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a bridge that is shaped like a cross
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Binding of the myosin heads sequentially prevents what?
myosin crossbridge binding
What are the key steps involved in the crossbridge cycle during muscle contraction?
During the crossbridge cycle in muscle contraction, the key steps involve the binding of myosin to actin, the power stroke where the myosin head pivots and pulls the actin filament, the release of ADP and inorganic phosphate, and the resetting of the myosin head for the next cycle.
What is the crossbridge cycle?
The crossbridge cycle is the cyclical formation of links between actin and myosin. This results in the sliding of thin filaments towards the M line of a sarcomere. The myosin head undergoes conformation changes which allows it to swivel back and forth. In its low energy form, myosin has a low affinity for actin. The ATP prepares myosin for binding with actin by moving it to its high energy form position. When myosin contracts, it has a high affinity for actin.
Cross-bridge formation that results in muscle contraction requires intracellular?
the major intracellular ion in crossbridge formation is what
What is the function of troponin?
Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction.
What is the difference between muscle dystrophy and muscle atrophy?
Myasthenia gravis: autoimmune to the ACh receptors at end plate (pathway >>> no muscle contraction) Muscular Dystrophy: genetic (X-linked) defect in dystrophin (pathway >>> myosin-actin crossbridge doesn't form > no muscle contraction)
What is contraction and relaxation?
Nerve impulse, or electrical signal, travels down the nerve to the terminal to cause the release of the neurotransmitter acetylcholine (ACh).ACh diffuses across the neuromuscular junction and binds to the receptor sites.Stimulation of the receptor sites causes an electrical impulse to form in the muscle membrane. The electrical impulse travels along the muscle membrane and penetrates deep into the muscle through the T-tubular system.The electrical impulse stimulates the sarcoplasmic reticulum to release calcium into the sarcomere (a contractile unit of a mofibril) area.Calcium allows the actin, myosin, and ATP to interact, causing crossbridge formation and muscle contraction. This process continues as long as calcium is available to the actin and myosin.Muscle relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum, away from the actin and myosin. When calcium moves in this way, the actin and myosin cannot interact, and the muscle relaxes.
What function do calcium ions perform during skeletal muscle contractions?
During skeletal muscle contraction calcium ions expose myosin binding sites on to the actin. When a nerve sends a signal to a muscle to do some "action" the release of acetylcholine at the neuromuscular junction triggers calcium to be released into the cytoplasm. At rest, the cell has a low concentration of calcium. When the calcium concentration rises, calcium diffuses over to the muscle protein fibers and causes a conformational change. Calcium binds to the troponin and causes it to rotate slightly which forms a crossbridge that drags along the actin fiber and shortens it thus creating muscle fiber contraction.
What is voluntary muscle?
Involuntary muscles are muscles that are not controllable consciously, and instead contract due to unconscious impulses sent by the autonomic nervous system or certain specialized cells or hormones. Both smooth muscle and cardiac muscle can be classified as involuntary muscles. Smooth muscle is comprised of spindle-shaped cells that have no striations and is found in numerous locations throughout the human body. Cardiac muscle is striated rather than smooth, and is found only within the walls of the heart. Smooth muscles are involuntary muscles composed of thick and thin protein filaments that are homologous to the organelles known as myofibrils found in skeletal muscles. The thin filaments are composed of a globular protein called actin, while the thick ones are made up of a motor protein called myosin. Smooth muscles require extracellular calcium ions to contract: the ions activate a nucleotide called Adenosine triphosphate (ATP), which then activates the myosin filaments. The myosin filaments attach to the actin filaments in a process known as the crossbridge cycle, which causes the thick and thin filaments to slide over each other and contract. When the myosin filaments release the actin filaments, the muscle relaxes.
How does striated muscle contract?
Striated muscles, like all other muscles, including cardiac and smooth, use a method called the sliding filament theory. This is the current model of how muscles contract and was developed by Hugh Huxley in the 1960's.
How do skeletal muscle fibers become multinucleated?
Being multinucleated enables anything to synthesize more protein. Also we know that skeletal muscle fibres are much longer than that of any other muscle fibre, so it needs a huge amount of actin and myosin protein to bring about required contraction of a muscle, as actin and myosin crossbridge is required for muscle contraction. Thus it can be postulated that this boon of being multinucleated is essential for a skeletal muscle to posses adequate amounts of actin and myosin for muscle contraction. By-- faireena singh ( mbbs first year cmc)
What happens during a single muscle contraction?
An action potential is sent down the axon. Once this action potential reaches the axonal knob, it signals the neurotansmitter acetylcholine (ACh) to be exocytosed via synaptic vesicles. Once the ACh travels across the synaptic cleft and binds to ACh receptors in the end plate, sodium channels open. The opening of these channels allows sodium to diffuse into the innermembrane space, down the gradient. Once this innermembrane space reaches threshold, the sodium channels become inactive. The inactivation of these sodium channels causes potassium channels to open. Potassium ions within the innermembrane space are now able to diffuse out, causing a voltage reversal (-90 to about +75 mV). This end plate potential (EPP) is what excites the muscle fiber. This first action potential then causes a chain reaction of other action potentials across the sarcolemma. This chain reaction spreads until it reaches the T tubules. Once this action potential flows down the T tubules, it causes calcium channels to open on the cisternal sacs of the sarcoplasmic reticulum. This allows calcium ions to flood the cytosol. Once in the cytosol, calcium will bind to tropnin. This causes troponin to release tropomyosin. Breaking this complex (troponin-tropomyosin) exposes the active site on an actin filament. If the ATP attached to myosin is hydrolyzed, myosin will convert to it's "cocked" position. The myosin will then bind to the unblocked active site of actin, creating a myosin-actin crossbridge. The myosin will then convert back to its bent, lower energy form and thus pull the actin filament to create a contraction.
