcium back using ATP
The reticulum's calcium pumps (SERCA) use ATP: contracting and relaxing both cost energy.