Notes on Sliding Filament Theory PDF

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Materials provided by Kate Strohm for Learn Direct, Unit 12 The Human Muscular and Skeletal Systems. The contraction of striated muscle fibres There are three main types of muscle cells in the body. Each type of muscle differs in the structure of their cells, their location within the body, their function and the way by which they are activated (i.e. contracted). Skeletal muscles Skeletal muscles are primarily used for movement around a joint and to hold a body part in a stable position. Muscles are attached to bone and work by voluntary control.

A diagram to show the main features of a skeletal muscle.

Each muscle contains many fibres. Each muscle fibre contains a bundle of approximately 4-20 myofibrils. Each myofibril is composed of thick (dark) and thin (light) myofilaments. The arrangement of these myofilaments gives the muscles its striated appearance and it is the movement of these myofilaments that cause the muscle to contract. Skeletal muscle contraction is the result of nervous stimulation. Contraction takes place via a mechanism called the Sliding Filament Theory. According to this theory, the thin filaments (also known as actin) slide past the thick filaments (also known as myosin) during muscle contraction. The thick and thin filaments found between a pair of Z lines forms a repeating pattern that is called a subunit, known as a sarcomere. The muscle fibre will contain many sarcomeres. Thick filaments extend from one end of the A band through the H zone and up to the other end of the A band. These filaments remain in relatively constant length during muscle contraction and relaxation. Thin and thick filament lies side-by-side in the A band. During muscle contraction, the thin filaments move into the A bands and the distance in the H zone is reduced. The I band shortens and the Z line comes in contact with the A bands.

A diagram to show the arrangement of the thick and thin filaments within the myofibril.

Movement of the thin filaments past the thick filament is caused by the formation of cross bridges. These cross bridges are formed when the myosin head binds to the actin. The formation of cross bridges produces a power stroke that forces the thin filaments to move past the thick filaments and towards the centre of the A band. An energy source known as adenosine triphosphate (ATP).

A schematic of the main features of a thin and thick filament cross bridge.

There are three main types of skeletal muscle fibres: 1.

2.

Slow twitch fibres (Type I): these contract slowly and repeatedly over long periods of time. They have a good blood supply, so they receive plenty of oxygen which is used to produce energy. These fibres are able to develop less force than the other two types and are most suited for endurance type activities (i.e. long walks or cycling). Fast twitch fibres (Type II): Type IIa have a fast contraction speed. These fibres are less reliant on oxygen, so they fatigue quicker than slow twitch fibres. They are best suited to speed, strength, and power-based activities such as weight training and longer sprints. Type IIb fibres have an extremely fast contraction speed and create very forceful contractions. They rely on readily available energy sources in the muscles, rather than those supplied by the blood. Because of this, they fatigue very quickly. Like type IIa fibres, type IIb fibres are best suited to speed, strength and power activities, as well as short sprints.

With the correct type of exercise training, muscle fibres within each type can be converted. Specifically type IIa can be converted to type IIb and vice versa. However, you cannot convert type I fibres to type II fibres and vice versa.

Cardiac muscles Cardiac muscles are only found I the walls of the heart and are used to force blood into the circulatory vessels. Cardiac muscle contractions are strong and use lots of energy. These contractions are continuous and occur with autonomic control (although contractions can be modified by external nervous and hormonal stimuli). Cardiac cells cannot regenerate, therefore damage to these cells can have disastrous consequences. Cardiac muscle fibres are long, cylindrical cells with one (sometimes two) nuclei. These nuclei are centrally located within the cell. Between the fibres, there is a connective tissue which has a rich capillary network that is able to meet the high metabolic demand of the continuous activity required by this type of muscle. Like skeletal muscle, cardiac muscle contains sarcomeres that are the contractile unit of each of the fibres and therefore contract in much the same way. Cardiac muscle cells are arranged in a branched shape so that each cell is in contact with multiple other cardiac cells. At the end of each fibre is an intercalated disk. This is a region of overlapping extensions of the cell membrane. The intercalated disks form junctions between the cells so that they cannot separate under the strain of pumping blood and electrochemical signals can be passed quickly from cell to cell. An important feature that is unique to cardiac muscle tissue is auto-rhythmicity. This means that this tissue is able to set its own contraction rhythm due to the presence of pacemaker cells that control the stimulation of the other cardiac muscle cells. These pacemaker cells receive input from the nervous system which serve to increase or decrease the heart rate depending on the body’s needs.

TeachPE.com – Anatomy and Physiology – Muscle Contraction and Sliding Filament Theory https://www.teachpe.com/anatomy-physiology/sliding-filament-theory

Sliding filament theory is the mechanism by which muscles are thought to contract at a cellular level. Each muscle fibre is made up of smaller fibres called myofibrils. These contain even smaller structures called actin and myosin filaments. These filaments slide in and out between each other to form a muscle contraction hence called the sliding filament theory. This diagram shows part of a myofibril called a sarcomere. This is the smallest unit of skeletal muscle that can contract. Sarcomeres repeat themselves over and over along the length of the myofibril.

Myofibril – a cylindrical organelle running the length of the muscle fibre, containing actin and myosin filaments. Sacromere – the functional unit of the myofibril, divided into I, A and H bands Actin – a thin, contractile protein filament, containing ‘active’ or ‘binding’ sites Myosin – a thick, contractile protein filament, with protrusions known as Myosin Heads Tropomyosin – an actin-binding protein that regulates muscle contraction Troponin – a complex of three proteins, attached to tropomyosin.

