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Muscle Contraction: Sliding Filament Theory
Transcript of Muscle Contraction: Sliding Filament Theory
I'm your tour guide - Guideo, pronounced "Guy-Doe" and I'm representing the Big Brain from NERVE INC, let's begin our journey! Sliding Filament Theory of
Muscle Contraction! Thank you for joining NERVE INCs tour about muscle contractions and the sliding filament theory of muscle contractions! We hope you learned something new and we hope to see you again some time in the future! :) Take care folks! General Knowledge of the Sarcomere Before beginning the tour, we must learn about the general anatomy of the sacromere. It is important to us at NERVE INC that we spread the knowledge! Ladies and gentlemen, as you can see, sarcomeres are made up of several proteins.
The Z-discs are perpendicular protein plates that form the lateral boundaries of the contractile unit.
Thin filaments (ACTIN) extend from the Z-discs to the centre of the sarcomere and ACTIN has 2 other vital proteins known as troponin and tropomyosin.
Thick filaments (MYOSIN) make up the centre of the sarcomere.
M-line proteins act as a stabilizer for the myosin in the middle of the sarcomere and titin anchor the thick filaments to the z-discs. Ahh, the majestic sarcomere. Quite amazing really, they are repeating subunits of a myofibril and are the smallest contractile unit of a muscle fibre. Thick and Thin Filaments The thin filaments play a vital role in the contraction of one's muscles. Let's take a look at the different components. The thin filament is most composed of G-Actin that polymerize to form the 2 twisted strands we see on the left. The whole thing is called an F-Actin.
Nebulin are proteins that act as guidelines for the F-Actin.
Tropomodulin and Cap-Z are attached to the medial and lateral ends of the actin respectively and actinin attaches the Cap-Z proteins to the Z-disc of the sarcomere.
The thin filaments also contain troponin and tropomyosin. Similar to the thin filaments, the thick filaments are also extremely important to muscle contraction. On the left, we see the golf-club like myosin molecule. Each myosin molecule consists of two heavy chain polypeptides, a tail, a head, 2 globular heads, a neck., and four light chain polypeptides to regulate the movement of the neck.
The heads also have actin binding sites and a region that functions as ATPase (binds to ATP) TROPONIN and TROPOMYOSIN Troponin which is on the tropomyosin on the actin, has a binding site for calcium. Tropomyosin is the cord-like structure that blocks the myosin head binding sites on the actin when the sarcomere is in a relaxed state. The two act as a lock and key mechanism and do not allow myosin heads to bind to the binding sites until calcium is released from the t-tubules in the sarcoplasmic reticulum. When calcium binding occurs on the troponin, the tropomyosin swivels off the actin and reveals the binding sites for the myosin heads. The troponin complex is what causes the tropomyosin away from its blocking position. Troponin's control of the tropomyosin is regulated by the sarcoplasmic calcium concentration. When there is an influx of calcium, the subunits of the troponin work to peel away the tropomyosin from the actin to allow for the myson cross-bridges (head) to bind to result in muscle contraction. Sarcoplasmic Reticulum The myofibrils inside the skeletal muscle fibres are surrounded by the sarcoplasmic reticulum. The t-tubules have a high concentration of calcium. The contraction process begins when electro-chemical impulses (action potentials!) travel down the t-tubules and open certain channels. The opening of these channels allow calcium ions to diffuse out and bind to troponin in nearby sarcomeres. Surface Striations Ladies and gentlemen, did you know that under a microscope, skeletal muscle fibres appear to be striped? A-bands are dark. I-bands are light. H-zones are slightly in the middle! If we look at the handy-dandy picture on the left, we can see exactly which bands are made up of which filaments! Pretty cool beans ain't it, folks? Blood Supply to Muscle Fibres Not only does contracting a muscle require energy, but it is also important for the muscle fibres to be well supplied with nutrient and oxygen filled blood! Arterioles bring nutrient filled and oxygenated blood directly to the muscle fibre. The capillary walls allow for said nutrients and gases to diffuse/exchange with nearby muscle fibres. When the blood is deoxygenated, the blood is carried away by venules. STEP 1: The big boss - Big Brain sends a nerve impulse from it's office, down to the Spinal Cord headquarters and the message goes to the axon terminal of the motor neuron. Recall the story of ACH the Mailman! The beginnings of the sliding filament theory and ACH the mailman are the same :) STEP 2: The message is carried through the synaptic cleft by our favourite mailman! ACH! Welcome back buddy! (: You were dearly missed. How have you been? Good? .... Oh that's great! Good for you ACH! ................ Ohoops, let's get back to the tour. Anyways, so then ACH travels across the synaptic cleft (how brave of your ACH, you inspire me)... and eventually binds to the receptors on the sarcolemma. Our favourite neighbourhood mailman, ACH, binds onto the receptors that graciously accept him and his other mailman friends. The presence of ACH causes the gates of special channels that go through the sarcolemma to open. This will allow sodium ions to enter the muscle fibre and for potassium ions to exit. The diffusion of these ions are in the favour of sodium, so more sodium ions enter the fibre than potassium ions exit. The sarcolemma naturally has a negative voltage. This imbalanced gain of positive ions causes the sarcolemma to depolarize. When the muscle fibre passes it's threshold potential, the action potential is produced which will lead to the muscle fibre's contraction. Repolarization begins when ACH and his mailmen friends stop crossing the synaptic cleft and then they all get slaughtered by acetylcholinesterase (ACHE). This causes the gate to close and through primary active transport, the negative voltage is once again generated in the sarcolemma. STEP 3: Folks, please recall the sarcoplasmic reticulum and the calcium filled t-tubules. Before the death of ACH, he indirectly triggered the sarcoplasmic reticulum to release calcium from it's t-tubules into the surrounding sarcoplasm/nearby sarcomeres due to the action potential that was generated. What a self-less mailman. We'll miss you dearly ACH. STEP 4: The released calcium binds onto the attachment sites of the troponin which are located on the actin's tropomyosin. Calcium Ions Calcium ions are now attached to the troponin. Moving along! Ooops, I guess our gardener took a vacation, just skip over the vegetation folks! :) STEP 5: The binding of the calcium on the troponin causes the troponin complex to change it's shape, causing the tropomyosin to be peeled back from the actin, revealing the actin binding sites for the myosin cross-bridges/heads! How exciting!!!! Covered up actin binding site JUST as the calcium finishes attaching. Exposed actin binding sites! Myosin heads are doing their happy dance right now because they can finally attach!!!!!! STEP 6: The actin binding sites are exposed now, and with the interaction of ATP molecles to provide energy for the myosin cross-bridge to attach onto the actin binding sites, we get what's called "CROSS BRIDGE BINDING" ! FASCINATING ISN'T IT? ATP! Cross-Bridge Binding! Recall the structure of a myosin molecule. It has the ability to move and pivot due to the tail and neck hinges. Though it's a little difficult to see in these two diagrams, the swinging of the cross-bridges that are binded to the actin are pulling towards the centre of the sarcomere - thus contracting the sarcomere and eventually the contracting of the entire muscle.
This swinging motion is called a POWER STROKE. Once cross-bridge binding has occured, it causes the ATP to be released from the myosin cross-bridge, this allows the power stroke to occur as the myosin moves into a lower energy state. The Contraction Cycle: After the myosin heads move into a lower energy state, if more ATP is present in the sacromere, it will attach again to the myosin heads, causing the myosin heads to move back into a high energy position. The cycling of ATP and movements of myosin heads continues as long as ATP is available.
As long as enough calcium ions are present, the muscle contraction will continue. Note: Without ATP binding to the myosin cross-bridge, calcium will not be released, resulting in the inhibiting of cross-bridge binding between the myosin and actin! STEP 7: Once the desired muscle contractions is completed to the Big Brain's satisfaction (for example, the bicep was curled woohoo!), then the boss sends another impulse down through Spinal Cord HQ and ACH gets destroyed by ACHE, which causes repolarization of the sarcolemma and results in the calcium returning back to the t-tubules of the sarcoplasmic reticulum. The absence of calcium means no cross-bridge binding will occur and the once contracted muscle can finally relax once more! :) Take a break, bicep! Non-contracted bicep muscle! :) FLEX ALLLLLL THE MUSCLES!
Including the bicep! :) Payments for the tour will be accepted by cheque or credit card only, okthanksbye. By: Karen Chum :) Irving - Exercise Science - 2013