T tubules, also known as transverse tubules, are inward folds of the sarcolemma.

Their function is to ensure that the entire muscle receives the impulse to contract simultaneously by spreading electrical impulses throughout the sarcoplasm of all the muscle fibres.

The sarcolemma is the plasma membrane that encloses muscle fibres in skeletal muscles.

Individual muscles are made up of numerous muscle fibres.

The muscle cells are fused into muscle fibres to avoid weakness at the junctions between cells and to increase overall muscle strength.

The cytoplasm in muscle fibres is called sarcoplasm.

Within the sarcoplasm, there are:

1. Multiple nuclei
2. Many mitochondria that provide ATP for muscle contraction
3. A modified version of the endoplasmic reticulum (sarcoplasmic reticulum), which contains calcium ions that are essential for muscle contraction

1. Actin - thin filament composed of two strands twisted around each other
2. Myosin - thicker filament composed of long rod-shapes with bulbous heads that project to the side

Myofibrils appear striped due to alternating light-coloured (I bands) and dark-coloured (A bands) bands.

I bands appear lighter because the thick and thin filaments do not overlap in this region.

The central, lighter-coloured region within each A band is called the H-zone.

The central line within each I band is called the Z-line.

The distance between adjacent Z-lines is called a sarcomere.

During muscle contraction, sarcomeres shorten, causing a change in the pattern of light and dark bands.