Muscle contraction The process of a muscle contracting can be divided into 5 sections: 1.

2.

3. 4.

5.

A nerve impulse arrives at the neuromuscular junction, which causes a release of a chemical called acetylcholine. The presence of Acetylcholine causes the depolarisation of the motor endplate which travels throughout the muscle by the transverse tubules, causing Calcium to be released from the sarcoplasmic reticulum. In the presence of high concentrations of calcium, the calcium binds to troponin, changing its shape and so moving tropomyosin from the active site of the actin. The myosin filaments can now attach to the actin, forming a cross-bridge. The breakdown of ATP releases energy which enables the myosin to pull the actin filaments inwards and so shortening the muscle. This occurs along the entire length of every myofibril in the muscle cell. The myosin detaches from the actin and the cross-bridge is broken when an ATP molecule binds to the myosin head. When the ATP is then broken down the myosin head can again attach to an actin binding site further along the actin filament and repeat the ‘power stroke’. This repeated pulling of the actin over the myosin is often known at the ratchet mechanism. This process of muscular contraction can last for as long as there are adequate ATP and calcium stores. Once the impulse stops the calcium is pumped back to the sarcoplasmic reticulum and the actin returns to its resting position causing the muscle to lengthen and relax.

It is important to realise that a single power stroke results in only a shortening of approximately 1% of the entire muscle. Therefore to achieve an overall shortening of up to 35% the whole process must be repeated many times. It is thought that whilst half of the cross-bridges are active in pulling the actin over the myosin, the other half are looking for their next binding site.

Stretched Muscle The diagram shows a stretched muscle where the I bands and the H zone is elongated due to the reduced overlapping of the myosin and actin filaments. There would be reduced muscle strength because few cross-bridges can form between the actin and myosin.

Partially contracted muscle This diagram shows a partially contracted muscle where there is more overlapping of the myosin and actin with lots of potential from cross bridges to form. The I bands and H zone is shortened.

Fully contracted muscle This diagram shows a fully contracted muscle with lots of overlap between the actin and myosin. Because the thin actin filaments have overlapped there is a reduced potential for cross bridges to form again. Therefore, there will be low force production from the muscle.

Lumen. Biology for Majors II. Module 23 – The Musculoskeletal System. Sliding filament model of contraction. https://courses.lumenlearning.com/wmbiology2/chapter/sliding-filament-model-of-contraction/

For a muscle cell to contract, the sarcomere must shorten. However, thick and thin filaments – the components of sarcomeres – do not shorten. Instead, they slide by one another, causing the sarcomere to shorten while the filaments remain the same length. The sliding filament theory of muscle contraction was developed to fit the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and relaxation. The mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement. When (a) a sarcomere (b) contracts, the Z lines move closer together and the I band gets smaller. The A band stays the same width and, at full contraction, the thin filaments overlap.

When a sarcomere shortens, some regions shorten whereas others stay the same length. A sarcomere is defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance between the Z discs is reduced. The H zone – the central region of the A zone – contains only thick filaments and is shortened during contraction. The I band contains only thin filaments and also shortens. The A band does not shorten – it remains the same length – but A bands of different sarcomeres move closer together during contraction, eventually disappearing. Thin filaments are pulled by the thick filaments toward the centre of the sarcomere until the Z discs approach the thick filaments. The zone of overlap, in which thin filaments and thick filaments occupy the same area, increases as the thin filaments move inward.

Lumen. Biology for Majors II. Module 23 – The Musculoskeletal System. ATP and Muscle Contraction https://courses.lumenlearning.com/wm-biology2/chapter/atp-and-muscle-contraction/ The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action requires energy, which is provided by ATP. Myosin binds to actin at a binding site on the globular actin protein. Myosin has another binding site for ATP at which enzymatic activity hydrolyses ATP to ADP, releasing an inorganic phosphate molecule and energy. ATP binding causes myosin to release actin, allowing actin and myosin to detach from each other. After this happens, the newly bound ATP is converted to ADP and inorganic phosphate. The enzyme at the binding site on myosin is called ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head into a ‘cocked’ position. The myosin head is then in a position for further movement, possessing potential energy, but

ADP and inorganic phosphate are still attached. If actin binding sites are covered and unavailable, the myosin will remain in the high energy configuration with ATP hydrolysed, but still attached. If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance between the actin and myosin molecules. Inorganic phosphate is then released, allowing myosin to expend the stored energy as a conformational change. The myosin head moves toward the M line, pulling the actin along with it. As the actin is pulled, the filaments move approximately 10nm toward the M line. This movement is called the power stroke, as it is the step at which force is produced. As the actin is pulled toward the M line, the sarcomere shortens and the muscle contracts. When the myosin head is ‘cocked’, it contains energy and is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the cross-bridge formed is still in place, and actin and myosin are bound together. ATP can then attach to myosin, which allows the cross-bridge cycle to start again and further muscle contraction can occur. The movement of the myosin head back to its original position is called the recovery stroke. Resting muscles store energy from ATP in the myosin heads while they wait for another contraction.

Regulatory Proteins When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation and preventing contraction in a muscle without nervous input. Troponin binds to tropomyosin and helps to position it on the actin molecule; it also binds calcium ions. To enable a muscle contraction, tropomyosin must change conformation, uncovering the myosin-binding site on an actin molecule and allowing cross-bridge formation. This can only happen in the presence of calcium, which is kept at extremely low concentrations in sarcoplasm. If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Cross-bridge cycling continues until Ca2+ ions and ATP are no longer available and tropomyosin again covers the binding sites on actin....