How a Muscle Contracts: Quick Revision

A brief note on How a Muscle Contracts. Read it before a test. Try each question at the end before you open its answer.

The 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 a subunit of troponin and unmasks the active sites on the actin.
• Motor unit: a motor neuron and all the fibres that it supplies.

The sliding filament theory

• The myosin head, energised by ATP, binds to the actin and makes a cross-bridge.
• The head pulls the actin toward the centre of the A band, and the Z lines move inward.
• The head releases ADP and Pi. A new ATP breaks the bridge. The ATP is hydrolysed again, and the cycle repeats.
• Relaxation: Ca$^{2+}$ is pumped back into the sarcoplasmic cisternae, and the sites on the actin are masked again.
• During contraction: the A band is the same. The I band and the H zone get shorter. The filaments keep their length.

Fatigue and fibres

• Fatigue: lactic acid builds up from the anaerobic breakdown of glycogen.
• Red fibres: much myoglobin, plenty of mitochondria, aerobic.
• White fibres: little myoglobin, fewer mitochondria, much sarcoplasmic reticulum, anaerobic.

A worked example

A relaxed muscle is 33 cm long. Its sarcomeres shorten from 22 u to 18 u.

1. The factor is $18/22$.
2. The length is $33\ \text{cm} \times 18/22 = 27\ \text{cm}$.

Practice with new numbers

A program to try

The program shows the bands of a model sarcomere as it shortens.

Mistakes to avoid

• Saying that the filaments get shorter. They slide.
• Saying that calcium binds to myosin. It binds to troponin.
• Forgetting the new ATP. It is needed to break the cross-bridge.
• Saying that white fibres have much myoglobin. They have very little.

Check yourself

1. Which neurotransmitter is released at the neuromuscular junction?

2. What happens to the A band, the I band and the H zone in a contracting sarcomere?

3. What does a new ATP do to a bound myosin head?

4. What builds up in a tired muscle, and from what?

Sources

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 10.
• NCERT (2023). Biology, Textbook for Class XI, chapter 17.