Structure of Skeletal Muscle
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of cell structure and proteins. You can test your knowledge on these below.
What organelle is responsible for producing ATP needed for muscle contraction?
Mitochondria.
What type of biological molecule are both actin and myosin made of?
Proteins (actin = globular, myosin = fibrous with globular heads).
What part of a eukaryotic cell provides internal support and is involved in movement?
The cytoskeleton (including microfilaments and microtubules).
Topic Explainer Video
Check out this @LauraDoesBiology video that explains structure of skeletal muscle or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Levels of Muscle Structure
Skeletal muscle is organised in a hierarchical structure from whole muscle to microscopic contractile units:
1. Muscle
- Attached to bones by tendons.
- Surrounded by connective tissue.
2. Muscle Bundle
- Groups of muscle fibres wrapped in connective tissue.
3. Muscle Fibre (Muscle Cell)
- Long, cylindrical, multi-nucleate (many nuclei) cells.
- Made up of many myofibrils.
- Surrounded by the sarcolemma (cell surface membrane).
- With transverse (T) tubules (folds of the cell membrane) into the sarcoplasm (the cytoplasm of the cell).
- Sarcoplasmic reticulum containing Ca2+.
- Many mitochondria.
4. Myofibril
- Thread-like structures composed of repeating sarcomeres.
- Contain actin and myosin protein filaments.
5. Sarcomere
- Functional unit of contraction.
- Contains organised actin and myosin filaments.
Key Structures in Muscle Fibre
| Structure | Description |
| Sarcolemma | Cell surface membrane of a muscle fibre. |
| Sarcoplasm | Cytoplasm of the muscle cell, contains many mitochondria. |
| Sarcoplasmic Reticulum | Stores and releases calcium ions during contraction. |
| T-tubules | Folds of sarcolemma that help transmit action potentials into the fibre. |
| Myofibril | Contractile unit containing sarcomeres with actin and myosin. |
| Mitochondria | Provide ATP for contraction (via aerobic respiration). |
| Multinuclei | Muscle fibres are formed by fusion of cells - many nuclei. |
Sarcomere Structure
| Band/Line | Description |
| Z-line | Ends of the sarcomere. Anchors actin filaments. |
| A-band | Dark band – contains overlapping actin and myosin filaments. |
| I-band | Light band – only actin present. |
| H-zone | Region in the centre of A-band with only myosin. |
| M-line | Centre of the sarcomere where myosin filaments are anchored. |
- The A band and I band are referred to as dark and light because this is how they appear using microscopes.
- The A band is darker as it contains myosin which is thicker.
- The I band is lighter as it contains only actin which is thinner.
When the muscle fibre contracts the sarcomere shortens. The myosin pulls on the actin filaments and they slide inwards.
- The Z lines get closer together.
- The H zone gets shorter.
- The I band gets shorter.
- The A band is equal to the length of the myosin filaments, so this stays the same.
Proteins Involved
| Protein | Role |
| Actin | Thin filament. Has binding sites for myosin heads. |
| Myosin | Thick filament. Contains heads that bind to actin and hydrolyse ATP. |
Key Terms
- Sarcomere: Contractile unit of the myofibril from Z-line to Z-line.
- Myofibril: Thread-like protein bundle inside muscle fibre.
- Sarcolemma: Muscle fibre membrane.
- Sarcoplasmic reticulum: Organelle storing Ca²⁺ ions.
- A-band: Area containing actin and myosin filaments.
- I-band: Region with actin only.
No answer provided.
Exam Tips
Be able to label a diagram of a sarcomere and describe what changes during contraction (e.g., I-band and H-zone shorten, A-band stays the same).
Be able to recognise the different parts of a sarcomere from a micrograph.
No answer provided.
Describe the structure of a myofibril and explain how its components are arranged in a sarcomere. (5 marks)
- A myofibril is made up of repeating units called sarcomeres.
- A sarcomere is the distance between two Z-lines.
- Actin filaments are thin and anchored to Z-lines.
- Myosin filaments are thick and found in the centre (A-band).
- The overlap between actin and myosin forms the A-band, while the I-band contains only actin and the H-zone contains only myosin.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!