A top-down view of skeletal muscle

A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same. Though the term 'contraction' implies a shortening or reduction, when used as a scientific term referring to the muscular system contraction refers to the generation of tension by muscle fibers. Locomotion in most higher animals is possible only through the repeated contraction of many muscles at the correct times. Contraction is controlled by the central nervous system, which comprises the brain and spinal cord. Voluntary muscle contractions are initiated in the brain, while the spinal cord initiates involuntary reflexes. http://training. ... http://training. ... A simplified, global view of a neuromuscular junction: 1. ... G-Actin (PDB code: 1j6z). ... Myosin is a motor protein filament found in muscle tissue. ... In a general sense, locomotion simply means active movement or travel, applying not just to biological individuals. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... For other uses, see Brain (disambiguation). ... The Spinal cord nested in the vertebral column. ... This article is about biological reflex. ...

Contractions, by muscle type

For voluntary muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. In the case of some reflexes, the signal to contract can originate in the spinal cord through a feedback loop with the grey matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli endogenous to the muscle itself. Other actions such as locomotion, breathing, chewing have a reflex aspect to them; the contractious can be initiated consciously or unconsciously, but are continued through unconscious reflex. For other uses, see Brain (disambiguation). ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. ... In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (CNS) which project their axons outside the CNS and directly or indirectly control muscles. ... Nerves (yellow) Nerves redirects here. ... A reflex action or reflex is a biological control system linking stimulus to response and mediated by a reflex arc. ... The Spinal cord nested in the vertebral column. ... The heart and lungs, from an older edition of Grays Anatomy. ... Cultured Smooth muscle of the aorta. ... This article or section is in need of attention from an expert on the subject. ...

There are three general types of muscle tissues:

• Skeletal muscle (voluntary and involuntary) contractions
• Cardiac muscle (involuntary) contractions
• Smooth muscle (involuntary) contractions.

Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope which is due to the highly organized alternating pattern of A band and I band. This article or section does not cite any references or sources. ... Cardiac muscle is a type of involuntary striated muscle found within the heart. ... Cultured Smooth muscle of the aorta. ... Structure of a skeletal muscle Skeletal muscle is a type of striated muscle, attached to the skeleton. ...

While nerve impulse profiles are, for the most part, always the same, skeletal muscles are able to produce varying levels of contractile force. This phenomenon can be best explained by Force Summation. Force Summation describes the addition of individual twitch contractions to increase the intensity of overall muscle contraction. This can be achieved in two ways: (1) by increasing the number and size of contractile unites simultaneously, called multiple fiber summation, and (2) by increasing the frequency at which action potentials are sent to muscle fibers, called frequency summation.

• Multiple Fiber Summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required.
• Frequency Summation - For skeletal muscles, the force exerted by the muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and during a contraction some fraction of the fibers in the muscle will be firing at any given time. Typically when a human is exerting a muscle as hard as they are consciously able, roughly one-third of the fibers in that muscle will be firing at once, but various physiological and psychological factors (including Golgi tendon organs and Renshaw cells) can affect that. This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon - the force generated by a 100% contraction of all fibers is sufficient to damage the body.

This article or section does not cite any references or sources. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Organ of Golgi (neurotendinous spindle) from the human tendo calcaneus. ... Renshaw cells are located in the spinal cord horn. ... An avulsion fracture is a bone fracture which occurs when a (usually small) piece of bone becomes bisected as a result of physical trauma to the ligament attached to it. ...

Skeletal muscle contractions

Skeletal muscles contract according to the sliding filament model: Image File history File links SlidingMyofibril. ... The sliding filament mechanism is a process used by muscles to contract. ...

