The signal from the nerve to the muscle

From a thought to a movement

You decide to pick up a pen. A signal leaves your brain, runs down your spinal cord and along a nerve to the muscles of your hand. A few moments later, the muscle fibres shorten. How does a signal become a contraction?

The contraction of a skeletal muscle starts with a signal from the central nervous system (CNS). The signal travels along a motor neuron, a nerve cell that carries orders to a muscle.

A motor neuron, together with all the muscle fibres that it supplies, is called a motor unit. When the neuron sends a signal, every fibre of its unit contracts.

The neuromuscular junction

The place where a motor neuron meets the sarcolemma of a muscle fibre is called the neuromuscular junction, or the motor end plate.

The steps are these.

1. A neural signal reaches the junction.
2. The signal makes the neuron release a neurotransmitter. At this junction it is acetylcholine.
3. Acetylcholine generates an action potential in the sarcolemma. An action potential is a quick, short change in the electrical state of a membrane. You will study it in the chapter on neural control.
4. The action potential spreads through the muscle fibre.
5. It causes the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.
6. Calcium ions bind to a subunit of troponin on the actin filaments. This removes the masking of the active sites for myosin.

Now the myosin heads can bind to the actin. This starts the sliding of the filaments, which is the subject of the next lesson.

Why calcium is the switch

In a resting fibre, the calcium ions are stored in the sarcoplasmic reticulum, and the sites on the actin are masked. The ions act as a switch. When they flood the sarcoplasm, the sites open. When they are pumped back, the sites are masked again.

You should find that a low calcium level opens only a few sites, and a high level opens nearly all of them. The curve is a teaching model. Real muscles do not have a number like this.

Try it in Python

The program prints the share of open sites at five calcium levels.

Practise with new numbers

Common mistakes

• Saying that the calcium comes from outside the fibre. It is released from the sarcoplasmic reticulum.
• Saying that calcium binds to myosin. It binds to a subunit of troponin on the thin filament.
• Saying that troponin masks sites on the myosin. It masks the sites on the actin.
• Saying that a sensory neuron sends the order to the muscle. The order goes along a motor neuron.
• Mixing the unit and the fibre. A motor unit is one neuron with all the fibres that it supplies.

Remember these

• Signal path: CNS, motor neuron, neuromuscular junction (motor end plate), acetylcholine, action potential in the sarcolemma.
• The action potential spreads through the fibre and releases Ca$^{2+}$ from the sarcoplasmic reticulum.
• Ca$^{2+}$ binds to troponin and unmasks the active sites on the actin.
• Motor unit: a motor neuron and the fibres that it supplies.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is a motor unit?

2. Where is the neuromuscular junction, and what is it also called?

3. Which neurotransmitter is released there, and what does it do?

4. Where do the calcium ions come from, and what do they bind to?

5. What does the binding of calcium to troponin do?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 10, Muscle Tissue, sections on muscle fibre contraction and relaxation.
• Alberts B. et al. (2015). Molecular Biology of the Cell, 6th edition, Garland Science, chapter 16, section on muscle contraction.
• NCERT (2023). Biology, Textbook for Class XI, chapter 17.