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Muscle contraction
A top-down view of skeletal muscle
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. Although the term 'contraction' implies shortening, when referring to the muscular system, it means muscle fibers generating tension with the help of motor neurons (the terms twitch tension , twitch force and fiber contraction are also used).
Voluntary muscle contraction is controlled by the central nervous system. Voluntary muscle 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 several muscle fibers. 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 proceeding in the body to the muscle itself.
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 several muscle fibers [1]^. 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 and chewing have a reflex aspect to them: the contractions can be initiated consciously or unconsciously.
There are three general types of muscle tissues:
- Skeletal muscle responsible for movement
- Cardiac muscle responsible for pumping blood
- Smooth muscle responsible for sustained contractions in the blood vessels, gastrointestinal tract, and other areas in the body
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.
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 [2]^ : (1) by increasing the number and size of contractile units 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. In a typical circumstance, when a human is exerting a muscle as hard as he/she is consciously able, roughly one-third of the fibers in that muscle will be firing at once, yet can be affected by various physiological and psychological factors (including Golgi tendon organs and Renshaw cells). This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon - the force generated by a 95% contraction of all fibers is sufficient to damage the body.
Skeletal muscle contractions
Skeletal muscles contract according to the sliding filament model :
- An action potential originating in the CNS reaches an alpha motor neuron, which then transmits an action potential down its own axon.
- The action potential propagates by activating voltage-gated sodium channels along the axon toward the synaptic cleft. Eventually, the action potential reaches the motor neuron terminal and causes a calcium ion influx through the voltage-gated calcium channels.
- The Ca2+^ influx causes vesicles containing the neurotransmitter acetylcholine to fuse with the plasma membrane, releasing acetylcholine out into the extracellular space between the motor neuron terminal and the motor end plate of the skeletal muscle fiber.
- The acetylcholine diffuses across the synapse and binds to and activates nicotinic acetylcholine receptors on the motor end plate of
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.
Classification of voluntary muscular contractions
Skeletal muscle contractions can be broadly separated into twitch and tetanic contractions. In a twitch contraction, a short burst of stimulation causes the muscle to contract, but the duration is so short that the muscle begins relaxing before reaching peak force. The shape of the graph of force vs time in a twitch contraction can give information about the relative rates of calcium release and re-uptake from the sarcoplasmic reticulum. If the stimulation is long enough, the muscle reaches peak force and plateaus at this level, resulting in a tetanic contraction. If the stimulation is not intense enough, force will oscilate during the plataeu and be submaximal, but with sufficient stimulation, there will be a constant force level until stimulation stops.
Voluntary muscular contractions can be further classified according to either length changes or force levels. In spite of the fact that the muscle actually shortens only in concentric contractions, all are typically referred to as "contractions".
- In concentric contraction, the force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts. This is what most people think of as a muscle contraction.
- In eccentric contraction, the force generated is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract. An eccentric contraction is used as a means of decelerating a body part or object, or lowering a load gently rather than letting it drop.
- In isometric contraction, the muscle remains the same length. An example would be holding an object up without moving it; the muscular force precisely matches the load, and no movement results.
- In isotonic contraction, the 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 on the muscle.
- In isovelocity contraction (sometimes called "isokinetic"), the muscle contraction velocity remains constant, while force is allowed to vary. True isovelocity contractions are rare in the body, and are primarily an analysis method used in experiments on isolated muscles that have been dissected out of the organism.
In reality, muscles rarely perform under any sort of constant force, velocity, or speed, but these contractions are useful for understanding overall muscle properties present in more complex contractions that occur in vivo. Cyclic in vivo contractions can be modeled using work loops.
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 on the 20 kilodalton (kDa) myosin light chains on amino acid residue-serine 19, initiating contraction and activating the myosin ATPase. The phosphorylation of caldesmon and calponin by various kinases is suspected to play a role in smooth muscle contraction.
Phosphorylation of the 20 kDa 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 decreases, the 20 kDa myosin light chains' phosphorylation decreases, and energy utilization decreases; however, force in tonic smooth muscle is maintained. During contraction of muscle, rapidly cycling crossbridges form between activated actin and phosphorylated myosin, generating force. It is hypothesized that the maintenance of force results from dephosphorylated "latch-bridges" that slowly cycle and maintain force. A number of kinases such as Rho kinase, 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.
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.
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.[3]^ Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or otherwise control the repositioning of a load. This can occur 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.[4]
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.[3]
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
These two fundamental properties of muscle have numerous biomechanical consequences, including limiting running speed, strength, and jumping distance and height.
