Muscle Structure and Function PDF

R.E.B, 4MedStudents.com, 2003 By: Dr. Safinaz Hamdy El Khoulany Lecturer of Medical Biochemistry and Molecular Biology 1 Structure of a Muscle Cell...

R.E.B, 4MedStudents.com, 2003 By: Dr. Safinaz Hamdy El Khoulany Lecturer of Medical Biochemistry and Molecular Biology 1 Structure of a Muscle Cell 2 3 R.E.B, 4MedStudents.com, 2003  Skeletal muscles consist of 100,000s of muscle cells (also known as 'muscle fibers') that perform the functions of the specific muscle of which they are a part. Musclefasiscles (Bundles)fibres(cells)Myofibrill (unit)Filaments (Thin and Thick) 4 5 Each muscle fibre ('muscle cell') is covered by a plasma membrane sarcolemma The cytoplasm present in muscle fibres (muscle cells) is called sarcoplasm The sarcoplasm present in muscle fibres contains many mitochondria, which are the energy-producing units within the cell. As they produce large amounts of 'Adenosine Triphosphate 'ATP‘ which is necessary for muscle contraction 6 Sarcoplasmic reticulum is simlar to smooth endoplasmic reticulum (SER). Sarcoplasmic reticulum extends throughout the sarcoplasm of muscle fibres. Its function is to store calcium ions, which are necessary for muscle contraction. Myoglobin is also present in the sarcoplasm of muscle fibres. This is a reddish pigment that results in the distinctive colour of skeletal muscle and stores oxygen - until it is required by the mitochondria for the ATP. 7 8  Myosin Actin The components of skeletal muscle cells that are specific to muscle tissue are called myofibrils which are cylindrical structures that extend along the complete length of the muscle fibre. Each myofibril consists of two types of protein filaments called : thick filaments/ Myosin thin filaments/Actin They overlap forming units called sarcomeres. 9  M-line Sarcomeres They are sections of myofibril within which the two filaments occur. They are separated from each other by areas of dense material called Z discs or line. They are described in terms of the bands: The 'A band' is a relatively darker area within the sarcomere that extends along the total length of the thick filaments. The 'H zone' is at the centre of the A band of each sarcomere in which there are only thick filaments. The 'I band' is the region between adjacent A bands, in which there are only thin filaments. ( Each I band extends across two adjacent sarcomeres.) 10 11 12  Contractile proteins of muscle fibers 1- Main proteins: Myosin and actin form the main contractile elements of muscles because their binding controls the state Actin of contraction / relaxation of the muscle Myosin 2- Regulatory proteins: Tropomyosin Troponin 13  Thick Filaments (Myosin) 55% of muscle proteins It is formed of two heavy chains and four light chains The two heavy chains have: 1-Globular heads with ATPase activity and interact with Actin 2-Long Alpha (α) helical tails that twist around each others The four light chains are around the heads of heavy chains 14  Thin Filaments (Actin) Actin molecules join together forming chains twisted into a helix configuration. These molecules are very important to the contraction mechanism of muscles because each actin molecule has a single 'myosin-binding site 15  Tropomyosin A fibrous protein consisting of two chains alpha and beta Attach to grooves in Actin filament 16  Troponin It binds calcium 17 In the resting muscle (relaxed) The Tropomyosin-Troponin complex rests in spiral groove of Actin filament covering the myosin-binding sites on the actin molecules It prevents binding of Actin to the globular head of Myosin 18 1- Nerve impulse and releases of nerotransmitters in neuromuscular junction Depolarization of membrane of muscle fibre and relrase of calcium from sarcoplasmic reticulum  Binding of calcium to troponin  conformational changes of troponin- tropomyosin complex  the modified complex drives itself deeper in the grooves of Actin helix  Exposure of Myosin binding sites  Binding of Actin to the globular head of Myosin 19  The sliding filament mechanism for muscle contraction Myosin The Myosin filaments attach M-line to Actin and pulls the Actin along its length Actin Actin filaments are attached to Z- lines Thus, the sarcomere shortens without change of thin or thick filament length as Z-lines become Thus, the sarcomere shorten closer to M- line from both sides when Actin filaments slide along the Myosin Contraction of sarcomere  filaments shortening of the whole m. fibre Cont. of all m.fibres  enough force to do action 20 21  Biochemical basis of The sliding filament mechanism for muscle contraction 1- A contraction begins when a bound ATP is hydrolyzed into ADP and inorganic phosphate 2-The Myosin head extends and attach to a binding site on Actin forming a cross-bridge 22  Biochemical basis of The sliding filament mechanism for muscle contraction 3- An action called “ the power stroke” is triggered allowing Myosin to pull Actin toward the M-line and thus shortening of sarcomere 4- ADP and Pi are released during the power stroke 23  Biochemical basis of The sliding filament mechanism for muscle contraction 5- the Myosin remains attached to actin until new molecule of ATP binds freeing the Myosin Myosin either go to another cycle of contraction or remains unattached allowing relaxation 24  Regulation of muscle contraction Muscle Contraction is controlled by : 1- Calcium in SR 2- regulatory proteins 25  Regulation of muscle contraction -When muscle is relaxed, Tropomyosin blocks the cross bridges binding sites on Actin -When Ca 2+ ions levels are high enough and ATP is present , Ca 2+ ions binds to Troponin exposing Myosine binding sites on Actin and thus cross bridges formation 26  Nerve impulse and releases of nerotransmitters in neuromuscular junction release which binds to receptors in m. membrane Depolarization of membrane of muscle fibre and release of calcium from sarcoplasmic reticulum  Binding of calcium to troponin  conformational changes of troponin- tropomyosin complex  the modified complex drives itself deeper in the grooves of Actin helix  Exposure of Myosin binding sites  Binding of Actin to the globular head of Myosin 27 Muscle metabolism 28  All cells use ATP as a fuel for their reactions and to perform work. The concentration of ATP within most cells is kept at this steady state level because new ATP is synthesized as fast as it is utilized. Muscle cells present a special case because they perform sustained periods of intense activity. During severe exercise, a muscle may utilize a hundred to a thousand times as much ATP as it does during rest. Thus, the supply has to adjust and meet these enormous demands. Thus, ATP in muscle is supplied via three separate sources: -Creatine phosphate - Anaerobic metabolism -Aerobic metabolism 29 30 31 32 33 Energy coverage under maximum workload 34 35 36