International A Level Biology / Muscles, movement and the heart

The structure of a skeletal muscle fibre

Edexcel International A Level Biology (YBI11), chapter 21: Muscles, movement and the heart, specification point 7.10. Free lesson.

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Skeletal muscle is organised in levels, from the whole muscle down to actin and myosin filaments. Knowing the parts of a muscle fibre and sarcomere helps you explain how the structure supports contraction.

How a muscle is organised

Each level sits inside the one above it. A muscle fibre is one very long cell, formed when many cells fuse, so it has many nuclei and is called multinucleate. It contains many myofibrils, which are chains of repeating sarcomeres. The sarcomere is the unit that shortens when a muscle contracts. A muscle is wrapped in connective tissue and divided into bundles of muscle fibres. A myofibril is inside a muscle fibre. It is not a cell itself. Each sarcomere contains thin actin filaments and thick myosin filaments.

What is inside a muscle fibre

The sarcolemma is the cell surface membrane, and the sarcoplasm is the fibre's cytoplasm. Myofibrils fill much of the fibre and contain the actin and myosin used in contraction. Capillaries may lie alongside a fibre, but they are not part of it. Many nuclei lie just under the sarcolemma because the fibre formed from fused cells. Mitochondria between the myofibrils carry out aerobic respiration, making the ATP used in contraction. The sarcoplasmic reticulum is a specialised network around each myofibril. It stores calcium ions ready for release. A T tubule is a fold of the…

How a sarcomere is arranged

A sarcomere runs from one Z line to the next. Actin filaments are anchored to the Z lines at each end and reach towards the middle. Myosin filaments sit in the middle, with actin overlapping their ends. A myofibril is made of sarcomeres arranged end to end. The I band contains actin only. A Z line splits each I band, with half on either side of the line. The A band is the full length of the myosin filaments, including the regions where actin overlaps them. The H zone, in the middle of the A band, contains myosin only. Under a microscope, the I bands look light and the A bands…

How to identify the bands

Use which filaments are present to name a band. The A band is not myosin alone: it includes the overlap with actin. Only the H zone has myosin without actin. A light band containing only thin filaments is the I band. The thin filaments are made of actin.

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