See also
- Exercise physiology
- Cramp
- Dystonia
- Fasciculation
- Hypnic jerk
- In_vitro_muscle_testing
- Myoclonus
- Spasm
- Supination
Additional images
Phase 1 Phase 2 Phase 3 Phase 4
References
[1] Tassinary & Cacioppo (2000), "The Skeletomotor system: surface electromyography", Handbook of psychophysiology, Second edition, Ed. John T. Cacioppo, Luois G. Tassinary, Gary G. Berntson [2] E. Shwedyk, R. Balasubramanian, R. N. Scott (1977), "A nonstationary model for the Electromyogram", IEEE Transactions on Biomedical Engineering, Vol. 24, No. 5, September [3] "Types of contractions" (http:/ / muscle. ucsd. edu/ musintro/ contractions. shtml). 2006-05-31.. Retrieved 2007-10-02. [4] Colliander EB, Tesch PA (1990). "Effects of eccentric and concentric muscle actions in resistance training". Acta Physiol. Scand. 140 (1): 31 – 9. doi:10.1111/j.1748-1716.1990.tb08973.x. PMID 2275403. [5] Brooks, G.A; Fahey, T.D. & White, T.P. (1996). Exercise Physiology: Human Bioenergetics and Its Applications. (2nd ed.).. Mayfield Publishing Co. [6] Alfredson H, Pietilä T, Jonsson P, Lorentzon R (1998). "Heavy-load eccentric calf muscle training for the treatment of chronic Achilles tendinosis" (http:/ / ajs. sagepub. com/ cgi/ pmidlookup?view=long& pmid=9617396). Am J Sports Med 26 (3): 360–6. PMID 9617396.. [7] Satyendra1,, L; Byl N (2006). Effectiveness of physical therapy for Achilles tendinopathy: An evidence based review of eccentric exercises (http:/ / iospress. metapress. com/ openurl. asp?genre=article& issn=0959-3020& volume=14& issue=1& spage=71). 14. pp. 71–80.. [8] Gordon, A. M., Huxley, A. F. & Julian, F. J. 1966 Variation in isometric tension with sarcomere length in vertebrate muscle fibres. J. Physiol-London 184, 170–192.
External links
- Animation: Myofilament Contraction (http://highered.mcgraw-hill.com/sites/0072495855/student_view0/ chapter10/animation__myofilament_contraction.html)
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Image:Skeletal muscle.jpg Source : http://en.wikipedia.org/w/index.php?title=File:Skeletal_muscle.jpg License : GNU Free Documentation License Contributors : Deadstar, Noca2plus, Rama, Raul654, Splette, 2 anonymous edits Image:Sarcomere.svg Source : http://en.wikipedia.org/w/index.php?title=File:Sarcomere.svg License : GNU Free Documentation License Contributors : David Richfield (Slashme user) Image:Muskel-molekulartranslation.png Source : http://en.wikipedia.org/w/index.php?title=File:Muskel-molekulartranslation.png License : Creative Commons Attribution-Sharealike 2. Contributors : Muskel-molekular.png: Hank van Helvete derivative work: GravityGilly Image:Lengthtension.jpg Source : http://en.wikipedia.org/w/index.php?title=File:Lengthtension.jpg License : Public Domain Contributors : User:Mokele Image:Forcevelocity.jpg Source : http://en.wikipedia.org/w/index.php?title=File:Forcevelocity.jpg License : Public Domain Contributors : User:Mokele Image:Querbrückenzyklus 1.png Source : http://en.wikipedia.org/w/index.php?title=File:Querbrückenzyklus_1.png License : GNU Free Documentation License Contributors : Original uploader was Moralapostel at de.wikipedia Image:Querbrückenzyklus 2.png Source : http://en.wikipedia.org/w/index.php?title=File:Querbrückenzyklus_2.png License : GNU Free Documentation License Contributors : Original uploader was Moralapostel at de.wikipedia Image:Querbrückenzyklus 3.png Source : http://en.wikipedia.org/w/index.php?title=File:Querbrückenzyklus_3.png License : GNU Free Documentation License Contributors : Original uploader was Moralapostel at de.wikipedia Image:Querbrückenzyklus 4.png Source : http://en.wikipedia.org/w/index.php?title=File:Querbrückenzyklus_4.png License : GNU Free Documentation License Contributors : Original uploader was Moralapostel at de.wikipedia
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