What is the sliding filament theory and how is tropomyosin involved?

Muscle fibers contain units known as Actin and Myosin. Actin is associated with Tropomyosin and Troponin's C,I, and T, together these structures form the thin filament. Myosin forms the thick filament and consists of Myosin heads which have an ATPase function. During the sliding filament model of muscle contraction, calcium efflux from the smooth endoplasmic reticulum occurs. These calcium ions bind to Troponin C, which causes a conformational change in the shape of the Troponin complex and also displaces Tropomyosin which covers the Myosin binding site on Actin proteins - allowing the formation of a cross bridge. Myosin ATPase hydrolyses ATP which yields the energy required for a power stroke to occur in which the thin and thick filaments slide past one another leading to muscle contraction.

The sliding filament theory describes how muscles contract. The two main components are myosin (a thick muscle filament which has characteristic heads protruding from it at regular intervals) and actin (a thin myofilament featuring a myosin binding site).

The myosin binding sites on actin are blocked by tropomyosin. When an action potential arrives at the muscle, this triggers calcium release from the sarcoplasmic reticulum. When the calcium binds to tropomyosin, it pulls it aside leaving the myosin binding sites on the actin free and available.

Myosin heads can now bind to the myosin binding sites on the actin. This is called an actin-myosin cross bridge. A molecule of ATP is hydrolysed which triggers bending of the myosin head, which pulls the actin along. The myosin head then detaches using a molecule of ATP, and binds to the actin at a point further along. The process of 'head bending' is repeated and thus the actin is pulled along by myosin and the muscle contracts.

The analogy of rowing is often used to describe the repeated motion of the myosin head binding, like a boat ploughing through water due to the repeated oar action.

The sliding filament theory is a model of how muscle contraction takes place at a molecular level in skeletal muscle fibres.

There are two types of filaments involved: actin (thin filament) and myosin (thick filament). The actin filament is tightly wrapped in a thinner filament called tropomyosin and also spherical proteins called troponin. The troponin is what tightly binds tropomyosin to the actin filament.

Calcium ions released from the sarcoplasmic reticulum target the troponin proteins and bind to them causing a change in shape. This ultimately loosens the tropomyosin filament thus the actin is less tightly wrapped. In addition to this, myosin binding sites on the actin filament are now exposed.

The sliding filament theory describes the action myosin makes as it "slides" past the actin filament to mediate muscle contraction. The myosin heads bind to the now exposed myosin binding sites on the actin and undergo a series of chemical reactions resulting in the myosin being pulled up each consecutive myosin binding site (much like a rowing team going up a river). Finally, ATP is required to shift the myosin heads back to their starting position to bind to the next myosin binding site.

For muscle contraction, there are actin and myosin filaments. Calcium binds to troponin and changes the shape of one of muscle filament bands. This causes tropomyosin to move and expose myosin binding sites on actin filaments. The actin and myosin filaments form a bridge and slide over each other which causes the muscle fibres to shorten and hence contract.

Tropomyosin is really important in muscles but it does not come alone as attached to tropomyosin is troponin. Both of these are attached to actin (the is the thin microfilament in muscles). When troponin is attached to tropomyosin it blocks the siding of actin over myosin so no muscle contraction can happen. However, when troponin is removed from tropomyosin this means tropomyosin can attach to myosin (forming an actin-myosin crossbridge) and allowing myosin to be slid over actin (like a rowing movement).

This answer is to make sure that your understanding is solid however together we would ensure that the model answer involves all the necessary exam technique.

The sliding filament theory is the process of how a muscle contracts. Muscles are made up of many smaller units called myofibrils each containing thousands of contracting units called sarcomeres. These sarcomeres contain myosin (thick) filaments and actin (thin) filaments. On the actin filament are proteins called Troponin and Tropomyosin. When Calcium is released from the sarcoplasmic reticulum, it binds to these proteins and causes them to change shape. This causes tropomyosin to move out of the myosin binding site on the actin filament. This means that the globular head of the myosin filament is free to bind to the actin filament. As it does so, through the use of ATP, it bends and moves the actin filament closer to the centre of the sarcomere, hence causing the myofibril to shorter and the muscle to contract. It then releases itself and binds to another actin filament closest to the sarcomere’s centre and will repeat this process of binding and releasing until the muscle is fully contracted.

The sliding filament theory describes how muscles contract. The two main components are myosin (a thick muscle filament which has characteristic heads protruding from it at regular intervals) and actin (a thin myofilament featuring a myosin binding site).

The myosin binding sites on actin are blocked by tropomyosin. When an action potential arrives at the muscle, this triggers calcium release from the sarcoplasmic reticulum. When the calcium binds to tropomyosin, it pulls it aside leaving the myosin binding sites on the actin free and available.

Myosin heads can now bind to the myosin binding sites on the actin. This is called an actin-myosin cross bridge. A molecule of ATP is hydrolysed which triggers bending of the myosin head, which pulls the actin along. The myosin head then detaches using a molecule of ATP, and binds to the actin at a point further along. The process of 'head bending' is repeated and thus the actin is pulled along by myosin and the muscle contracts.

The analogy of rowing is often used to describe the repeated motion of the myosin head binding, like a boat ploughing through water due to the repeated oar action.

