Course 16.2Lesson 2 of 3

The sliding filament theory and the cross-bridge cycle

In the sliding filament theory, the myosin heads bind to the actin, pull it toward the centre of the A band, let go and bind again, using the energy of ATP. The thin filaments slide over the thick ones, so the sarcomere gets shorter. The A band keeps its length, but the I band and the H zone shorten.

about 30 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

Filaments that slide

When your muscle shortens, nothing in it gets shorter. The thin and the thick filaments keep their lengths. They slide past each other, like the two parts of a telescope. This is the sliding filament theory of muscle contraction.

One turn of the cross-bridge cycle

Calcium ions bind to troponin, and the active sites on the actin are uncovered.
The head of the myosin, which has been energised by the hydrolysis of ATP, binds to the exposed site on the actin. This makes a cross-bridge.
The head pulls the attached actin filament toward the centre of the A band. The Z line fixed to this actin is pulled inward too. The sarcomere gets shorter. This is the contraction.
The myosin head releases ADP and Pi and goes back to its relaxed state.
A new ATP binds to the head, and the cross-bridge breaks.
The head hydrolyses the ATP again. The cycle of making and breaking a cross-bridge repeats, and the actin slides further.

It goes on while calcium ions are present. So ATP is used in each turn of the cycle: its energy powers the head, and a new ATP is needed to break the bridge.

What gives relaxation

The cycle stops when the calcium ions are pumped back into the sarcoplasmic cisternae, the stores of the sarcoplasmic reticulum. The sites on the actin are masked again. The Z lines return to their first positions. This is relaxation.

What changes and what does not

Three drawings of one sarcomere, one under the other, to the same scale: relaxed, partly contracted and fully contracted. Each has two Z lines, thick filaments in red in the middle and thin filaments in green that start at the Z lines. In the relaxed sarcomere, a gap, the H zone, lies in the middle, and the light I bands are wide. In the partly contracted sarcomere, the thin filaments have slid inward until their tips just meet, and the H zone has gone. In the fully contracted sarcomere, the Z lines are close to the ends of the thick filaments, the tips of the thin filaments overlap in the middle, and the I bands are very short. The A band has the same length in all three. On the right are the lengths in model units, u: sarcomere 22, 20 and 17; I band 6, 4 and 1; H zone 2, 0 and 0; A band 16 in all.

Part	When the muscle contracts
A band	keeps its length
I band	gets shorter
H zone	gets shorter, and may disappear
Sarcomere	gets shorter
Length of each filament	does not change

The A band keeps its length because the thick filaments do not shorten. The I band shortens because the thin filaments slide in, and the Z lines come closer to the A band.

In a relaxed muscle, the thin filaments do not reach the centre, and the H zone is a gap. As the muscle contracts, the tips of the thin filaments meet. Then they may overlap. The H zone is gone.

The graph shows the three lengths of the model sarcomere for sarcomeres of different lengths.

A line graph of the lengths of the zones of a model sarcomere against the length of the sarcomere, from 16 to 26 model units, u. The A band is a horizontal line at 16 u. The I band is a rising straight line from 0 at a sarcomere length of 16 u to 10 u at 26 u. The H zone is zero up to a sarcomere length of 20 u, and then rises in a straight line to 6 u at 26 u. A dashed vertical line at 20 u is marked: the tips of the thin filaments meet.

This part is not in the NEET syllabus, but A-Level and IB ask it. After death, the cells stop making ATP. The myosin heads stay bound to the actin, because a new ATP is needed to break the bridges. The muscles stay stiff. This stiffness is called rigor mortis.

You should find that the A band is always $16$ u, and that the H zone has vanished once the shortening is about 9 per cent.

Try it in Python

The program prints the bands of a model sarcomere at five amounts of shortening.

Practise with new numbers

Common mistakes

Saying that the filaments get shorter. They slide. Their lengths do not change.
Saying that the A band gets shorter. The A band is as long as the thick filaments, so it keeps its length.
Saying that the H zone gets wider. It gets narrower and may vanish.
Thinking that ATP is needed only to start the pull. A new ATP is also needed to break the cross-bridge.
Saying that relaxation needs no work. The calcium ions must be pumped back into the sarcoplasmic cisternae.

Remember these

The thin filaments slide toward the centre of the A band, pulled by the myosin heads. The Z lines move inward, and the sarcomere gets shorter.
Cycle: calcium binds troponin, the head binds the actin (a cross-bridge), the pull, release of ADP and Pi, a new ATP breaks the bridge, the ATP is hydrolysed and the cycle repeats.
During contraction: the A band is unchanged. The I band and the H zone get shorter.
Relaxation: the calcium ions are pumped back, and the sites on the actin are masked again.

Check yourself

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

In the sliding filament theory, what slides over what?

Which bands and zones get shorter in a contracting sarcomere, and which keeps its length?

What does a new ATP molecule do when it binds to a myosin head?

What is the first thing that happens before the head can bind to the actin?

What brings about the relaxation of the muscle?

References

Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 10, Muscle Tissue.
Huxley H. E. and Hanson J. (1954). Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation. Nature, 173, 973 to 976.
NCERT (2023). Biology, Textbook for Class XI, chapter 17.