Lecture 6-Molecular basis of contraction Flashcards
What are the three types of muscle?
-skeletal striated(voluntary) -cardiac, striated (involuntary) -smooth, unstriated (involuntary)
What is a muscle fibre?
-single muscle cell, multinucleate
What is a myofibril?
– componenent of the muscle cell, there are many myofibrils in one -90% volume of muscle fibre -cylindrical intracellular organelle -the more there is the greater the the contractile strength and tension -contractile element
What does a myofibril consist of?
-thick and thin filaments
What are thick filaments made of?
-protein myosin
What are thin filaments made of?
-protein actin
Why does skeletal muscle appear striated?
-alternating light and dark bands of actin and myosin = the myofilaments which slightly overlap
What is an A band?
-dark band in muscle -stack of thick filaments and a portion of thin filaments that overlap at both ends of the thick filament
What is the H zone?
-lighter area within the middle of the A band -where the thin filaments don’t reach
What is the I band?
-thin filaments that don’t project into the A band
What is a Z line?
-in the middle of the I band, dense vertical line -flat cytoskeletal disk(protein) connects thin filaments of two adjoining sarcomeres
What is a sarcomere?
- -the area between two Z lines -functional unit of skeletal muscle=smallest unit of a muscle fibre that can contract
What is a functional unit of an organ?
-the smallest unit that can perform the function of that organ
How does a muscle increase in size during growth?
-by adding new sarcomeres
What is a M line?
-proteins that hold the thick filaments together vertically in each stack -vertically down in the A band within the center of the H zone
What is a cross bridge?
-fine cross bridges between thick and thin filaments in the region where they overlap (within the A band)
What is myosin?+structure
-protein, has two subunits each shaped like a golfclub with two heads -the tails intertwined around each other, heads sticking out at one end -tails oriented towards the center of the filament and the heads out -the heads form the cross bridges between the thick and thin filaments
How many and which binding sites do the myosin heads have?
-each head has two binding sites = actin binding site =ATPase binding site
How is myosin linked to the Z line?
-elastic protein titin
What do the thin filament consist of?
3 proteins -actin -tropomyosin -troponin
What is actin?
-spherical protein -forms backbone of a thin filament= two actin strands wrapped around each other(like DNA) -two sites for myosin binding (weak= electrostatic, strong= with myosin cross bridge)
Why can’t actin bind with the cross bridge when muscle fibre relaxed?
-because of the position of tropomyosin and troponin
What is tropomyosin?
-threadlike protein -end to end alongside the groove of the actin molecule -in this position it covers the binding sites needed for muscle contraction
What is troponin?
-protein complex: consists of three polypeptide units -one binds to tropomyosin -one binds to actin -one that binds to Ca2+ (calcium)
Describe role of calcium in tropomyosin and troponin interaction?
-when Ca2+ not bound to troponin then the troponin stabilises tropomyosin in its blocking position of over actin’s cross bridge binding sites -when Ca2+ bound, troponin changes shape so tropomyosin slides away from actin binding sites and myosin can bind resulting in muscle contraction
What happens during contraction?
-the thin filaments in each side of a sarcomere slide towards the A band center -as they slide the thin filaments pull the Z discs closer together= sarcomere shortening -H zone much smaller -I band narrows -the thin nor the thick filaments don’t change length during contraction=accomplished by thin filaments sliding closer together -the A band remains the same
How are thin filaments pulled inwards relative to the thick filaments during contraction?
-after Ca2+ bound to troponin the actin sites empty -myosin heads (or cross bridges) „walk“ along the actin filament pulling it inwards
Do the myosin heads act together or independently?
-independently -only one head attaching at one given time
What happens when actin and myosin make contact at a cross bridge?
-the bridge changes shape, bending 45 degrees inwards =power stroke! -pulls the thin filament inwards
What happens when actin and myosin stop making contact at a cross bridge?
-the myosin heads go back to their position and can bind to the next actin
How many powerstrokes do we need in muscle contraction?
-many many many money;)
What stimulates skeletal muscle to contract?
-release of Acetylcholine (Ach) at junction between motor neuron terminals and muscle fibres= leads to AP conducted over the muscle cell
What is a T tubule?
T= transverse -at the end of the A band the surface membrane dips deeply into the larger muscle fiber to form a transvere tubule that runs perpendicularly from the surface of the muscle cell membrane to the central portions of the muscle
Why are T tubules important for AP spread?
-because the T tubule membrane is continuous with the surface membrane= so AP spreading on T tubule is a way of getting it to the middle -local AP in the T tubule causes permeability changes in the sarcoplasmic reticulum
What is the sarcoplasmic reticulum?
-modified endoplasmic reticulum, important for turning muscle on/off -interconnected tubules surrounding each myofibril like a sleeve -longitudinal down the myofibril but not continuous(separate ones around each A and I band) -at the end of each= lateral sacs = store Ca2+
What does spread of AP across T tubules cause in terms of Ca2+?
-triggers release of Ca2+ via the Ca2+-ATPase pump in the lateral sacs in the sarcoplasmic reticulum into the cytosol to trigger contraction (the pump later reuptakes Ca2+)
What does Ca2+ do after being released from the sarcoplasmic reticulum?
-binds to troponin on thin filaments
What does Ca2+ binding to troponin cause?
-causes the tropomyosin to change shape, physically moving it from its blocking position, uncovering the binding sites on actin for myosin cross bridges.
What happens to Ca2+ when APs stop?
-then Ca2+ is taken up by the sarcoplasmic reticulum, with no Ca2+ on troponin, tropomyosin moves back to block the myosin cross bridge binding site on actin -contraction stops and the thin filaments passively slide back to their relaxed position
What is the ATPase binding site on myosin for?
-an anzymatic binding site for the binding of ATP which it splits into ADP and P releasing energy in the process -the ATP binds before the cross bridge is formed and the ADP and P remain attached = dissociate when power stroke to provide space so more energy can be created -and the generated energy produces high energy form of myosin = ready to act when the actin sites are free!
What does acetylcholinesterase do?
-reuptakes Ach in the neuromuscular junction and stops the propagation of AP
What does Pervalbumin do?
-binds the Ca2+
Does relaxation require energy?
-yes -ATP needed for actin and myosin to separate
How is contraction stopped?
- not enough Ca2+ - decrease in ATP
What splits the ATP on the myosin ATPase site?
-Magnesium (Mg2+)
What causes detachment of the myosin head from the actin site?
-Mg detached and ADP detached
What are the components of a tendon?
-collagen fibres -elastin fibres -peritenon -formation and repair of fibres
What are the parts of a mucle cell?
-nucleus -sarcolemma: T tubules -sarcoplasm: mitochondria and sarcoplasmic reticulum -myofibrils = the fibres
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