Skeletal Muscles: MDCAT Biology notes
Skeletal muscles MDCAT notes: muscle fibre structure, sarcolemma, sarcoplasmic reticulum, T-tubules, myofibrils, sarcomere bands, actin and myosin.
Organization of a skeletal muscle
A skeletal muscle is made of bundles of long, cylindrical muscle fibres, which are the muscle cells. Each fibre is multinucleate, because it forms by the fusion of many cells. The levels of organization are:
Muscle → bundles of fibres → muscle fibre (cell) → myofibrils → myofilaments (actin and myosin)
Parts of a muscle fibre
| Structure | What it is |
|---|---|
| Sarcolemma | The cell membrane of the muscle fibre, a modified plasma membrane |
| Sarcoplasm | The cytoplasm, which surrounds the myofibrils. It is rich in glycogen, myoglobin and many mitochondria |
| Sarcoplasmic reticulum | A modified smooth ER wrapped around each myofibril. It stores and releases $\mathrm{Ca^{2+}}$ |
| Transverse (T) tubules | Inward folds of the sarcolemma that carry the nerve impulse deep into the fibre |
| Myofibrils | Long contractile threads made of sarcomeres placed end to end |
Because the SR is so extensive, skeletal muscle cells have more SER and less RER.
Energy and oxygen stores
- Myoglobin stores oxygen inside the muscle. Haemoglobin is in red blood cells, not in muscle fibres.
- Creatine phosphate is a quick energy store. It regenerates ATP during contraction.
Myofilaments
- Thick filaments: made of myosin, whose heads form cross-bridges.
- Thin filaments: made of two chains of actin twisted together, with the regulatory proteins tropomyosin and troponin.
The sarcomere
A sarcomere is the contractile unit of a myofibril. It is the region between two successive Z lines, and the Z line marks the boundary between one sarcomere and the next.
| Region | Contains | Appearance |
|---|---|---|
| Z line | The anchor for thin (actin) filaments | Boundary line |
| I band | Actin only | Light band, bisected by the Z line |
| A band | The whole length of the myosin, overlapping with actin at both ends | Dark band |
| H zone | Myosin only, with no overlap | Lighter centre of the A band |
| M line | Proteins holding the myosin filaments together | Middle of the H zone |
The alternating light I bands and dark A bands give skeletal muscle its striations.
Relaxed and contracted sarcomeres
- Relaxed: the H zone and I bands are broad. Tropomyosin covers the binding sites on actin, and $\mathrm{Ca^{2+}}$ stays stored in the SR.
- Contracted: actin slides over myosin. The H zone and I bands shorten or disappear, and the Z lines move closer together. The A band length stays the same.
Worked example
Question: In a cross-section through one region of a sarcomere, you see both thick and thin filaments. Which band is it?
Answer: The overlap zone of the A band. The I band shows only thin filaments and the H zone shows only thick ones.
Common MDCAT traps
- A sarcomere runs Z line to Z line, not between M lines or I bands.
- Thick filaments are myosin and thin filaments are actin.
- The sarcolemma is the membrane, and the sarcoplasm is the cytoplasm.
- T-tubules conduct the impulse. The SR stores calcium.
- Skeletal fibres contain myoglobin, not haemoglobin.
Quick revision
- Sarcomeres in series make up a myofibril.
- The A band contains overlapping actin and myosin.
- The H zone contains only myosin.
- The sarcoplasmic reticulum is modified ER that stores $\mathrm{Ca^{2+}}$.
- Creatine phosphate is the muscle's energy reserve.