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What is the sliding filament theory? The sliding filament theory was first solely introduced in 1954 by two research teams one consisting of Andrew Huxley & Rolf Niedergerke, These two men were from the University of Cambridge, and the other team consisted of Hugh Huxley & Jean Hanson from the Massachusetts Institute of technology. It was originally conceived by Hugh in 1953.  The sliding filament theory is a mechanism in our bodies in which the muscles contract at a cellular level. To better understand this topic, we need to understand the structure of skeletal muscles as it is very helpful. Each muscle is made up of a number of muscle fibres, each bundle of muscle fibres contains anywhere from 10-100 individual fibres. Each muscle fibre itself contains cylindrical organelles known as Myofibryls which contain bundles of proteins known as Actin and Myosin, surrounding Myofibryl there is a network of tubules and channels called the sarcoplasmic reticulum where calcium is stored, and each Myofibryl can be broken down into functional repeating segments called sarcomere. If we look at a 2-dimensional model of a sarcomere they consist of Actin and Myosin when a nerve impulse arrives at a muscle it causes a release of a chemical called Acetylcholine this presence of this causes depolarization enabling calcium to be released from the sarcoplasmic reticulum, the calcium binds troponin changing its shape and moving tropomyosin from the active site of the actin the Myosin filaments now attach to the actin forming a cross bridge. The breakdown of ATP=ADP + P + Energy which enables the myosin to pull the actin filaments inwards contracting the muscle this occurs along the entire length of every myofibril in the muscle cell. When an ATP molecule binds to the myosin head the myosin detaches for the actin the cross-bridge is broken. When the ATP is then broken down the myosin head can again attach to the actin-binding site further along the actin filament and repeat the process, this repeated pulling of the actin over the myosin is often known as “The ratchet mechanism”, this process of muscular contraction can last for as long as there are adequate ATP and CA+ stored. Once the nerve impulse stops the CA+ is pumped back to the sarcoplasmic-reticulum and the actin returns to its resting position causing the muscle to lengthen and relax.

In conclusion, The sliding filament theory is a theory in which the body's muscles contract via the fibres in those muscles which contain the Myfibryls which contain the proteins that are Actin and Myosin and the whole action of contraction comes down to these two proteins as its based on the actin and myosin filaments that slide past each other and cause sarcomere shortening which translates to muscles contraction and muscle fibre shortening, two other things that are required for the process are ATP and CA+ (calcium) stores in our bodies. So this conclusion is just me telling you what basic knowledge you’d need to know to understand “The Sliding Filament Theory”.

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Sliding Filament Theory

(Summary)

I  will summarize the sliding filament theory from the website study smarter.co.uk .

Definition - The muscles will contract through the overlapping of actin and myosin filaments. This will result in a shortening of the muscle fibre length. The cellular movement is controlled by actin (thin filaments) and myosin (thick filaments).

Skeletal muscle structures are muscle cells that are long and cylindrical. They are often alluded to as muscle fibres of myofibres. In this article they state that “Skeletal muscle fibres are multinucleated cells, meaning that they consist of multiple  nuclei (singular nucleus) because of the fusion of hundreds of precursor muscle cells ( embryonic myoblasts) during early development.” This states that these particular muscles can be very big in humans.

The muscle fibres contain of plasma membrane, the muscle fibre in this is called the “sarcolemma”. The cytoplasm is called the “sarcoplasm”. The myofibres contains a “smooth endoplasmic reticulum called the sarcoplasmic reticulum (SR), adapted for storing, releasing and resorbing calcium ions.” Myofibres will also include myofibrils, thick myosin and thin actin myofilaments.

The thick and thin myofilaments is called sarcomere and it is the contractile unit of a myofibre. They are four proteins found in sarcomeres these are actin, myosin, tropomyosin and troponin, these proteins play a critical role in regulating the function of actin filaments in muscle contraction .”During muscle relaxation, tropomyosin binds along actin filaments blocking the actin-myosin interactions.” This is a quote directly from this website.

Tropooin is composed of three subunits

1.    Tropooin T.  bind to tropomyosin.

2.    Tropooin I. bind to actin filaments

3.    Tropooin C. binds to calcium ions

Actin-“Since actin and its associated proteins form filaments thinner in size than the myosin, it is referred to as the thin filament.”

