Ultrastructure of Striated Muscle
- Striated muscle makes up the muscles in the body that are attached to the skeleton
- 'Striated' means it is striped/streaky in appearance
Striated Muscle Appearance (Stained)
Skeletal striated muscle as seen under the microscope, after staining
Image by Nephron, CC BY-SA 3.0, via Wikimedia Commons
- Striated muscle is made up of muscle fibres
- A muscle fibre is a highly specialised cell-like unit:
- Each muscle fibre contains an organised arrangement of contractile proteins in the cytoplasm
- Each muscle fibre is surrounded by a cell surface membrane
- Each muscle fibre contains many nuclei – this is why muscle fibres are not usually referred to as cells
- The different parts of a muscle fibre have different names from the equivalent parts of a normal cell:
- Cell surface membrane = sarcolemma
- Cytoplasm = sarcoplasm
- Endoplasmic reticulum = sarcoplasmic reticulum (SR)
- The sarcolemma has many deep tube-like projections that fold in from its outer surface:
- These are known as transverse system tubules or T-tubules
- These run close to the SR
- The sarcoplasm contains mitochondria and myofibrils
- The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction
- Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction
- The membranes of the SR contain protein pumps that transport calcium ions into the lumen of the SR
The Ultrastructure of Striated Muscle Diagram
The ultrastructure of striated muscle and of a section of muscle fibre
Myofibrils
- Myofibrils are located in the sarcoplasm
- Each myofibril is made up of two types of protein filament:
- Thick filaments made of myosin
- Thin filaments made of actin
- These two types of filament are arranged in a particular order, creating different types of band and line
Myofibrils Parts & Descriptions Table
| Part of myofibril | Description |
| H band | Only thick myosin filaments present |
| I band | Only thin actin filaments present |
| A band | Contains areas where only myosin filaments are present and areas where myosin and actin filaments overlap |
| M Line | Attachment for myosin filaments |
| Z line | Attachment for actin filaments |
| Sarcomere | The section of myofibril between two Z lines |
Myofibril Structure Diagram
The structure of a myofibril
