Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

6.3.3Organisms Respond to Changes in their Environments (A Level only)

Skeletal Muscle Under a Microscope — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 29 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

6.3.3 Skeletal Muscle Under a Microscope — AQA A Level Biology 7402 notes: Interpret the banding pattern of a sarcomere in micrographs and explain how the bands change during contraction.When viewed with a light microscope or in an electron micrograph, skeletal muscle appears str…

  • sarcomere gets shorter
  • I-band gets narrower
  • H-zone gets narrower
  • A-band stays the same length
  • Cardiac muscle
  • Smooth muscle

Section 1 of 9The banding pattern

When viewed with a light microscope or in an electron micrograph, skeletal muscle appears striped, which is why it is also called striated muscle. The pattern is caused by the arrangement of actin and myosin within the sarcomere. Under the electron microscope, the following features can be identified:

RegionAppearanceWhat it contains
A-bandDarkThe whole length of the myosin filaments, including where they overlap actin
I-bandLightActin only, no myosin; spans two adjacent sarcomeres across the Z-line
H-zoneLighter central region of the A-bandMyosin only, no actin
Z-lineThin dark lineBoundary of the sarcomere; actin filaments attach here
M-lineThin line in the middleMiddle of the myosin filaments

A sarcomere is measured from one Z-line to the next.

Section 2 of 9Changes during contraction

When a muscle contracts, the actin and myosin filaments slide past each other. The filaments themselves do not change length. The result is a characteristic change in the bands:

1Sarcomere gets shorter
The sarcomere gets shorter (the Z-lines move closer).
2I-band gets narrower
The I-band gets narrower, because the actin filaments move in towards the centre and overlap more myosin.
3H-zone gets narrower
The H-zone gets narrower and may disappear, because the actin filaments approach each other in the centre.
4A-band stays the same length
The A-band stays the same length, since it is the length of the myosin filaments, which do not change.

These observations provided evidence for the sliding filament model: if the filaments shortened, the A-band would also shorten.

Section 3 of 9Preparing and viewing a slide

Skeletal muscle for microscopy is fixed, sectioned and stained.

1
Longitudinal sections (cut along the fibres) show the stripes,
  • whereas transverse sections show circular cross-sections of fibres with nuclei at the edge.
2
When using a light microscope, a student may use a stain, focus first at low power,
  • then move to high power, and draw a clear, labelled diagram in pencil with continuous lines and no shading.
3
Each fibre appears as a long, parallel-sided cell with multiple flattened nuclei at the edge,
  • whereas cardiac muscle has branched fibres and a single central nucleus.
4
Electron micrographs are needed to see the individual filaments and the Z-line in detail.

Section 4 of 9Calculations with micrographs

Micrograph questions often ask for the length of a sarcomere or the change in length.

1
Use magnification = image size / actual size.
2
Measure the sarcomere in the image using a ruler (in mm), convert to micrometres (multiply by 1000), and divide by the magnification.
3
For example, a sarcomere measuring 24 mm on a micrograph at ×10 000 has an actual length of 24 000 µm / 10 000 = 2.4 µm.
4
To find the percentage change in length on contraction: (original − new) / original × 100.
  • If the relaxed sarcomere is 2.4 µm and the contracted is 1.8 µm, the decrease is 25%.

Section 5 of 9Interpreting exam diagrams

If a diagram shows a sarcomere with labelled bands, remember the rules: A-band equals the length of myosin; I-band equals actin only; H-zone equals myosin only.

Interpreting exam diagrams
To work out whether a muscle is relaxed or contracted, compare the I-band and H-zone: wide bands indicate relaxed muscle, narrow or missing bands indicate contraction.
If a muscle is stretched strongly, the overlap decreases and the force of contraction falls, because fewer myosin heads can bind actin.
Linking the observed pattern to the sliding filament mechanism is a common high-mark answer.

Section 6 of 9Distinguishing muscle types in micrographs

Students should be able to tell skeletal muscle from the other types of muscle in a micrograph. Skeletal muscle has long, unbranched, striated fibres with many nuclei at the edge. Cardiac muscle has striated, branched fibres connected by intercalated discs, and has one or two central nuclei per cell. Smooth muscle has short, spindle-shaped cells without stripes and with a single central nucleus. The striations of skeletal and cardiac muscle are due to the regular arrangement of sarcomeres, and smooth muscle lacks this arrangement.

When asked to make a labelled drawing, use a sharp pencil, show only the features seen and include a scale or magnification. When comparing micrographs of relaxed and contracted muscle, give the evidence for your conclusion from the bands, using measurements where possible.

Section 7 of 9Exam-style questions

  1. A muscle contracts. State and explain what happens to the width of the I-band and the A-band. [3]3 marks

  2. A sarcomere in an electron micrograph measures 30 mm. The magnification is ×12 500. Calculate the actual length in µm. [2]2 marks

  3. Relaxed sarcomere length is 2.4 µm and contracted is 1.8 µm. Calculate the percentage decrease. [2]2 marks

Section 8 of 9Key terms

TermMeaning
A-bandDark band containing myosin filaments
I-bandLight band containing actin only
H-zoneCentral region of the A-band with only myosin
Z-lineBoundary of a sarcomere to which actin is attached
M-lineLine in the middle of the sarcomere
Striated muscleMuscle showing a banding pattern, including skeletal muscle
MagnificationImage size divided by actual size

Section 9 of 9Exam technique

1
Saying the A-band gets shorter in contraction; it stays the same.
2
Thinking the filaments themselves shorten.
3
Confusing the I-band with the H-zone
  • the I-band contains only actin;
    • the H-zone only myosin.
4
Forgetting to convert mm to µm in magnification calculations.
5
Describing the dark band as actin; the A-band is defined by myosin.

Last updated: