Myofibril

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Basic rod-like unit of a muscle cell

Anatomy

Latin	myofibrilla

A myofibril is a basic rod-like unit of a muscle cell. Muscles are composed of tubular cells called myocytes, also known as muscle fibers, and these cells in turn contain many chains of myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as alternating dark and light bands. These bands are responsible for the striated appearance of skeletal and cardiac muscle.

Structure

Myofibrils are made up of long proteins including actin, myosin, and titin, and other proteins that hold them together. The myofibrils are surrounded by the sarcoplasmic reticulum, which is a type of smooth endoplasmic reticulum specialized to store and release calcium ions. The release of calcium ions from the sarcoplasmic reticulum triggers muscle contraction.

Each myofibril is composed of many sarcomeres, the basic contractile units of muscle. Sarcomeres are delineated by Z lines, to which the actin filaments are anchored. The myosin filaments are anchored to the M line in the center of the sarcomere. The arrangement of actin and myosin filaments within the sarcomere gives the muscle its striated appearance.

Function

The primary function of myofibrils is to perform muscle contraction via the sliding filament model. In this model, the actin and myosin filaments slide past each other to shorten the overall length of the sarcomere, leading to muscle contraction. This process is powered by the hydrolysis of adenosine triphosphate (ATP).

Types of Myofibrils

There are two main types of myofibrils:

• **Slow-twitch (Type I)**: These myofibrils are more efficient at using oxygen to generate more ATP for continuous, extended muscle contractions over a long time. They are used in endurance activities.
• **Fast-twitch (Type II)**: These myofibrils are better at generating short bursts of strength or speed than slow-twitch fibers. However, they fatigue more quickly.

Clinical Significance

Disorders of myofibrils can lead to various muscle diseases. For example, mutations in the genes encoding myofibrillar proteins can cause cardiomyopathy and muscular dystrophy. Understanding the structure and function of myofibrils is crucial for developing treatments for these conditions.

See also

• Muscle contraction
• Muscle fiber
• Sarcomere
• Actin
• Myosin
• Sarcoplasmic reticulum