1. An action potential originating in the CNS reaches an alpha motor neuron, which then transmits an action potential down its own axon.
2. The action potential activates voltage-dependent calcium channels on the axon, and calcium rushes in.
3. Calcium causes vesicles containing the neurotransmitter acetylcholine to fuse with the plasma membrane, releasing acetylcholine into the synaptic cleft between the motor neuron terminal and the motor end plate of the skeletal muscle fiber.
4. The acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptor on the motor end plate. Activation of the nicotinic receptor opens its intrinsic sodium/potassium channel, causing sodium to rush in and potassium to trickle out. Because the channel is more permeable to sodium, the muscle fiber membrane becomes more positively charged, triggering an action potential.
5. The action potential spreads through the muscle fiber's network of T-tubules, depolarizing the inner portion of the muscle fiber.
6. The depolarization activates L-type voltage-dependent calcium channels (dihydropyridine receptors) in the T tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.
7. Activated voltage-gated calcium channels physically interact with calcium-release channels to activate them, causing the sarcoplasmic reticulum to release calcium.
8. The calcium binds to the troponin C present on the actin-containing thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin sterically obstructs binding sites for myosin on the thin filament; once calcium binds to the troponin C and causes an allosteric change in the troponin protein, troponin T allows tropomyosin to move, unblocking the binding sites.
9. Myosin (which has ADP and inorganic phosphate bound to its nucleotide binding pocket and is in a ready state) binds to the newly uncovered binding sites on the thin filament (binding to the thin filament is very tightly coupled to the release of inorganic phosphate). Myosin is now bound to actin in the strong binding state. The release of ADP and inorganic phosphate are tightly coupled to the power stroke (actin acts as a cofactor in the release of inorganic phosphate, expediting the release). This will pull the Z-bands towards each other, thus shortening the sarcomere and the I-band.
10. ATP binds myosin, allowing it to release actin and be in the weak binding state (a lack of ATP makes this step impossible, resulting in the rigor state characteristic of rigor mortis). The myosin then hydrolyzes the ATP and uses the energy to move into the "cocked back" conformation. In general, evidence (predicted and in vivo) indicates that each skeletal muscle myosin head moves 10-12 nm each power stroke, however there is also evidence (in vitro) of variations (smaller and larger) that appear specific to the myosin isoform.
11. Steps 9 and 10 repeat as long as ATP is available and calcium is present on thin filament.
12. While the above steps are occurring, calcium is actively pumped back into the sarcoplasmic reticulum. When calcium is no longer present on the thin filament, the tropomyosin changes conformation back to its previous state so as to block the binding sites again. The myosin ceases binding to the thin filament, and the contractions cease.

The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in the sarcoplasm. The active pumping of calcium ions into the sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This causes the removal of calcium ions from the troponin. Thus the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... Alpha motor neurons (α-MNs) are large lower motor neurons of the brainstem and spinal cord. ... An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... This article needs to be cleaned up to conform to a higher standard of quality. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... Illustration of the major elements in a prototypical synapse. ... A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscles surface. ... Nicotinic acetylcholine receptors, or nAChRs, are ionotropic receptors that form ion channels in cells plasma membranes. ... For sodium in the diet, see Edible salt. ... General Name, symbol, number potassium, K, 19 Chemical series alkali metals Group, period, block 1, 4, s Appearance silvery white Standard atomic weight 39. ... A T-tubule (or Transverse tubule), is a deep invagination of the plasma membrane found in skeletal and cardiac muscle cells. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... The dihydropyridine receptor (DHPR) is a voltage-dependent calcium channel found in the transverse tubule of muscles. ... Ryanodine receptors form a class of calcium channels in various forms of muscle. ... The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e. ... Troponin Troponin C is a part of the troponin complex. ... G-Actin (PDB code: 1j6z). ... A diagram of the structure of a Myofybril Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cells. ... In biochemistry, allosteric regulation is the regulation of an enzyme or protein by binding an effector molecule at the proteins allosteric site (that is, a site other than the proteins active site). ... Troponin Tropomyosin, along with the troponin, regulate the shortening of the muscle protein filaments actin and myosin. ... Adenosine diphosphate, abbreviated ADP, is a nucleotide. ... A cofactor is any substance that needs to be present in addition to an enzyme to catalyze a certain reaction. ... Image of sarcomere A sarcomere is the basic unit of a cross striated muscles myofibril. ... Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide that is most important as a molecular currency of intracellular energy transfer. ... This article is about the sign of death. ... Sodium-Potassium pump, an example of Primary active transport secondary active transport Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. ...

Classification of voluntary muscular contractions

Voluntary muscular contractions can be classified several ways.

One of these categorizes them as either eccentric or concentric.

• In the case of eccentric contraction, the force generated is insufficient to overcome the resistance placed on the muscle and the muscle fibers lengthen as they contract.
• In the case of concentric contraction, the force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts.

Alternatively, muscle contractions can be categorized as isometric or isotonic.

• An isometric contraction occurs when the muscle remains the same length despite building tension; an example of this is muscle contraction in the presence of an afterload.
• Isotonic contractions occur when tension in the muscle remains constant despite a change in muscle length. This can occur only when a muscle's maximal force of contraction exceeds the total load (preload and afterload) on the muscle.

Isometrics is a form of exercise involving the contraction of a muscle without the shortening of the angle of the joint. ... In cardiac physiology, afterload is the tension produced by a chamber of the heart in order to contract. ... The term Isotonic refers to a muscle contraction, either eccentric or concentric, that is not speed limited. ...

