Skeletal Muscle

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Ultrastructure and histology of skeletal muscle (F

Skeletal muscle is a type of striated muscle.

Striated muscle is composed of elongated cells (muscle fibres) which contract and relax.

Muscle fibres are multinucleated (have many nuclei).

Each muscle fibre is surrounded by a membrane called the sarcolemma. The sarcolemma invaginates to form transverse tubules (T tubules). These help depolarisation to spread throughout the fibre.

The sarcoplasm (cytoplasm of the muscle fibre) contains many nuclei, sarcoplasmic reticulum and mitochondria.

Within each muscle fibre, there are a series of myofibrils that run the length of the muscle fibre. They show a pattern of light and dark bands called anisotrophic or A bands and isotropic or I bands respectively. 

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Ultrastructure and histology of a myofibril (F224.

Each I band on the myofibril has a central line called the Z line. The distance between two adjacent Z lines is a sarcomere (2.5 micrometres relaxed).

Each A band has a lighter region in the centre which is called the H zone. This H zone may have a central dark line within itself, called the M line.

The pattern in the band results due to the arrangement of the proteins actin and myosin.

Myosin - made up of thick filaments

Actin - made up of thin filaments

A bands- actin and myosin

I bands - actin only

During contraction of the muscle fibre, the actin filaments slide between the myosin filaments. Myosin filaments are regularly arranged in a lattice, when there is overlap between actin and myosin, there are 6 actin around 1 myosin.                             

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A sarcomere diagram (F224.1.2 n)

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