Myofilaments are the three protein filaments of myofibrils in

muscle cell A muscle cell, also known as a myocyte, is a mature contractile Cell (biology), cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal muscle, skeletal, smooth muscle, smooth, and Cardiac muscle, cardiac ...

s. The main

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s involved are

myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

, and

titin Titin (; also called connectin) is a protein that in humans is encoded by the ''TTN'' gene. The protein, which is over 1 μm in length, functions as a molecular spring that is responsible for the passive elasticity of muscle. It comprises 2 ...

. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act together in

muscle contraction Muscle contraction is the activation of Tension (physics), tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in musc ...

, and in order of size are a thick one of mostly myosin, a thin one of mostly actin, and a very thin one of mostly titin. Types of muscle tissue are striated

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

and

cardiac muscle Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall o ...

, obliquely striated muscle (found in some

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s), and non-striated

smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...

. Various arrangements of myofilaments create different muscles. Striated muscle has transverse bands of filaments. In obliquely striated muscle, the filaments are staggered. Smooth muscle has irregular arrangements of filaments.

Structure

There are three different types of myofilaments: thick, thin, and elastic filaments. *Thick filaments consist primarily of a type of

, a motor protein – myosin II. Each thick filament is approximately 15 nm in diameter, and each is made of several hundred molecules of myosin. A myosin molecule is shaped like a golf club, with a tail formed of two intertwined chains and a double globular head projecting from it at an angle. Half of the

myosin head The myosin head is the part of the thick myofilament made up of myosin that acts in muscle contraction, by sliding over thin myofilaments of actin. Myosin is the major component of the thick filaments and most myosin molecules are composed of a ...

s angle to the left and half of them angle to the right, creating an area in the middle of the filament known as the ''M-region'' or ''bare zone''. *Thin filaments, are 7 nm in diameter, and consist primarily of the protein

, specifically filamentous

F-actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

. Each F-actin strand is composed of a string of subunits called globular G-actin. Each G-actin has an active site that can bind to the head of a myosin molecule. Each thin filament also has approximately 40 to 60 molecules of tropomyosin, the protein that blocks the active sites of the thin filaments when the muscle is relaxed. Each tropomyosin molecule has a smaller calcium-binding protein called

troponin Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that are integral to muscle contraction in skeletal muscle and cardiac muscle, but not smooth muscle. Measurements of cardiac-spe ...

bound to it. All thin filaments are attached to the Z-line. *Elastic filaments, 1 μm in diameter, are made of

, a large springy protein. They run through the core of each thick filament and anchor it to the Z-line, the end point of a

sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal striated muscle, Skeletal muscles are composed of tubular ...

. Titin also stabilizes the thick filament, while centering it between the thin filaments. It also aids in preventing overstretching of the thick filament, recoiling like a spring whenever a muscle is stretched.

Function

The protein complex composed of actin and myosin, contractile proteins, is sometimes referred to as actomyosin. In striated

skeletal A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fram ...

and

, 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

(up to several centimeters in some skeletal muscle cells). The contractile nature of this protein complex is based on the structure of the thick and thin filaments. The thick filament,

, 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,

, 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.

Muscle fiber contraction

The axon terminal of a

motor neuron A motor neuron (or motoneuron), also known as efferent neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly o ...

releases the

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

, which diffuses across the

synaptic cleft Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in neuromuscular junction, muscles or glands. Chemical synapses allow neurons to form biological neural ...

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

Calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

ions are then released from the sarcoplasmic reticulum into the sarcoplasm and subsequently bind to

. Troponin and the associated tropomyosin undergo a conformational change after calcium binding and expose the

binding sites on

, the thin filament. The filaments of actin and myosin then form linkages. After binding, myosin pulls actin filaments toward each other, or inward. Thus muscle contraction occurs, and the sarcomere shortens as this process takes place.

Muscle fiber relaxation

The

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

acetylcholinesterase Acetylcholinesterase (HUGO Gene Nomenclature Committee, HGNC symbol ACHE; EC 3.1.1.7; systematic name acetylcholine acetylhydrolase), also known as AChE, AChase or acetylhydrolase, is the primary cholinesterase in the body. It is an enzyme th ...

breaks down acetylcholine and this ceases muscle fiber stimulation. Active transport moves calcium ions back into the sarcoplasmic reticulum of the muscle fiber. ATP causes the binding between actin and myosin filaments to break. Troponin and tropomyosin revert to their original conformation and thereby block binding sites on the actin filament. The muscle fiber relaxes and the entire sarcomere lengthens. The muscle fiber is now prepared for the next contraction.

Response to exercise

The changes that occur to the myofilament in response to exercise have long been a subject of interest to exercise physiologists and the athletes who depend on their research for the most advanced training techniques. Athletes across a spectrum of sporting events are particularly interested to know what type of training protocol will result in maximal force generation from a muscle or set of muscles, so much attention has been given to changes in the myofilament under bouts of chronic and acute forms of exercise. While the exact mechanism of myofilament alteration in response to exercise is still being studied in mammals, some interesting clues have been revealed in

Thoroughbred The Thoroughbred is a list of horse breeds, horse breed developed for Thoroughbred racing, horse racing. Although the word ''thoroughbred'' is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thorough ...

race horses. Researchers studied the presence of mRNA in skeletal muscle of horses at three distinct times; immediately before training, immediately after training, and four hours after training. They reported statistically significant differences in mRNA for genes specific to production of

. This study provides evidence of the mechanisms for both immediate and delayed myofilament response to exercise at the molecular level. More recently, myofilament protein changes have been studied in humans in response to resistance training. Again, researchers are not completely clear about the molecular mechanisms of change, and an alteration of fiber-type composition in the myofilament may not be the answer many athletes have long assumed. This study looked at the muscle specific tension in the quadriceps femoris and vastus lateralis of forty-two young men. Researchers report a 17% increase in specific muscle tension after a period of resistance training, despite a decrease in the presence of MyHC, myosin heavy-chain. This study concludes that there is no clear relationship between fiber-type composition and in vivo muscle tension, nor was there evidence of myofilament packing in the trained muscles.

Research

Other promising areas of research that may illumine the exact molecular nature of exercise-induced protein remodeling in muscle may be the study of related proteins involved with cell architecture, such as desmin and

dystrophin Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. This complex is variously known as the costa ...

. These proteins are thought to provide the cellular scaffolding necessary for the actin-myosin complex to undergo contraction. Research on desmin revealed that its presence increased greatly in a test group exposed to resistance training, while there was no evidence of desmin increase with endurance training. According to this study, there was no detectable increase in dystrophin in resistance or endurance training. It may be that exercise-induced myofilament alterations involve more than the contractile proteins actin & myosin. While the research on muscle fiber remodeling is on-going, there are generally accepted facts about the myofilament from the American College of Sports Medicine. It is thought that an increase in muscle strength is due to an increase in muscle fiber size, not an increase in number of muscle fibers and myofilaments. However, there is some evidence of animal satellite cells differentiating into new muscle fibers and not merely providing a support function to muscle cells. The weakened contractile function of skeletal muscle is also linked to the state of the myofibrils. Recent studies suggest that these conditions are associated with altered single fiber performance due to decreased expression of myofilament proteins and/or changes in myosin-actin cross-bridge interactions. Furthermore, cellular and myofilament-level adaptations are related to diminished whole muscle and whole body performance.

References

* Muscle :: Diversity of Muscle—Britannica Online Encyclopedia." Encyclopedia - Britannica Online Encyclopedia. Web. * 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
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