Smooth muscle contraction

The interaction of sliding actin and myosin filaments is similar in smooth muscle. There are differences in the proteins involved in contraction in vertebrate smooth muscle compared to cardiac and skeletal muscle. Smooth muscle does not contain troponin, but does contain the thin filament protein tropomyosin and other notable proteins-caldesmon and calponin. Contractions are initiated by the calcium activated phosphorylation of myosin rather than calcium binding to troponin. Contractions in vertebrate smooth muscle are initiated by agents that increase intracellular calcium. This is a process of depolarizing the sarcolemma and extracellular calcium entering through L type calcium channels, and intracellular calcium release predominately from the sarcoplasmic reticulum. Calcium release from the sarcoplasmic reticulum is from Ryanodine receptor channels (calcium sparks) by a redox process and Inositol triphosphate receptor channels by the second messenger inositol triphosphate. The intracellular calcium binds with calmodulin which then binds and activates myosin-light chain kinase. The calcium-calmodulin-myosin light chain kinase complex phosphorylates myosin, specifically on the 20 kilodalton (kd) myosin light chains on amino acid residue-serine 19 to initiate contraction and activate the myosin ATPase. The phosphorylation of caldesmon and calponin by various kinases is suspected to play a role in smooth muscle contraction. The name sarcolemma is used to describe the cell membrane of a muscle fibre or muscle cell. ... oommen sir is a fool. ... Part of the myosin structure, atoms in the heavy chain are colored red on the left-hand side, and atoms in the light chains are colored orange and yellow. ...

Phosphorylation of the 20 kd myosin light chains correlates well with the shortening velocity of smooth muscle. During this period there is a rapid burst of energy utilization as measured by oxygen consumption. Within a few minutes of initiation the calcium level markedly decrease, the 20 kd myosin light chains phosphorylation decreases, and energy utilization decreases, however there is a sustained maintenance of force in tonic smooth muscle. During contraction of muscle, rapidly cycling crossbridges form between activated actin and phosphorylated myosin generating force. The maintenance of force is hypothesized to result from dephosphorylated "latch-bridges" that slowly cycle and maintain force. A number of kinases such as ROCK, Zip kinase, and Protein Kinase C are believed to participate in the sustained phase of contraction, and calcium flux may be significant.

Invertebrate smooth muscles

In invertebrate smooth muscle, contraction is initiated with calcium directly binding to myosin and then rapidly cycling cross-bridges generating force. Similar to vertebrate tonic smooth muscle there is a low calcium and low energy utilization catch phase. This sustained phase or catch phase has been attributed to a catch protein that is similar to myosin light chain kinase and titin called twitchin.

Contractions

Concentric contraction

A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force. A top-down view of skeletal muscle Muscle (from Latin musculus little mouse [1]) is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ...

During a concentric contraction, a muscle is stimulated to contract according to the sliding filament mechanism. This occurs throughout the length of the muscle, generating force at the musculo-tendinous junction, causing the muscle to shorten and changing the angle of the joint. In relation to the elbow, a concentric contraction of the biceps would cause the arm to bend at the elbow and hand to move from near to the leg, to close to the shoulder (a biceps curl). A concentric contraction of the triceps would change the angle of the joint in the opposite direction, straightening the arm and moving the hand towards the leg. The sliding filament mechanism is a process used by muscles to contract. ... A tendon (or sinew) is a tough band of fibrous connective tissue that connects muscle to bone and is built to withstand tension. ... Elbow redirects here. ... Look up Biceps in Wiktionary, the free dictionary. ... Look up ARM in Wiktionary, the free dictionary. ... The biceps curl is sometimes performed on the preacher bench, which helps to keep the upper arm motionless. ... The triceps brachii muscle is a large three-headed skeletal muscle found in humans. ...

Eccentric contraction

During an eccentric contraction, the muscle elongates while under tension due to an opposing force being greater than the force generated by the muscle.^[1] Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to 'brake' or slow the movement of a joint, either involuntarily (when attempting to move a weight too heavy for the muscle to lift) or voluntarily (when the muscle is 'smoothing out' a movement). Over the short-term, strength training involving both eccentric and concentric contractions appear to increase muscular strength more than training with concentric contractions alone.^[2] Strength training is the use of resistance to muscular contraction to build the strength, endurance and size of skeletal muscles. ... The magnitude of physical strength, often referred to as just strength, determines the ability of a person or animal to exert force on physical objects using muscles. ...

During an eccentric contraction of the biceps muscle, the elbow starts the movement while bent and then straightens as the hand moves away from the shoulder. During an eccentric contraction of the triceps muscle, the elbow starts the movement straight and then bends as the hand moves towards the shoulder. Desmin, titin, and other z-line proteins are involved in eccentric contractions, but their mechanism is poorly understood in comparison to cross-bridge cycling in concentric contractions.^[1] In human anatomy, the biceps brachii is a muscle located on the upper arm. ... This article is about the joint in the arm. ... This article is about the body part. ... The triceps brachii muscle is a large three-headed skeletal muscle found in humans. ... Intermediate filaments are one component of the cytoskeleton - important structural components of living cells. ... Titin, also known as connectin, (UniProt name: Q10466_HUMAN; accession number: Q10466) is a protein that is important in the contraction of striated muscle tissues. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ...