The sliding filament theory is a suggested mechanism of contraction of striated muscles, actin and myosin filaments to be precise, which overlap each other resulting in the shortening of the muscle fibre length. Actin (thin) filaments combined with myosin (thick filaments) conduct cellular movements

The sliding filament theory explain how muscles in the human body contract to produce force.

The steps of this theory are outlined below:

• Muscle Activation: The motor nerve stimulates a motor impulse to pass down a neuron to the neuromuscular junction. It stimulates the sarcoplasmic reticulum to release calcium into muscle cells.

• Muscle Contraction: Calcium floods into the muscle cell and it binds with troponin allowing actin and myosin to bind.  The myosin and actin cross-bridges bind and contract using ATP.

• Recharging: ATP is resynthesized which allows actin and myosin to maintain their strong binding state.

• Relaxation: Relaxation takes place when stimulation of the nerve stops.  Calcium is then pumped back into the sarcoplasmic reticulum which breaks the link between actin and myosin. Myosin and actin return to their unbound state causing the muscle to relax. Alternatively, relaxation (failure) also occurs when ATP is no longer available.

Hope this helps.

The sliding filament theory explains how muscles contract once the nervous system receives signals to do. Muscles are made up of contractile units called sarcomeres with thin actin and thick myosin filaments (with the myosin heads binding to the actin-binding site) which then overlap resulting in the shortening of the filament and hence a contraction.

The actin filaments are closely linked with tropomyosin and troponin which are 2 proteins involved in regulation. Once, Ca2+ enters the cell (due to the nervous system) it binds to troponin which then changes shape causing tropomyosin to move. Tropomyosin normally blocks the binding site on actin so movement causes the binding site to be open and therefore allowing myosin to bind.

the sliding filament theory refers to the muscle contraction and how after the muscle stretches how the shortening of the muscle fibre so the ovelapping of the actin and myosin filament is called 'sliding' becuse the slide above eachother and contract the muscle.

Tropomyosin helps the myosin filaments and allows the sliding to take place so it acts as a supporting protein in this case.

The sliding filament theory is the principle of how the skeletal muscles in our body contract and relax, in other words how they function . It all begins with an action potential to the muscle which releases Ca2+ ions from the sarcoplasm reticulum into the cytoplasm. Tropomyosin is essentially a lock that hides the actin away from the myosin heads (Which is what allows muscle contraction to begin) so when Ca2+ ions are released they bind to a site on the tropomyosin called the troponin C. This allows the "lock" i.e the tropomyosin to shift allowing the myosin head to bind to "exposed" acting. Once bound, ATP is used allowing a "power stroke" or a "slide" to occur. This stroke is when the myosin head pulls on the actin filament causing the muscle to shorten. When the muscle shortens, the H and I bands between Z lines shorten too. This means a contraction is taking place. After the slide, the myosin head detaches and attaches itself to the next exposed actin and the next power stroke occurs. This repeats as long as there are Ca2+ ions present.

Hello,

The sliding filament theory is a way of explaining how muscles contract. For the AQA Biology specification, you are required to know this theory and may be asked application questions about it. The actual process is a bit more complicated than how AQA explains and involves another protein called troponin, but if you are studying AQA only worry about tropomyosin.

Tropomyosin is a protein which blocks the myosin binding sites on actin and prevents the formation of a cross-bridge. This is important to ensure that the muscles are only contracting when there is a stimulus.

When there is a stimulus, calcium ions are released from the sarcoplasmic reticulum and these calcium ions bind to tropomyosin. This shifts the position of tropomyosin and exposes the myosin binding sites on actin. As a result, myosin is now able to bind to actin to form a cross-bridge. Remember this can only take place when the muscle is stimulated (by an action potential).

ADP and Pi (from the hydrolysis of ATP) bind to the myosin head and cause the myosin heads to bend or 'nod forwards' pulling the actin filament along the myosin filament. This is the muscle contracting.

The cross bridge must then break so that the muscle can relax and this happens when ATP binds to the myosin head.

Calcium ions then activate ATPase enzymes to hydrolyse the ATP --> ADP + Pi which returns the myosin head to its initial position.

The process repeats as long as there are action potentials stimulating the muscle to contract.

Sliding filament theory or power stroke explains how actin and myosin filaments slide over each other to produce a muscle contraction in skeletal muscle. Once a nerve impulse/action potential travels down the the T-tubule deep into a muscle fibre, it causes the release of calcium ions from sarcoplasmic reticulum. The calcium ions bind to troponin (a protein molecule attached to tropomyosin) which causes a conformational change and pulls the tropomyosin filaments aside. This exposes the myosin head binding site on actin filaments, The myosin head is now able to bind onto actin filament forming a cross bridge pulling actin filament. ATP molecule attaches to myosin head, hydrolysis of which causes the myosin head to detach and come to original shape before binding to the next myosin head binding site.

The sliding filament theory explains when a muscle fibre contracts is when myosin filament pull the actin closer together and this causes the sarcomere to shorten. When all the sarcomeres in a muscle fiber shorten, the fiber contracts. In resting state the tropomyosin blocks the binding of myosin to actin so no cross bridges are formed.