Myosin-“Myosin strands are thicker due to their larger size and multiple heads that protrude outwards.”

Source of energy for muscle contraction.”Energy in the form of ATP is needed for the movement of myosin heads and the active transportation of Ca ions into the sarcoplasmic reticulum.”

This type of energy can be portrayed in three ways aerobic respiration of glucose and oxygen oxidative phosphorylation in the mitochondria. Step two is the anaerobic respiration of glucose, furthermore, the regeneration of ATP using Phosphocreatine.

Moving on I will go over a brief description of the 9  steps of the sliding filament theory from studymind.co.uk

Step 1:” An action potential crosses the neuromuscular junction. The muscle cell depolarizes passes down the sarcolemma and spreads down the transverses tubules to the (SR)

Step 2: “Calcium channels open releasing stored calcium ions into the sarcoplasm.”

Step 3: “Actin-myosin binding sites are exposed”

Step 4: “Myosin heads bind with the actin-myosin binding sites this is called actin-myosin cross-bridges.

Step 5:” ATP Is converted into ADP + PI

Step 6: “ ATP is pulled past the myosin. Energy will be released from the ATP changes the angle of the myosin head, this allows actin to slide over the myosin and contain the muscle, this is called power stroke.”

Step 7: “Actin-myosin cross bridges break.”

Step 8: “ Myosin head attaches to a different actin-myosin binding site.”

Step 9: “The cycle is repeated.”

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1. The sliding filament theory states that when a muscle  contracts, it lengthens because of the interaction of actin and myosin molecules 

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2.According to the sliding  filament theory, actin filaments actively attach to and pull on myosin filaments.

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4. which of the following ions is responsible for unmasking of active binding sites on actin for forming cross-bridge during muscle contraction

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3.which of the following is correct for muscle contraction?

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5.Which of the following shows ATPase activity during muscle contraction?

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7.this is a contractile protein of muscles 

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8. which of the following is a functional unit of striated muscles?

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9.when actin filaments is dragged towards m-line it is known as ?

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10.accumulation of ____________ causes muscle fatigue 

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I will summarise the sliding filament theory through a podcast called. "NASM-CPT PODCAST WITH RICK RICHEY".

he starts off by talking about the muscle fibers and cells in the body that contain protein. These are called, actin and myosin. 1 is a thick filament and the other is a thin filament. Rick teaches us an easy way to remember by saying . "Myosin sounds like muscles, everyone wants big muscles, therefore myosin is a thick filament", he then goes on to say that actin is a thin filament. He then says that actin and myosin work together to contract and attach through the sarcomere, which is the functional unit of the cell. This goes from Z line to Z line. The 2 lines make zig-zag appearance in our body, and when a muscle contracts it brings the z lines closer together, myosin and actin attach via something called cross bridging. The myosin head reaches across and grabs the actin which pulls the Z lines closer together, but there is some force fields in the way blocking the actin, these are called troponin and tropomyosin. Rick then talks about ATP, which is the energy currency of the cell, while this forefield is happening, they need to come into play, along with calcium to break down these force fields. These calcium ions cause the muscle to contract, which excites the muscles causing them to contract, and then the Z lines come closer together. Calcium moves troponin out of the way, whenever they connect it pulls the other force field (tropomyosin), away as well. This is when the actin and myosin make a connection, as there is no force field blocking it anymore. He now explains how the actin and myosin separate again, this happens by the energy currency of the cell. (ATP), attaching to the myosin head, and it releases and lets go. Once it lets go, it goes back into its original position again, so whenever it touches the actin again calcium ions get released and the troponin and tropomyosin move away, then we see the cross bridging taking place again , which allows the filament to slide.

So to summarise everything he has said I am going to break it down into 8 easy steps,

Step 1 , The action protentional happens

Step 2 , The calcium ions get released by the sarcomeres

Step 3, The calcium ions moves the troponin out of the way, which also makes the tropomyosin move out of the way.

Step 4, The myosin head binds to the actin

Step 5, The ATP, converts to ADP

Step 6, The ATP connects to the myosin head causing it to move away from the actin

Step 7, The cross bridges that were made, now break and everything resets back to normal

Step 8, The sarcomere returns back and it is ready to repeat the cycle again.