Extrafusal muscle fibers

Extrafusal Muscle Fibers

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Illustration of extrafusal muscle fibers

Extrafusal muscle fibers are the skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They are the most numerous type of muscle fiber and are responsible for the force exerted by contraction of the muscle.

Structure

Extrafusal muscle fibers are surrounded by a layer of connective tissue called the endomysium. These fibers are bundled together into fascicles by a thicker connective tissue layer called the perimysium. The entire muscle is then wrapped in a layer of connective tissue known as the epimysium.

Function

Extrafusal muscle fibers are responsible for generating the force for muscle contraction. They do this by shortening in length, which pulls on the tendons attached to the muscle, and thus moves the bone.

Types

There are two types of extrafusal muscle fibers: slow-twitch fibers and fast-twitch fibers. Slow-twitch fibers are more efficient at using oxygen to generate fuel for continuous, extended muscle contractions over a long period. They fire more slowly than fast-twitch fibers and can go for a long time before they fatigue. Fast-twitch fibers are much better at generating short bursts of strength or speed than slow-twitch fibers. However, they fatigue more quickly.

Clinical significance

Damage to the extrafusal muscle fibers can result in a loss of muscle strength and can lead to muscle weakness or muscle atrophy. This can occur as a result of various conditions, including neuromuscular diseases, muscular dystrophy, and nerve damage.

See also

• Intrafusal muscle fiber
• Motor unit
• Muscle contraction
• Muscle spindle

External links

• Extrafusal muscle fibers at the US National Library of Medicine Medical Subject Headings (MeSH)