Muscles undergoing heavy eccentric loading suffer greater damage when overloaded (such as during muscle building or strength training exercise) as compared to concentric loading. When eccentric contractions are used in weight training they are normally called "negatives". During a concentric contraction muscle fibers slide across each other pulling the Z-lines together. During an eccentric contraction, the filaments slide past each other the opposite way, though the actual movement of the myosin heads during an eccentric contraction is not known. Exercise featuring a heavy eccentric load can actually support a greater weight (muscles are approximately 10% stronger during eccentric contractions than during concentric contractions) and also results in greater muscular damage and delayed onset muscle soreness one to two days after training. Exercise that incorporates both eccentric and concentric muscular contractions (i.e. involving a strong contraction and a controlled lowering of the weight) can produce greater gains in strength than concentric contractions alone.^[3][4] The caveat for this is that heavy eccentric contractions can easily lead to over-training since they are so demanding. This article does not cite its references or sources. ... Strength training is the use of resistance to muscular contraction to build the strength, endurance and size of skeletal muscles. ... Delayed Onset Muscle Soreness (DOMS) is the pain or discomfort often felt 24 to 72 hours after exercising and subsides generally within 2 to 3 days. ...

Eccentric contractions in movement

Eccentric contractions normally occur as a braking force in opposition to a concentric contraction to protect joints from damage. During virtually any routine movement, eccentric contractions assist in keeping motions smooth, but can also slow rapid movements such as a punch or throw. Part of training for rapid movements such as pitching during baseball involves reducing eccentric braking allowing a greater power to be developed throughout the movement. This article does not cite any references or sources. ...

Eccentric contractions are being researched for their ability to speed rehab of weak or injured tendons. Achilles tendinitis has been shown to benefit from high load eccentric contractions.^[5][6] Achilles tendinitis is inflammation of the Achilles tendon. ...

Isometric contraction

Main article: Isometric exercise

An isometric contraction of a muscle generates force without changing length. An example can be found in the muscles of the hand and forearm grip an object; the joints of the hand do not move but muscles generate sufficient force to prevent the object from being dropped. Isometrics is a form of exercise involving the contraction of a muscle without the shortening of the angle of the joint. ... For other uses, see Hand (disambiguation). ... // The Human Forearm The forearm is the structure on the upper limb, between the elbow and the wrist. ... For other uses, see Joint (disambiguation). ...

See also

• Spasm
• Hypnic jerk
• Dystonia
• Myoclonus
• Cramp

A spasm is a sudden, involuntary contraction of a muscle, a group of muscles, or a hollow organ, or a similarly sudden contraction of an orifice. ... A hypnic or hypnagogic jerk is an involuntary muscle twitch (more generally known as myoclonus or a myoclonic twitch) which often occurs during the transition from wakefulness to sleep (see hypnagogia). ... Dystonia (literally, abnormal muscle tone) is a generic term used to describe a neurological movement disorder involving involuntary, sustained muscle contractions. ... Myoclonus is brief, involuntary twitching of a muscle or a group of muscles. ... This article is about muscular pain. ...

External links

• Animation: Myofilament Contraction

Additional images

Phase 1 Image File history File links No higher resolution available. ...

Phase 2 Image File history File links No higher resolution available. ...

Phase 3 Image File history File links No higher resolution available. ...

Phase 4 Image File history File links No higher resolution available. ...

References

1. ^ ^{a b} Types of contractions (2006-05-31). Retrieved on 2007-10-02.
2. ^ Colliander EB, Tesch PA (1990). "Effects of eccentric and concentric muscle actions in resistance training". Acta Physiol. Scand. 140 (1): 31–9. PMID 2275403. 
3. ^ Brooks, G.A; Fahey, T.D. & White, T.P. (1996). Exercise Physiology: Human Bioenergetics and Its Applications. (2nd ed.).. Mayfield Publishing Co. 
4. ^ Colliander EB, Tesch PA (1990). "Effects of eccentric and concentric muscle actions in resistance training". Acta Physiol. Scand. 140 (1): 31–9. PMID 2275403. 
5. ^ Heavy-Load Eccentric Calf Muscle Training For the Treatment of Chronic Achilles Tendinosis on-line
6. ^ Effectiveness of physical therapy for Achilles tendinopathy: An evidence based review of eccentric exercises on-line