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Skeletal Muscle Fiber



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Quick Facts

A myofibril is one of the slender threads that can be made visible in a muscle fiber by maceration in certain acids. They run parallel with the long axis of the fiber, and are composed of numerous myofilaments (Dorland, 2011).

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Myofibrils have a diameter of approximately 1 µm and are composed of numerous sarcomeres, the functional contractile units of striated muscle. Sarcomeres are approximately 2 µm long (Standring, 2016). The sarcomere is composed of recurring light and dark bands, giving skeletal muscle its striated appearance. The dark regions of the sarcomere are called A-bands. These regions appear dark because they refract polarized light, due to a property known as anisotropy. A-bands are composed of thick filaments of the protein myosin and are dissected by the M-line. The number of sarcomeres in each myofibril varies depending on the size and length of the muscle fiber (Laing and Nowak, 2005).

Key Features/Anatomical Relations

Myofibrils are arranged in bundles enclosed by a plasma membrane called the sarcolemma. This structure is collectively called a muscle fiber. A connective tissue layer, the endomysium, and a basal lamina surround the sarcolemma.

Each myofibril is made up of numerous sarcomeres that are positioned end to end. The light regions of the sarcomere do not refract light and are considered isotropic. These regions are called I-bands. I-bands are composed of thin filaments of the protein actin and its associated proteins, such as tropomyosin and troponin. I-bands are dissected by a Z-line. The area between successive Z-lines are called sarcomeres. The sections of thick filament that are not superimposed by the thin filaments when the myofibril is relaxed is called the H-band (Laing and Nowak, 2005).


The protein bands of the myofibril facilitate contraction and relaxation of the muscle by gliding the thin actin and thick myosin filaments over each other.

List of Clinical Correlates

—Myofibrillar Myopathy

—Actin Myopathy

—Hyaline Body Myopathy

—Muscular Dystrophy


Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Laing, N. G. and Nowak, K. J. (2005) 'When contractile proteins go bad: the sarcomere and skeletal muscle disease', Bioessays, 27(8), pp. 809-22.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series: Elsevier Limited.

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