Muscle contraction

Muscle contraction

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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 with the help of motor neurons. 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 (CNS), which comprises the brain and spinal cord. Voluntary muscle contractions are initiated in the brain, while the spinal cord initiates involuntary reflexes.

 

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 contractions can be initiated consciously or unconsciously, but are continued through unconscious reflex.

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.

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 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. 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.

[edit] Skeletal muscle contractions

Skeletal muscles contract according to the sliding filament model:

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.

[edit] 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.

[edit] 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.

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.

[edit] 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.

[edit] Contractions

[edit] 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.

[edit] 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 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.^[2]

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]

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.

[edit] 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.

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]

[edit] 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.

[edit] See also

• Spasm
• Hypnic jerk
• Dystonia
• Myoclonus
• Cramp

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Mechanism of Skeletal Muscle Contraction

The sliding filament theory: It is well accepted that this is the basic mechanism for muscle contraction. Two groups in England, A.F. Huxley and Niedergerke (1954), and H.E. Huxley and Hanson (1954) are the founders of the theory. Their classical experiments are shown in Figures ME1 and ME 2:

Fig. ME1 shows a living frog muscle fiber under the interference microscope. A bands appear dark, I bands are light. The length of the sarcomeres, A and I bands were measured on densitometer tracings. The fiber was stimulated electrically and it was allowed to shorten. on the left side for each frame two numbers are given. The upper numbers indicate the sarcomere length, which in successive times during contraction decreased from 3.10 to 2.93 to 2.70 to 2.37m . The lower numbers indicate A band length, 1.43, 1.45, 1.50, and 1.48 m , that means, it was unchanged during contraction.

Fig. ME2 shows the pictures of glycerol extracted psoas fibrils under phase contrast microscope. A bands are dark, I bands are light, in the middle of I bands the Z lines are seen. Four sarcomeres of one fibril are pictured during ATP induced contraction. A bands did not change in length, but the I bands shortened, in the last frame the I bands disappeared.

Fig. ME1. Changes in sarcomere length and in I band length during contraction of an electrically stimulated frog fiber. (From A.F. Huxley and Niedergerke, 1954).

Fig. ME2. Changes in sarcomere length and in I band length during contraction of a glycerol-extracted fibril contracted by ATP. (From H.E. Huxley and Hanson, 1954).

Further experimentation revealed that during contraction the length of the actin containing thin filaments and the length of the myosin containing thick filaments remain constant. Thus, during contraction the length of the sarcomere and I band decrease, the overlap between thick and thin filaments increases, the length of the thick and thin filaments remains unchanged. Consequently, the filaments must slide past each other.

Length-tension relationship: The physiological interpretation of the sliding filament theory was tested by measuring the tension of a single muscle fiber at different sarcomere length (Gordon et al., 1966). Figure ME3 illustrates the experiment. Maximum tension was obtained at rest length, between 2.0-2.25 m , when all crossbridges were in the overlap region between thick and thin filaments. When the muscle fiber was stretched so that the sarcomere length increased from 2.25 to 3.675 m and consequently the number of crossbridges in the overlap region decreased from maximum to zero, the tension fell from 100% to 0.

The crossbridges are uniformly distributed along the thick filaments with the exception of a short bare zone in the middle. The crossbridges seem to be identical and are the site of the interaction between thick and thin filaments. The tension is the algebraic sum of the tension produced at each individual site. At or above rest length the tension is directly proportional to the number of crossbridges in the overlap region between thick and thin filaments.

Below rest length, when the thin filaments meet in the center of A band or they start to interact with the oppositely directed crossbridge sites past the bare zone (in the middle of the sarcomere), tension drops off.

 

Fig. ME3. Length-tension relationship of a single frog semitendinosus muscle fiber. (From Gordon et al., 1966). The numbers 1 through 6 on the length tension curve correspond to the numbers on the schematic diagram of thick and thin filament arrangement. In this way the relationship between thick and thin filaments can be compared to the tension at various sarcomere length.

Crossbridge cycle and its relation to actomyosin ATPase: A scheme for the coupling of ATP hydrolysis to the crossbridge cycle is shown in Fig. ME4. The following major steps are involved:

1. ATP dissociates actomyosin into actin and myosin; i.e. the thick filaments will be detached from the thin filaments. ATP binds to the myosin head in the thick filaments.
2. ATP is hydrolyzed by myosin; the products ADP and P_i are bound to myosin. The energy released by the splitting of ATP is stored in the myosin molecule. The myosin.ADP.P_i complex is a high-energy state; this is the predominant state at rest.
3. Upon muscle stimulation, the inhibition of actin-myosin interaction, imposed by the regulatory proteins, is lifted and consequently the myosin with bound ADP and P_i attaches to actin. It is believed that the angle of crossbridge attachment is 90^o.
4. The actin-myosin interaction triggers the sequential release of P_i and ADP from the myosin head, resulting in the working stroke. It is thought that the energy stored in the myosin molecule brings about a conformational change in the crossbridge tilting the angle from 90^o to 45^o. This tilting pulls the actin filament about 10 nm toward the center of the sarcomere, while the energy stored in myosin is utilized.

With a new ATP a new cycle may begin and the cycling may continue till the regulatory mechanism stops the interaction of actin and myosin. As shown in Fig. ME4, ATP is needed for step 1 that is for the detachment of myosin from actin. In case of ATP depletion, the cycle is arrested. When actin and myosin are permanently bound in the absence of ATP, the muscle becomes rigid. This state is called rigor mortis.

Fig. ME4. Crossbridge cycle and its relation to actomyosin ATPase. (Courtesy of Dr. Jack Rall).

Summary of Events in Skeletal Muscle Contraction

Excitation

The sarcolemma is depolarized and the action potential propagates

The action potential spreads inside along the T-tubules

The signal is transmitted from T-tubule to terminal sacs of sarcoplasmic reticulum

Calcium is released from sarcoplasmic reticulum into sarcoplasm

Contraction

Calcium binds to troponin

Cooperative conformational changes take place in troponin-tropomyosin system

The inhibition of actin and myosin interaction is released

Crossbridges of myosin filaments are attached to actin filaments

Tension is exerted, and/or the muscle shortens by the sliding filament mechanism

Relaxation

Calcium is pumped into sarcoplasmic reticulum

Crossbridges are detached from the thin filaments

Troponin-tropomyosin regulated inhibition of actin and myosin interaction is restored

Active tension disappears and the rest length is restored

 

References

Gordon, A.M., Huxley, A.F., and Julian, F.J. (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibres. J. Physiol., 184, 170-192.

Huxley, A.F. and Niedergerke, R. (1954). Structural changes in muscle during contraction. Interference microscopy of living muscle fibres. Nature, 173, 971-973.

Huxley, H.E. and Hanson, J. (1954). Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation. Nature, 173, 973-976.

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