Chapter 6.docx

6 Pages
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Department
Kinesiology & Health Science
Course Code
KINE 2011
Professor
Olivier Birot

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Chapter 6 – Muscle Physiology - Muscle contraction: when muscles develop tension and shorten - Muscle is the largest group of tissue in our body o Make up about 50% of our body mass; 40% made up by skeletal muscle alone (mechanical and involved in homeostasis) - Striated muscle: o Cardiac muscle (involuntary) o Skeletal muscle (voluntary) - Unstriated muscle: o Smooth muscle (involuntary) Structure of Skeletal Muscle Striated Muscle - Muscle fibre/myofibre: a single muscle cell - Striated muscle under a microscope – alternating light bands perpendicular to the long axis o Seen in cardiac and skeletal muscle Skeletal Muscle - A myofibre is formed by the fusion of undifferentiated and mononucleated cells called myoblasts o Myoblast = one single cylindrical, relatively large and elongated, multinucleated cell  In adults – 20-100um diameter, 20 cm length - Satellite cells can be used in cases of muscle injury o New fibres can form from these undifferentiated cells - Muscle: a number of muscle fibres bound together by connective tissue o Bound to bone by bundles of collagen fibres (tendons) - Why we called them „striated‟ muscles: o They have an arrangement of numerous thin and thick filaments  Composed of the contractile proteins actin (thin filaments) and myosin (thick filaments)  Are arranged in a repeating pattern along the length of the myofibril o Sarcomere: one unit of this repeating pattern o Myofibrils (in the cytoplasm) make up 80% of the volume of the muscle - Epimysium: surrounds an entire muscle - Perimysium: surrounds groups of myofibrils (groups them into bundles) - Endomysium: surrounds each individual myofibril A and I Bands - A band: wide, dark bands (thick filaments, myosin) located in the centre of the sarcomere o Thick filaments of myosin are anchored in place by titin fibres - Z line: two sets of thin filaments, one at each end of a sarcomere, that are anchored to a network of connecting proteins – one sarcomere = between two Z lines o The other extremities (actin) overlap a portion of the thick filaments  Twice as many actin filaments as myosin filaments (actin surrounds myosin)  Actin are anchored to the Z lines  Tropomyosin and troponin bind to the actin filaments  Are important regulatory molecules for the contraction - I band: lighter band; between the ends of the A bands when thin filaments do NOT overlap the thick ones - H zone: centre of the A band; portion without any thin filaments (no overlap of filaments) - The sarcomere is the functional unit of skeletal muscle o Is the smallest component that can contract in the myofibre - Cross-bridge: portion of thick myosin molecule that extends to reach the thin actin filaments o Enables a muscle contraction o Myosin cross-bridges bind to actin then flex to slide actin  Sliding movement – actin move towards the centre of the sarcomere  Make the H zone smaller  Each myosin can interact with 6 actin filaments  Each actin can interact with 3 myosin filaments How a Muscle Contracts - Muscle contraction does not necessarily mean muscle shortening o Contraction: activation of force-generating sites within muscle fibres  Followed by relaxation - Sliding filament mechanism: when force generation produces shortening of skeletal muscle fibres, an overlapping thick and thin filaments in each sarcomere move past each other o This is the basis for the shortening of skeletal, smooth and cardiac muscle o Changes in the membrane potential of muscle are linked to internal changes in calcium release (and contraction)  Myosin and actin interactions are regulated by the availability of Ca ions - Structure of actin molecule: globular molecule – single polypeptides that polymerize (come together) to generate a long actin filament; eventually consist of 2 intertwined helical chains o Each actin molecule contains a binding site for myosin - Structure of myosin molecule: o Two large polypeptide heavy chains + four smaller light chain = a large molecule with two globular heads and one long tail  Globular heads = the cross-bridge  Each head has two binding sites: o One for actin o One for ATP – also has enzymatic activity (ATPase) which produces energy for the contraction o The myosin molecules on the two ends of each thick filament are oriented in opposite directions  Results in the power strokes of the cross-bridges moving the thin filaments towards the centre of the sarcomere  The actual length of the fibres does not change, they are simply moving towards each other Molecule Basis of Skeletal Muscle Contraction - Sequence of events (cross-bridge cycle): o Binding between the myosin cross-bridge and the thin actin filament o Movement of the cross-bridge (power stroke), pulling the actin inward o Detachment of cross-bridge from actin, return to original conformation o Re-energizing of the cross-bridge so that it can attach to a more distal thin filament and repeat the cycle Excitation-Contraction Coupling - Calcium will initiate the cross-bridge cycling by entering the cytoplasm o By excitation-contraction coupling:  The action potential in the plasma membrane of a myofibril produces the initiation of the cross-bridge cycling - Transverse (T-) tubules: directly link the plasma membrane and the lateral sacs (store calcium) o Run perpendicularly from the surface of the muscle cell membrane into the central portion of the muscle fibre (deep into the muscle) - Release of calcium from the sarcoplasmic reticulum: o Cytosolic calcium concentration is low  Action potential produces a rapid increase in the cytosolic calcium concentration  The source of calcium = the sarcoplasmic reticulum o Similar to the endoplasmic reticulum – forms sleeve-like segments around each myofibril with lateral sacs on the ends  Lateral sacs are connected to each other by smaller tubular elements - T-tubules bring action potentials into the interior of the skeletal muscle fibres, so that the wave of depolarization passes close to the sarcoplasmic reticulum, stimulating the release of calcium ions o Extensive meshwork of sarcoplasmic reticulum assures that when it releases calcium ions, these ions can readily diffuse to all of the troponin sites - In relaxed skeletal muscle, tropomyosin blocks the cross-bridge binding site on actin o Partially covers the myosin-binding site on each actin molecule - Tropomyosin is held in this inhibitory binding position by the troponin molecule o Troponin has three subunits:  Subunit T – interacts with the tropomyosin molecule  Sub unit I – has an inhibitory grip that prevents tropomyosin from moving along actin  Subunit C – is the binding site for calcium o Tropomyo
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