Myofilament

Myofilament
Myofilament
Latin	myofilamentum
Code	TH H2.00.05.0.00006

Myofilaments, the filaments of myofibrils constructed from proteins,. The principal types of muscle are striated muscle, obliquely striated muscle and smooth muscle. Various arrangements of myofilaments create different muscles. Striated muscle has transverse bands of filaments. In obliquely striated muscle, the filaments are staggered,and smooth muscle has irregular arrangements of filaments.

There are three different types of myofilaments: thick, thin, and elastic filaments.

• Thick filaments consist primarily of the protein myosin. Each thick filament are approximately 15 nm in diameter, and each is made of several hundred molecules of myosin.
• Thin filaments, 7 nm in diameter, consist primarily of the protein actin. All thin filaments are attached the Z disc.
• Elastic filaments, 1 nm in diameter, are made of titin, a large springy protein. They flank each thick filament and anchor it to the Z disc, the end point of a sarcomere.

The protein complex composed of actin and myosin, contractile proteins, is sometimes referred to as "actomyosin". In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length in the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length of the myofibril. These subunits are called sarcomeres.

The contractile nature of this protein complex is based on the structure of the thick and thin filaments. The thick filament, myosin, has a double-headed structure, with the heads positioned at opposite ends of the molecule. During muscle contraction, the heads of the myosin filaments attach to oppositely oriented thin filaments, actin, and pull them past one another. The action of myosin attachment and actin movement results in sarcomere shortening. Muscle contraction consists of the simultaneous shortening of multiple sarcomeres, .

What follows is a summary of muscle fiber contraction & relaxation,

MUSCLE FIBER CONTRACTION

1. The axon terminal of a motor neuron releases the neurotransmitter, acetylcholine.

2. Acetylcholine diffuses across the synaptic cleft and binds to the muscle fiber membrane.

3. This depolarizes the muscle fiber membrane, and the impulse travels to the muscle's sarcoplasmic reticulum via the transverse tubules.

4. Calcium ions are then released from the sarcoplasmic reticulum into the sarcoplasm and subsequently bind to troponin.

5. Troponin and the associated tropomyosin undergo a conformational change after calcium binding and expose the myosin binding sites on actin, the thin filament.

6. The filaments of actin and myosin then form linkages.

7. After binding, myosin pulls actin filaments toward each other, or inward.

8. Thus muscle contraction occurs, and the sarcomere shortens as this process takes place.

MUSCLE FIBER RELAXATION

1. The enzyme acetylcholinesterase breaks down acetylcholine and this ceases muscle fiber stimulation.

2. Active transport moves calcium ions back into the sarcolplasmic reticulum of the muscle fiber.

3. ATP causes the binding between actin and myosin filaments to break.

4. Troponin & tropomyosin revert to their original conformation and thereby block binding sites on the actin filament.

5. The muscle fiber relaxes and the entire sarcomere lengthens.

6. The muscle fiber is now prepared for the next contraction.

References

2. Muscle :: Diversity of Muscle -- Britannica Online Encyclopedia." Encyclopedia - Britannica Online Encyclopedia. Web. 3. Saladin, Kenneth S. "Myofilaments." Anatomy & Physiology: the Unity of Form and Function. 5th ed. New York: McGraw-Hill, 2010. 406-07. Print.

External links

• Diagrams and explanations at biomol.uci.edu
• MeSH Myofilaments