Ultrastructure of Striated Muscle
- Striated muscle makes up the muscles in the body that are attached to the skeleton
- 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 to 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 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 bands and line
Myofibrils parts & descriptions table
The structure of a myofibril
