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Muscle Contraction – Flashcards

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8 July 2022

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Myosin heads have binding sites for _____.

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ATP and actin

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The "motor unit" in vertebrate skeletal muscle refers to _____.

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one motor neuron and all of the muscle fibers on which it has synapses

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The muscles of a recently deceased human can remain in a contracted state, termed rigor mortis, for several hours, due to the lack of _____.

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ATP needed to break actin-myosin bonds

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Contraction of a muscle cell begins with input from a motor neuron across the synaptic cleft and results in a change in the cytosolic _____ ion in the muscle cell

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Ca2+

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Myofibrils

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A skeletal muscle cell is composed of a large number of contractile fibrils

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Sarcomere

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The basic unit of contraction in a myofibril is a sarcomere. The sarcomeres are arranged end to end along the entire length of each myofibril.

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Sarcoplasmic Reticulum

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Each myofibril is partially surrounded by a specialized form of endoplasmic reticulum, called the sarcoplasmic reticulum. The sarcoplasmic reticulum is a specialized form of ER that surrounds each myofibril.

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Function of sarcoplasmic reticulum

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The sarcoplasmic reticulum functions to control cytosolic Ca2+ levels in the muscle cell. Changes in cytosolic Ca2+ concentrations couple action potentials to muscle contraction.

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Funcitonal components of sarcomere:

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The thin filaments and thick filaments and their associated proteins are the key functional components of the sarcomere.

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An action potential traveling along the motor neuron initiates an action potential in the muscle cell, which leads to ___________.

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Muscle contration

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At the synaptic terminal of the _________________, an action potential causes the release of neurotransmitters, which subsequently trigger an action potential on the plasma membrane of the ____________.

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motor neuron, muscle cell

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In the interior of the muscle cell, the action potential initiates changes in the ________.

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sarcoplasmic reticulum

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The role of T tubules in conducting an action potential:

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T tubules are infoldings of the plasma membrane that encircle the myofibrils and are in contact with the sarcoplasmic reticulum. Without T tubules, the muscle cell would not be able to contract. T tubules carry action potentials into the interior of the muscle cell via voltage-gated Na+ and K+ channels.

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The T tubules play two important roles in linking an action potential to muscle contraction.

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1) T tubules propagate the action potential from the plasma membrane into the interior of the muscle cell via voltage-gated Na+ and K+ channels. 2) An action potential carried by a T tubule regulates the opening and closing of Ca2+ channels in the sarcoplasmic reticulum. The resulting change in cytosolic Ca2+ concentration triggers contraction of the myofibrils.

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The concentration of Ca2+ ions in the sarcoplasmic reticulum is typically much _______ than the Ca2+ concentration in the cytosol. This concentration gradient is key to the movement of Ca2+ in the muscle cell in response to an action potential.

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higher,

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Myosin Head

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motor protein that drives contraction

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Calcium Binding Sites

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found on troponin proteins

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Myosin-Binding Sites

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found on actin proteins of the thin filament

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Tropomyosin

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protein that controls access to myosin-binding sites

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Thick Filament

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composed of myosin proteins, whose heads bind to actin and pull the thin filament toward the center of the sarcomere pulls thin filament toward center of sarcomere during contraction

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Thin Filament

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composed of actin and regulatory proteins that respond to changes in the Ca2+ concentration in the sarcomere. pulled toward centrer of sarcomere during contraction.

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Relaxed muscle

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In this state, the myosin molecules of the thick filaments are not in contact with the actin of the thin filaments, and ADP and Pi are bound to the myosin heads. An action potential on the muscle cell plasma membrane initiates contraction of the sarcomeres from this relaxed state.

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1) A relaxed muscle recieves an action potential leading to:

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the release of calcium ions

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2) The release of calcium ions leads to

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the binding of myosin and actin, forming a cross-bridge.

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3) the binding of myosin and actin leads to:

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myosin releasing phosphate

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4) myosinn releasing phosphate leads to:

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The power stroke

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What is the power stroke?

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myosin pulls actin toward center of sarcomere

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5) the power stroke leads to:

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myosin binds ATP

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6) myosin binding ATP leads to:

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detachment of myosin and actin

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7) the detachment of myosin and actin leads to:

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myosin hydrolyzing ATP to ADP and Pi

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8) myosin hydrolyzing ATP to ADP and Pi leads to:

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extension: myosin head extends/pivots

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The sequence of events involved in the regulation of skeletal muscle contraction by Ca2+ ions:

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1) An action potential causes the release of Ca2+ ions from the sarcoplasmic reticulum (SR), resulting in an increase in Ca2+ in the cytosol and sarcomeres. 2) Ca2+ binds to troponin complexes of the thin filaments, causing tropomyosin to move and expose the myosin-binding sites on actin. 3) The myosin heads repeatedly bind and release actin, pulling the thin filaments toward the center of the sarcomere. 4) After the action potential is completed, the Ca2+ channels close and ATP-dependent Ca2+ pumps transport Ca2+ back into the SR, resulting in a decrease in Ca2+ in the cytosol and sarcomeres. 5) The lower cytosolic Ca2+ level causes Ca2+ to be released from troponin. As a result, tropomyosin moves and covers the myosin-binding sites on actin, blocking the myosin heads from binding to actin. This blockage of the myosin-binding sites, in conjunction with ATP binding to the myosin heads, causes the muscle to relax.

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Tetanus

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muscle twitch and sustained contraction

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What does it mean when saying that muscle contration is a graded process?

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you can voluntarily alter the strength and extent of contraction of your skeletal muscles, such as your biceps. Increasing the strength and extent of contraction occurs by increasing the number of muscle cells that receive action potentials. In addition, increasing the number of action potentials sent to a muscle cell can also increase muscle tension,

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why does a series of closely spaced action potentials causes a sustained contraction rather than a series of closely spaced twitches?

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When a series of action potentials is closely spaced, there is not sufficient time for Ca2+ uptake into the sarcoplasmic reticulum between action potentials, and Ca2+ remains bound to troponin throughout the series.

Muscle Contraction – Flashcards

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